Luet oppimateriaalin englanninkielistä versiota. Mainitsit kuitenkin taustakyselyssä osaavasi suomea. Siksi suosittelemme, että käytät suomenkielistä versiota, joka on testatumpi ja hieman laajempi ja muutenkin mukava.

Suomenkielinen materiaali kyllä esittelee englanninkielisetkin termit.

Kieli vaihtuu A+:n sivujen yläreunan painikkeesta. Tai tästä: Vaihda suomeksi.

Scala Reference

After studying the actual chapters of O1’s ebook and then turning to a programming problem, you may find yourself thinking “How was I supposed to write that thing again?” When that happens, you may want to check this page. The sections below summarize selected features of the Scala language and its standard libraries. There are short, isolated examples of each feature.

The reference doesn’t cover the entire Scala language; it focuses on topics that are covered in O1. In addition to standard Scala constructs, a few of the main tools in O1’s own auxiliary library are included.

This appendix of the ebook is just a list of tools. It won’t teach you any principles, or concepts, nor will it tell you what you may want to use all these constructs for; those are the sorts of things that you can learn in the ebook proper. The ordering of the sections on this page is not identical to the order in which the constructs appear in the ebook chapters.

Can’t find what you’re looking for?

You may find it through these links:

• glossary,
• O1’s style guide,
• other resources.

In the long run, you may want to learn to read the Scala Language Specification and the Scala API documentation. Parts of those documents are hard for a beginner programmer to make sense of, however.

If you wanted to find something on this page that isn’t here, you can let us know through the feedback form at the bottom of the page or directly via email to juha.sorva@aalto.fi.

The Very Basics

Numbers

Basic arithmetic (Chapter 1.3):

100 + 1res0: Int = 101
1 + 100 * 2res1: Int = 201
(1 + 100) * 2res2: Int = 202

Dividing an Int (integer) with another Int chops off any decimals and effectively rounds towards zero:

76 / 7res3: Int = 10

The modulo operator % produces the remainder of a division (Chapter 1.7):

76 % 7res4: Int = 6

Doubles have decimals (up to a limit):

76.0 / 7.0res5: Double = 10.857142857142858

See Chapter 5.4 for the constraints that govern numerical data types’ range and precision. For the methods available on these types, see Chapter 5.2.

Characters and strings

A String is a sequence of characters (Chapter 1.3). Strings have the operators + and *:

"mam" + "moth"res6: String = mammoth
"moth" * 3res7: String = mothmothmoth

The Char type represents individual characters (Chapter 5.2). A Char literal goes in single quotation marks:

'a'res8: Char = a
'!'res9: Char = !

For more on strings and characters, see Methods on string objects, Collection Basics, and Processing Collections with Higher-Order Methods further down on this page.

Variables

A variable definition (Chapter 1.4):

val myNumber = 100myNumber: Int = 100

You may mark a data type on the variable explicitly, as shown below, but due to Scala’s type inference, you often don’t need to:

val anotherVariable: Int = 200anotherVariable: Int = 200

You can use a variable’s name as an expression. Such an expression can be part of a longer expression:

myNumberres10: Int = 100
myNumber + anotherVariable + 1res11: Int = 301

You can use val or var to define a variable. The value of a val never changes, but the value of a var may be assigned a new value, which replaces the old one:

var changeable = 100changeable: Int = 100
changeable = 150changeable: Int = 150
changeable = changeable + 1changeable: Int = 151

The command just above assigns the variable a new value that is obtained from the variable’s old value with a simple computation. There is a shorthand for such commands, which combines the assignment with the arithmetic operator (Chapter 4.1):

changeable += 10changeable: Int = 161
changeable -= 100changeable: Int = 61
changeable *= 2changeable: Int = 122

Comments

Program code may contain comments, which don’t affect the program’s behavior (Chapter 1.2).

// This is a single-line comment. It starts with a couple of slashes and runs until the end of the line.

val myVariable = 100  // You can follow a line of code with a comment.

/* A comment like this, which starts with a slash and
   an asterisk, may be split over multiple lines of code.
   The comment ends in the same characters in reverse order. */

An initial /** marks a documentation comment (Chapter 3.2):

/** This description of the variable below will appear in the documentation. */
val myText = "I have been documented."

The Scaladoc tool extracts such comments from Scala code and uses them in the documents that it creates.

Packages and Libraries

Using packages

When you want to use one of the tools (functions, classes, etc.) from Scala’s standard library (Chapter 3.2) or from some other package, you can prefix the tool’s name with the name of the package. Here, we access the abs function in package scala.math to compute an absolute value:

scala.math.abs(-50)res12: Int = 50

Since the contents of the package scala are always available in all Scala programs, we’re allowed to leave out the first bit and just refer to the subpackage math:

math.abs(-50)res13: Int = 50

The universal package scala contains basic data types such as Int and Double, collection types such as Vector and List, and the output function println. You can use these tools without specifying the package. For instance, even though you could write scala.Int, you don’t have to.

To avoid having to write a package name repeatedly, you can import:

importing from a package

You can import the tools you need from a package (Chapter 1.6):

import scala.math.absimport scala.math.abs
abs(-50)res14: Int = 50
abs(100)res15: Int = 100

Now we don’t need the package name.

This gives us access to all the tools in the package:

import scala.math._import scala.math._

It’s common to write import statements at the top of the code file, which makes the imported tools available within the entire file. You can also import within a specific context; for example, starting a function body with an import makes the imported tools available in that function only.

Defining a package

You can sort your own code in packages by marking the package at the top of each file (Chapter 2.6). Here’s an example:

package mystuff.subpackage.subsubpackage

You then need to store these files in nested folders so that the folder names match the packages.

Scala moreover allows us to use individual objects as packages; see the section Package objects, below.

Commonly used functions from scala.math

A few frequently used functions from scala.math:

import scala.math._import scala.math._
val absoluteValue = abs(-50)absoluteValue: Int = 50
val power = pow(10, 3)power: Double = 1000.0
val squareRoot = sqrt(25)squareRoot: Double = 5.0
val sine = sin(1)sine: Double = 0.8414709848078965
val greaterNumber = max(2, 10)greaterNumber: Int = 10
val lesserNumber = min(2, 10)lesserNumber: Int = 2

In the same package, you’ll find other trigonometric functions (cos, atan, etc.), cbrt (cubic root), hypot (hypotenuse of two given legs), floor (rounding down), ceil (rounding up), round (rounding to nearest integer), log and log10 (logarithms). The complete list is in Scala’s documentation.

The other sections on this page introduce contents from the Scala API’s other packages as appropriate for each topic.

The text console: println, readLine

You can use println to generate a custom printout in the text console or the REPL:

println(100 + 1)101
println("llama")llama

Below are a few examples of reading keyboard input in the text console (Chapter 2.7). These examples assume an earlier import scala.io.StdIn._.

println("Please write something on the line below this prompt: ")
val textEnteredByUser = readLine()

If you don’t want a line break between the prompt and the input, you can use print instead:

print("Please write something after this prompt, on the same line: ")
val textEnteredByUser = readLine()

This does the same thing:

val textEnteredByUser = readLine("Please write something after this prompt, on the same line: ")

readLine returns a String. You can also read an input and immediately interpret it as a number:

val intInput = readInt()
val doubleInput = readDouble()

These two last commands cause a runtime error if the characters in the input don’t correspond to a valid number.

Function Basics

A simple function

An example function from Chapter 1.7:

def average(first: Double, second: Double) = (first + second) / 2

We need to annotate each parameter variable with its type.

Note the required punctuation.

When the function body consists of just a single expression, the function’s return value is determined by evaluating that expression.

Calling a function

average(10.0, 12.5)res16: Double = 11.25

A function call is an expression. Its value is the value that the function returns.

Multiple lines in a function

When the function body consists of several consecutive commands, you need to put them in curly brackets. Here’s an example from Chapter 1.7:

def incomeTax(income: Double, thresholdIncome: Double, baseRate: Double, additionalRate: Double) = {
  val baseTax = min(thresholdIncome, income)
  val additionalTax = max(income - thresholdIncome, 0)
  baseTax * baseRate + additionalTax * additionalRate
}

You still need the equals sign.

The expression that is evaluated last determines the function’s return value.

If the function is an effectful one, it’s customary to write the curly brackets around the function body even when it’s not required; see the style guide.

Function parameters

The example functions above had a single parameter list (in round brackets after the function name). That parameter list may be empty (Chapter 2.6):

def printStandardMessage() = {
  println("This is printed out every time we call printStandardMessage() .")
}

A function may not have a parameter list at all. (This is more common when the function is a method on an object; Chapter 2.2.)

def returnStandardText = "Calling returnStandardText always yields this string."

Or there may be multiple parameter lists (Chapter 6.1):

def myFunc(first: Int, second: String)(additionalParameter: Int) = first * additionalParameter + second

myFunc(10, "llama")(100)res17: String = 1000llama

Return values

In all of the examples above, we left the function’s return type implicit, which we can do because of type inference. But we may choose to explicitly mark the return type (Chapter 1.8), as shown here:

def average(first: Double, second: Double): Double = (first + second) / 2

def returnStandardText: String = "Calling returnStandardText always yields this string."

In certain contexts, an explicit return type annotation is mandatory. Primarily, this happens when a function calls a function of the same name; that is, the function calls either:

• another function with the same name but different parameters (when overloading a name; Chapter 4.1); or
• itself (in a recursive function; Chapter 12.1).

Explicitly returning a value

It’s possible (but not usual in Scala) to explicitly instruct a function to return a value. The return command (Chapter 8.3) interrupts the function call and returns a value.

def incomeTax(income: Double, thresholdIncome: Double, baseRate: Double, additionalRate: Double): Double = {
  val baseTax = min(thresholdIncome, income)
  val additionalTax = max(income - thresholdIncome, 0)
  return baseTax * baseRate + additionalTax * additionalRate
}

We follow return with the expression whose value should be returned.

A function that uses return needs a return type annotation.

Singleton Objects

Defining an object: methods, variables, and this

Here is a definition of an singleton object taken from an example in Chapter 2.2. (That chapter discusses the example in more detail.)

object employee {
  var name = "Edelweiss Fume"
  val yearOfBirth = 1965
  var monthlySalary = 5000.0
  var workingTime = 1.0

  def ageInYear(year: Int) = year - this.yearOfBirth

  def monthlyCost(multiplier: Double) = this.monthlySalary * this.workingTime * multiplier

  def raiseSalary(multiplier: Double) = {
    this.monthlySalary = this.monthlySalary * multiplier
  }

  def description =
    this.name + " (b. " + this.yearOfBirth + "), salary " + this.workingTime + " * " + this.monthlySalary + " e/month"

}

We follow the object keyword with a name that we’ve chosen for our object.

The curly brackets are mandatory. An object body, unlike a function definition, doesn’t have an equals sign before the body.

Variables store data associated with the object. Some of the variables may be immutable (val) and others mutable (var).

Methods are functions that are attached to objects. Each method definition starts with def.

The this keyword refers to the object itself: the object whose method is being executed. For instance, the value of this.name is the value of the object’s own name variable. (It’s not strictly necessary to always include the word this in all such expressions; see Chapter 2.2.)

Using a singleton object: dot notation

You can access the variables of an object:

employee.monthlySalaryres18: Double = 5000.0
employee.workingTime = 0.6employee.workingTime: Double = 0.6

And call the object’s methods:

employee.raiseSalary(1.1)employee.ageInYear(2021)res19: Int = 56

Launching an application: app objects

An app object (Chapter 2.7) is a singleton object that serves as an application’s entry point:

object MyTestProgram extends App {
  println("These lines of code are executed when the application is launched.")
  println("This simple app does nothing more than print out these lines of text..")
  println("In a richer app, we could call other objects’ methods here.")
}

extends App makes this an app object. (To be more precise, it mixes in the App trait; see Traits, further down.)

Package objects

A singleton object can serve as a package for assorted tools such as functions, objects, and classes (Chapter 5.2). Such a package object can be defined as a regular singleton object. Here, we define a package named mystuff.experiment:

package mystuff

object experiment {
  def doubled(number: Int) = number * 2
  def tripled(number: Int) = number * 3
}

The functions are defined in an object named experiment, which we intend to use as a package object.

The above code needs to be stored within a folder named mystuff; we might name the file experiment.scala, for instance. We can now import the tools in our object:

import mystuff.experiment._import mystuff.experiment._
doubled(10)res20: Int = 20
tripled(10)res21: Int = 30

An alternative way to define a package object

Another way to define a package object is to repeat the package keyword as shown:

package mystuff

package object experiment {
  def doubled(number: Int) = number * 2
  def tripled(number: Int) = number * 3
}

A package object thus defined needs to be placed in a folder that matches the name of the package; it might go in mystuff/experiment/package.scala, for example. People often name such files package.scala, but a different name will also work.

Classes (and more about objects)

Defining a class

Here is an example class from Chapter 2.4; it represents employees. Each instance of this class is a distinct object of type Employee and has its own attributes:

class Employee(nameParameter: String, yearParameter: Int, salaryParameter: Double) {

  var name = nameParameter
  val yearOfBirth = yearParameter
  var monthlySalary = salaryParameter
  var workingTime = 1.0

  def ageInYear(year: Int) = year - this.yearOfBirth

  // Etc. Other methods go here.
}

The keyword class precedes the class name. The curly brackets around the class body are mandatory.

Constructor parameters: when we instantiate this class with new, we need to pass in a name, a year, and a salary.

The code inside the curly brackets (apart from the method definitions) serves as a constructor: it initializes each new instance. Here, we assign values to the object’s instance variables, taking most of those values from the constructor parameters; however, we set the working time of any new Employee object as 1.0, independently of any parameters.

Method definitions in a class work just like they work in singleton objects. In a class, too, this refers to the specific object that runs the method. For instance, ageOfYear computes an employee’s age from the yearOfBirth of whichever Employee object we invoke ageOfYear on.

There is a more compact notation for class definitions (Chapter 2.4):

class Employee(var name: String, val yearOfBirth: Int, var monthlySalary: Double) {

  var workingTime = 1.0

  def ageInYear(year: Int) = year - this.yearOfBirth

  // Etc. Other methods go here.
}

We’ve combined the definitions of three instance variables and the three constructor parameters that they receive their values from.

The fourth instance variable doesn’t receive a value directly from a constructor parameter. We define it separately, as before.

Creating and using instances

We can use the above Employee class as shown below (Chapter 2.3):

new Employee("Eugenia Enkeli", 1963, 5500)res22: o1.Employee = o1.Employee@1145e21

We use new to instantiate the class, following it with the name of the class and values for the constructor parameters.

The expression’s value is a reference to a new object: an instance of the class.

We can store such a reference to a newly created object in a variable. Then we can use the variable’s name to access the object:

val justHired = new Employee("Teija Tonkeli", 1985, 3000)justHired: o1.Employee = o1.Employee@704234
justHired.ageInYear(2021)res23: Int = 36
println(justHired.description)Teija Tonkeli (b. 1985), salary 1.0 * 3000.0 e/month

Tailoring an instance

It’s possible to define methods that are specific to an individual instance of a class.

Here’s an example class from Chapter 2.4:

class Person(val name: String) {
  def say(sentence: String) = this.name + ": " + sentence
  def reactToKryptonite = this.say("What an odd mineral.")
}

A regular Person doesn’t know how to fly, but the following specific Person instance does. Moreover, one of its methods works differently than the same method on other Person objects.

val realisticSuperman = new Person("Clark") {
  def fly = "WOOSH!"
  override def reactToKryptonite = "GARRRRGH!"
}

We create a Person as usual except that we tailor this instance within the curly brackets that follow.

This one Person instance has fly as an additional method.

The other instance-specific method replaces the more generic definition in the Person class, a fact that we need to mark with the override keyword.

(This is actually an example of inheritance; see Chapter 7.3._)

Scala’s basic types as objects; operator notation

Basic types such as Int, Double, and String are classes, too, and the operations defined on them are methods (Chapter 5.2). For instance, when we use the + method to add two numbers, it’s possible to use dot notation:

1.+(1)res24: Int = 2

The more familiar expression 1 + 1 also works: when a method takes exactly one parameter, we can opt to use operator notation and omit the dot and the brackets. This also works on methods that we wrote ourselves:

justHired ageInYear 2021res25: Int = 36

Image Manipulation with O1Library

The IntelliJ module O1Library is a software library that has been designed for O1 and that we use frequently. It contains an assortment of tools for graphical programming, among other things.

The relevant contents of O1Library are introduced in various chapters of the ebook; you can also look at the module’s documentation. What appears below is a short summary of some of the features you’re most likely to need in O1:

Colors: o1.Color

The Color class represent colors (Chapter 1.3). The o1 package provides many specific instances of this class as constants:

import o1._import o1._
Redres26: Color = Red
CornflowerBlueres27: Color = CornflowerBlue

These named color constants cover all the colors listed in W3C’s CSS Color Module standard and some others as well.

You can also define a color as a combination of its RGB components (Chapter 5.4). Each component is a number between 0 and 255, inclusive. Below, we create a fairly bright color that is especially high in red and blue:

val preciselyTheColorWeWant = Color(220, 150, 220)preciselyTheColorWeWant: Color = Color(220, 150, 220)

You can access the color’s individual components:

preciselyTheColorWeWant.redres28: Int = 220
CornflowerBlue.blueres29: Int = 237

In addition to their R, G, and B components, colors have an opacity value (sometimes called the alpha channel):

Red.opacityres30: Int = 255
val translucentRed = Color(255, 0, 0, 100)translucentRed: Color = Color(255, 0, 0, opacity: 100)

A color with an opacity of only a hundred is fairly translucent. An opacity of zero would have made it completely transparent; An opacity of 255 means the color is ompletely opaque, which is the default.

Locations: o1.Pos

The class o1.Pos represents locations in a two-dimensional coordinate system (Chapter 2.5).

 val first = new Pos(15.5, 10)first: Pos = (15.5,10.0)
val second = Pos(0, 20)second: Pos = (0.0,20.0)

A Pos object is essentially a pair of coordinates, each of which is a Double.

You don’t need to use new to instantiate Pos (because the class comes with a factory method; see Chapter 5.3).

You can examine each coordinate separately:

first.xres31: Double = 15.5
first.yres32: Double = 10.0

You can compute on Pos objects:

val distanceAlongX = second.xDiff(first)distanceAlongX: Double = 15.5
val distanceAlongY = second.yDiff(first)distanceAlongY: Double = -10.0
val distanceAsCrowFlies = first.distance(second)distanceAsCrowFlies: Double = 18.445866745696716
val aBitToTheRight = first.addX(1.5)aBitToTheRight: Pos = (17.0,10.0)
val adjustedBoth = aBitToTheRight.add(10, -5)adjustedBoth: Pos = (27.0,5.0)

None of the above methods changes the existing Pos objects; neither does any other method. The add method, for example, doesn’t modify the existing Pos but generates a new one. Pos objects are immutable.

For more methods, see, e.g., Chapter 3.1 and the documentation.

Pictures: o1.Pic

The class o1.Pic represents images.

You can load an image from a file or a network address (Chapter 1.3):

val loadedFromFileInModule = Pic("face.png")loadedFromFileInModule: Pic = face.png
val loadedFromAbsoluteFilePath = Pic("d:/kurssi/GoodStuff/face.png")loadedFromAbsoluteFilePath: Pic = d:/kurssi/GoodStuff/face.png
val loadedFromTheNet = Pic("https://en.wikipedia.org/static/images/project-logos/enwiki.png")loadedFromTheNet: Pic = https://en.wikipedia.org/static/images/project-logos/enwiki.png

The file you load may be within the same IntelliJ module as the code, in the pics folder of the O1Library module, or somewhere else in the program’s classpath.

Pics have a width and a height in pixels:

loadedFromTheNet.widthres33: Double = 135.0
loadedFromTheNet.heightres34: Double = 155.0

To display an image, you can use o1.show or the method of the same name on Pic objects:

show(loadedFromTheNet)loadedFromTheNet.show()

There are several functions available that generate images of geometric shapes. Here are a few examples:

val myCircle = circle(250, Blue)myCircle: Pic = circle-shape
val myRectangle = rectangle(200, 300, Green)myRectangle: Pic = rectangle-shape
val myIsoscelesTriangle = triangle(150, 200, Orange)myIsoscelesTriangle: Pic = triangle-shape
val myStar = star(100, Black)myStar: Pic = star-shape
val myEllipse = ellipse(200, 300, Pink)myEllipse: Pic = ellipse-shape

The Pic methods that combine images by placing them relative to each other (Chapter 2.3) see a lot of use in O1. Here are some examples:

val circleBesideRect = myCircle.leftOf(myRectangle)circleBesideRect: Pic = combined pic
val circleBelowRect = myCircle.below(myRectangle)circleBelowRect: Pic = combined pic
val circleInFrontOfRect = myCircle.onto(myRectangle)circleInFrontOfRect = combined pic

The methods don’t modify any existing image; they create new Pic objects.

You can also place an image against a background image (Chapter 2.5):

val littlePic = rectangle(10, 20, Black)littlePic: Pic = rectangle-shape
val littlePicAgainstBg = myRectangle.place(littlePic, Pos(30, 80))littlePicAgainstBg: Pic = combined pic
val withAnAddedCircle = littlePicAgainstBg.place(myCircle, Pos(150, 150))withAnAddedCircle: Pic = combined pic

We must inform place where in the background image it should place the front image. Here, we do that by passing in a pair of coordinates (in which x grows rightwards and y downwards). The front image’s middle will appear at those coordinates in the combined image.

The largish circle doesn’t entirely fit against the background. place discards the part that doesn’t fit from the result.

Here is a partial list of the methods available on Pic objects:

• Placement on a single plane: above, below, leftOf, rightOf (Chapter 2.3).
• Placement in front of and behind: onto, against, place (Chapters 2.3 and 2.5).
• Placement using anchors (e.g., “Put the top-left corner of this pic at the center of that pic’s top edge.”): see the end of Chapter 2.5.
• Rotation: clockwise, counterclockwise (Chapter 2.3).
• Mirroring: flipHorizontal, flipVertical (Chapter 2.3).
• Scaling: scaleBy (Chapter 2.3), scaleTo.
• Selecting a part: crop (Chapter 2.5).
• Shifting along a coordinate axis: shiftLeft, shiftRight (Chapter 3.1).
• Examining individual pixels: pixelColor (Chapter 5.4).
• Transforming by pixel: transformColors, combine (Chapter 6.1).
• Generating from pixels: Pic.generate (Chapter 6.1).

The complete list is in the Scaladocs.

Other classes in package o1

In addition to Color, Pos, and Pic, the o1 package contains other tools that are useful for creating graphical programs. In particular:

• The class view View provides a framework for writing GUIs. See the section Graphical User Interfaces further down on this page.
• The class Direction represents (arbitrary) directions in a two-dimensional, Pos-based coordinate system (see Chapters 3.6 and 4.4 and the docs).
• The class Grid represents two-dimensional grids of elements (Chapter 7.4; Scaladocs). It’s works in combination with two additional classes:
  • class GridPos, which represents locations on a grid (Chapters 6.3 and 7.4; Scaladocs); and
  • class CompassDir, which represents the main compass directions in a grid-based coordinate system (Chapter 6.3; Scaladocs).
• The class Anchor represents “anchoring points” of images within other images and can make it easier to lay out Pics relative to each other (Chapter 2.5; Scaladocs).

Truth Values

The Boolean type

You can represent truth values with the Boolean data type (Chapter 3.3). There are exactly two values of this type, true and false, each of which has its own Scala literal.

falseres35: Boolean = false
val theValueOfThisVariableIsTrue = truetheValueOfThisVariableIsTrue: Boolean = true

Relational operators

Relational operators produce Booleans (Chapter 3.3):

10 <= 10res36: Boolean = true
20 < (10 + 10)res37: Boolean = false
val age = 20age: Int = 20
val isAdult = age >= 18isAdult: Boolean = true
age == 30res38: Boolean = false
20 != ageres39: Boolean = false

You need a “double-equals” to check for equality.

The operator != checks if the values are not equal.

Logical operators

Logical operators (from Chapter 5.1):

Operator	Name	Example	Meaning
&&	and	someClaim && otherClaim	“Are both Booleans true?”
||	or	someClaim || otherClaim	“Is at least one of the Booleans true?”
^	exclusive˽or (xor)	someClaim ^ otherClaim	“Is exactly one of the Booleans true?”
!	not (negation)	!someClaim	“Is the Boolean false?”

Examples:

val dividend = 50000dividend: Int = 50000
var divisor = 100divisor: Int = 100
!(divisor == 0)res40: Boolean = true
divisor != 0 && dividend / divisor < 10res41: Boolean = false
divisor == 0 || dividend / divisor >= 10res42: Boolean = true
dividend / divisor >= 10 || divisor == 0res43: Boolean = true

The operators && and || are non-strict: if the subexpression on the left is enough to determine the value of the entire logical expression, the subexpression on the right isn’t evaluated at all:

divisor = 0divisor: Int = 0
dividend / divisor >= 10 || divisor == 0java.lang.ArithmeticException: / by zero
...
divisor == 0 || dividend / divisor >= 10res44: Boolean = true
divisor != 0 && dividend / divisor < 10res45: Boolean = false

Dealing with Missing Values

Option, Some, and None

The following example function has the return type Option[Int] (Chapter 4.3). The function either divides two numbers and returns the result wrapped in a Some object, or returns None in case the operation is impossible:

def divide(dividend: Int, divisor: Int) =
  if (divisor == 0) None else Some(dividend / divisor)

divide(100, 5)res46: Option[Int] = Some(20)
divide(100, 0)res47: Option[Int] = None

Here, we use Option as a wrapper for a String:

var test: Option[String] = Nonetest: Option[String] = None
test = Some("like it hot")test: Option[String] = Some(like it hot)

A variable of type Option[String] can refer either to the singleton object None — in which case there is no string there — or a Some object that contains a string.

In the square brackets we put a type parameter. This type parameter indicates the type of the value that may or may not be present in the Option wrapper.

If we were to omit the type annotation, the computer wouldn’t be able to tell which sort of Option we’d like as the type of test.

Scala makes it possible to use the null reference instead of Options; however, it’s highly unadvisable that you do so (Chapter 4.3).

Methods on Option objects

The methods isDefined and isEmpty check whether an Option wrapper is full or empty:

val wrappedNumber = Some(100)wrappedNumber: Option[Int] = Some(100)
wrappedNumber.isDefinedres48: Boolean = true
wrappedNumber.isEmptyres49: Boolean = false
None.isDefinedres50: Boolean = false
None.isEmptyres51: Boolean = true

getOrElse returns the value stored in an Option wrapper. When we call it, we need to pass in a parameter expression that determines what the method should return in case the wrapper is empty:

wrappedNumber.getOrElse(12345)res52: Int = 100
None.getOrElse(12345)res53: Int = 12345

The similar method orElse returns the Option object itself, in case it’s a Some, and the value of the method parameter in case the Option is None. That is, the difference to getOrElse is that orElse doesn’t unwrap the value:

wrappedNumber.orElse(Some(54321))res54: Option[Int] = Some(100)
None.getOrElse(Some(54321))res55: Option[Int] = Some(54321)

More on Option

The following are also useful for working with Options:

• the selection command match, discussed soon below; and
• various higher-order methods, which are discussed further down on this page at Option as a collection.

Selection: if and match

Selecting with if

The if command (Chapter 3.4) evaluates a conditional expression that evaluates to true or false, then selects one of two options on that basis:

val number = 100number: Int = 100
if (number > 0) number * 2 else 10res56: Int = 200
if (number < 0) number * 2 else 10res57: Int = 10

The round brackets around the conditional expression are mandatory. You can use any Boolean expression as a condition.

You can use an if expression as you assign to variables or pass parameters to functions, just like you can use other expressions:

val selected = if (number > 100) 10 else 20selected: Int = 20
println(if (number > 100) 10 else 20)20

When a branch of an if contains multiple commands, you need to split the branch across multiple lines and put it in curly brackets (and it’s customary to do that anyway in case the if is effectful; see the style guide):

if (number > 0) {
  println("The number is positive.")
  println("More specifically, it is: " + number)
} else {
  println("The number is not positive.")
}The number is positive.
More specifically, it is: 100

When all you want to do is to cause an effect in case the condition is true — and nothing otherwise — you can omit the else:

if (number != 0) {
  println("The quotient is: " + 1000 / number)
}
println("The end.")The quotient is: 10
The end.

The final println isn’t part of the if; it follows the if. This is why the above program always finishes with "The end.", no matter whether number holds zero or not. If number had been zero, that would have been the program’s only output.

Combining ifs

One way to select among multiple alternatives is to put an if in another if’s else branch:

val number = 100number: Int = 100
if (number < 0) "negative" else if (number > 0) "positive" else "zero"res58: String = positive
if (number < 0) {
  println("The number is negative.")
} else if (number > 0) {
  println("The number is positive.")
} else {
  println("The number is zero.")
}The number is positive.

There are other ways to nest an if inside another, too:

if (number > 0) {
  println("Positive.")
  if (number > 1000) {
    println("More than a thousand.")
  } else {
    println("Positive but no more than a thousand.")
  }
}  Positive.
Positive but no more than a thousand.

In this example, the outer if has no else branch at all. If number hadn’t been positive, nothing would have been printed out.

In the example above, the else branch connected with the “closer” of the two ifs. That else branch was selected precisely because the outer condition was true but the inner one wasn’t. In the next example, which places the curly brackets differently, the inner if has no else branch but the outer one does:

if (number > 0) {
  println("Positive.")
  if (number > 1000) {
    println("More than a thousand.")
  }
} else {
  println("Zero or negative.")
}Positive.

For a further discussion, see Chapter 3.4. The end of Chapter 3.5 lists some examples of errors that you may make when you use an if to determine the function’s return value.

Selecting with match

The match command (Chapters 4.3 and 4.4) evaluates an expression and then checks a list of possible matches for that value. It selects the first one that matches. Here’s what the command looks like in general terms:

  expression E match {
    case pattern A => code to run if E’s value matches pattern A
    case pattern B => code to run if E’s value matches pattern B (but not A)
    case pattern C => code to run if E’s value matches pattern C (but not A or B)
    And so on. (Usually, you’ll seek to cover all the possible cases.)
  }

The value of the expression that precedes the match keyword is compared to...

... so-called patterns, which describe different cases.

Here’s an example as concrete code:

val cubeText = number * number * number match {
  case 0         => "number is zero and so is its cube"
  case 1000      => "ten to the third is a thousand"
  case otherCube => "number " + number + ", whose cube is " + otherCube
}

We examine the value of the arithmetic expression in order to select one of the cases.

match checks the patterns in order until it finds one that matches the value of the expression. Here, we have a total of three patterns.

Even a simple literal can be used as a pattern. Here, we’ve used a couple of Int literals. The first case is a match if the cube of number equals zero; the second matches if it equals one thousand.

You can also enter a new variable name as a pattern; here, we’ve picked the name otherCube. Such a pattern will match any value; in this example, the third case will always be selected if the cube wasn’t zero or one thousand.

Whenever such a pattern matches, we get a new local variable that stores the actual value that matched the pattern. We can use the variable name to access the value.

One use for match is to extract a value from an Option wrapper:

// We need this function for the example of match below.
def divide(dividend: Int, divisor: Int) =
  if (divisor == 0) None else Some(dividend / divisor)

divide(firstNumber, secondNumber) match {
  case Some(result) => "The result is: " + result
  case None            => "No result."
}

The pattern defines the structure of the matched object: a Some will have some value inside it. That value is automatically extracted and stored in the variable result.

(However, for working with Options, higher-order methods are often even better than match; see Chapter 8.2 and Option as a collection, below.)

Here’s one more example that demonstrates some more features of match. The example is from Chapter 4.4, which you can visit for more optional material on this versatile command.

def experiment(someSortOfValue: Any) =
  someSortOfValue match {
    case text: String              => "it is the string " + text
    case number: Int if number > 0 => "it is the positive integer " + number
    case number: Int               => "it is the non-positive integer " + number
    case vector: Vector[_]         => "it is a vector with " + vector.size + " elements"
    case _                         => "it is some other sort of value"
  }

Our example function’s parameter has the type Any, which means that we can pass a value of any type as a parameter.

The patterns have been annotated with data types. Each of these patterns only matches values of a particular type.

The condition (pattern guard) narrows down the case: we select this branch only if the value is greater than zero (and doesn’t equal 1000, which we already covered in another case). Note that we use the familiar if keyword, but this isn’t a standalone if command.

The underscore pattern matches any value and is selected if neither of the two preceding cases is. We could have written the name of a variable here (as we did in the earlier example), but if we have no use for the value of the variable, an underscore will do.

Scopes and Access Modifiers

Program components — variables, functions, classes, and singleton objects — each have a scope that depends on where that component is defined (Chapter 5.6). The programmer may further adjust scope by adding access modifiers such as private (Chapter 3.2).

The scope of a class and its members

class MyClass(constructorParameter: Int) {

  val publicInstanceVariable = constructorParameter * 2
  private val privateInstanceVariable = constructorParameter * 3

  def publicMethod(parameter: Int) = parameter * this.privateMethod(parameter)

  private def privateMethod(parameter: Int) = parameter + 1 + this.privateInstanceVariable

}

The scope of a public instance variable encompasses the entire class. Moreover, the variable is accessible from outside the class, too, as long as we have an instance of the class available: myObject.publicInstanceVariable. Similarly, we can call a public method anywhere within the class or outside of it. Instance variables and methods are public unless otherwise specified.

The scope of a private instance variable or a private method is limited to the enclosing class.

This class itself is public, so we’re free to use it anywhere in the program.

Method implementations are always inaccessible from outside the method.

The scope of local variables

Mouse over the boxes below to highlight the corresponding scope within the program.

def myFunc(param: Int) = {
  var local = param + 1
  var anotherLocal = local * 2
  if (local > anotherLocal) {
    val localToIf = anotherLocal
    anotherLocal = local
    local = localToIf
  }
  anotherLocal - local
}

A parameter variable such as param is defined throughout the function’s body. It is accessible anywhere within that scope.

The scope of a variable defined at the outermost level within the function, such as local, runs until the end of the function body.

The same goes for anotherLocal.

When an outer command contains a variable definition, the variable’s scope extends only until the end of that command. For instance, here we have a variable localToIf whose scope is limited by the surrounding if.

You’ll find a few more complex examples in Chapter 5.6.

Local functions

As discussed in Chapter 6.4, you can also define functions as local to other functions. Here’s a very simple example:

def outerFunc(number: Int) = {
  def inner(original: Int) = original * 2
  inner(number) + inner(number + 1)
}

inner is defined within the other function’s body and is meant to be used only within the containing function.

The outer function (and only it) may call its auxiliary function. In this example, that happens twice.

Companion objects

As an exception to the general rules outlined above, a class and its companion object have access to each other’s private members. Here’s a summary of an example from Chapter 5.3:

object Customer {
  private var createdInstanceCount = 0
}

class Customer(val name: String) {
  Customer.createdInstanceCount += 1
  val number = Customer.createdInstanceCount

  override def toString = "#" + this.number + " " + name
}

A companion object is a singleton object that has been given precisely the same name as a class and that is defined in the same file with that class.

You can use a companion object to store variables or methods (such as this instance counter) that pertain to a class in general rather its individual instances. Only a single copy of createdInstanceCount exists in memory, since the customer object is a singleton. This contrasts with the names and numbers of the various Customer instances.

The Customer class and its companion object are “friends” and have access to each other’s private members.

Pairs and Other Tuples

A tuple is an immutable structure that consists of two or more values that may or may not have the same data type (Chapter 8.4). You can use round brackets and commas to define a tuple:

val quartet = ("This tuple has four members of different types.", 100, 3.14159, false)quartet: (String, Int, Double, Boolean) = (This tuple has four members of different types.,100,3.14159,false)

A tuple’s members are numbered from one(!) upward. You access them via names that start with an underscore:

quartet._1res59: String = This tuple has four members of different types.
quartet._3res60: Double = 3.14159

Pairs are tuples with two members. Both members of this pair are strings:

val pair = ("laama", "llama")pair: (String, String) = (laama,llama)

You can assign the members of a pair to multiple variables with a single command:

val (finnish, english) = pairfinnish: String = laama
english: String = llama

Instead of the brackets and the comma, you can define a pair like this:

val identicalPair = "laama" -> "llama"identicalPair: (String, String) = (laama,llama)

The latter notation is particularly popular when using pairs to store the keys and values of a Map; see Maps, below.

More about Strings

Methods on string objects

This section lists examples of selected methods on Strings (Chapters 3.3 and 5.2). Strings are introduced above at Characters and strings; for still more methods, see the sections Collection Basics and Processing Collections with Higher-Order Methods further down on this page (since strings are collections, too).

There are two ways to check a string’s length (size):

val myString = "Olavi Eerikinpoika Stålarm"myString: String = Olavi Eerikinpoika Stålarm
myString.lengthres61: Int = 26
myString.sizeres62: Int = 26

Changing letter case:

"five hours of Coding can save 15 minutes of Planning".toUpperCaseres63: String = FIVE HOURS OF CODING CAN SAVE 15 MINUTES OF PLANNING
"five hours of Coding can save 15 minutes of Planning".toLowerCaseres64: String = five hours of coding can save 15 minutes of planning
"five hours of Coding can save 15 minutes of Planning".capitalizeres65: String = Five hours of Coding can save 15 minutes of Planning

Selecting a part of a string:

"Olavi Eerikinpoika Stålarm".substring(6, 11)res66: String = Eerik
"Olavi Eerikinpoika Stålarm".substring(3)res67: String = vi Eerikinpoika Stålarm

Splitting a string:

"Olavi Eerikinpoika Stålarm".split(" ")res68: Array[String] = Array(Olavi, Eerikinpoika, Stålarm)
"Olavi Eerikinpoika Stålarm".split("la")res69: Array[String] = Array(O, vi Eerikinpoika Stå, rm)

Removing leading and trailing whitepace:

val myText = "   whitespace trimmed from around    the string but not the middle  "myText: String = "   whitespace trimmed from around    the string but not the middle  "
myText.trimres70: String = whitespace trimmed from around    the string but not the middle

Interpreting the characters in a string as a number:

"100".toIntres71: Int = 100
"100".toDoubleres72: Double = 100.0
"100.99".toDoubleres73: Double = 100.99
"one hundred".toIntjava.lang.NumberFormatException: For input string: "one hundred"
...
" 100".toIntjava.lang.NumberFormatException: For input string: " 100"
...
" 100".trim.toIntres74: Int = 100

You can do the above more safely with the Option-suffixed methods:

"100".toIntOptionres75: Option[Int] = Some(100)
"one hundred".toIntOptionres76: Option[Int] = None
"100.99".toDoubleOptionres77: Option[Double] = Some(100.99)

Comparing strings by the Unicode alphabet:

"abc" < "bcd"res78: Boolean = true
"abc" >= "bcd"res79: Boolean = false
"abc".compare("bcd")res80: Int = -1
"bcd".compare("abc")res81: Int = 1
"abc".compare("abc")res82: Int = 0
"abc".compare("ABC")res83: Int = 32
"abc".compareToIgnoreCase("ABC")res84: Int = 0

The sign indicates the result of the comparison.

Embedding values in a string

You can embed any expression’s value in a string (Chapter 1.4).

val number = 100number: Int = 100
val stringWithEmbeddedValues = s"The variable stores $number, which is slightly less than ${number + 1}."stringWithEmbeddedValues: String = The variable stores 100, which is slightly less than 101.

Note the leading s.

You can follow a dollar sign with a variable name. That variable’s value is then embedded in the string.

Use curly brackets to delimit the embedded expression as needed.

You can use the plus operator to combine a string with values of different types, such as Ints:

val theSameUsingPlus = "The variable stores " + number + ", which is slightly less than " + (number + 1) + "."theSameUsingPlus: String = The variable stores 100, which is slightly less than 101.
"the number is " + numberres85: String = the number is 100
"kit" + 10res86: String = kit10

Those examples appended values to the ends of strings. However, the other way around — with the number before the plus — isn’t okay:

number + " is the number"number + " is the number"
       ^
warning: method + in class Double is deprecated (since 2.13.0):
Adding a number and a String is deprecated. Use the string interpolation `s"$num$str"`

Special characters in strings

A backslash character marks a special character within a string (Chapter 5.2):

val newline = "\n"newline: String =
"
"
println("first row\nsecond row")first row
second row
val tabulator = "first\tsecond\tthird"tabulator: String = first   second    third
"here's a double quotation mark \" and another \""res87: String = here's a double quotation mark " and another "
"here's a backslash \\ and another \\"res88: String = here's a backslash \ and another \

If you triple the double quotes around a string literal, you can write special characters without “escaping” them with the backslash:

"""This string contains a quotation mark " and
a backslash \ on two separate rows."""res89: String =
This string contains a quotation mark " and
a backslash \ on two separate rows.

The toString method

All Scala objects have a parameterless method named toString. It returns a description of the object as a string:

100.toStringres90: String = 100
false.toStringres91: String = false

All custom classes and objects that you write have a toString method, too (because they inherit it; see Inheritance, below):

class MyClass(val variable: Int)defined class MyClass
val myObj = new MyClass(10)myObj: MyClass = MyClass@56181
myObj.toStringres92: String = MyClass@56181
myObjres93: MyClass = MyClass@56181

The default toString method generates strings that look like this (Chapter 2.5)

The REPL uses toString as it describes objects. What you see above is three outputs obtained by calling toString thrice.

You can override the default implementation of toString (see Chapter 2.5 and Inheritance, below):

class Experiment(val value: Int) {
  override def toString = "THE OBJECT'S VALUE IS " + this.value
}defined class Experiment
val testObj = new Experiment(11)testObj: Experiment = THE OBJECT'S VALUE IS 11

toString also gets called whenever we print out an object or combine an object with a string:

println(testObj)THE OBJECT'S VALUE IS 11
testObj + "!!!"res94: String = THE OBJECT'S VALUE IS 11!!!
s"testObj's toString returns something that we embed here $testObj in the middle of this string."res95: String = testObj's toString returns something that we embed here THE OBJECT'S VALUE IS 11 in the middle of this string.

Collection Basics

Basic use of a buffer

Buffers are a type of collection (Chapters 1.5 and 4.2). The corresponding type Buffer is in scala.collection.mutable:

import scala.collection.mutable.Bufferimport scala.collection.mutable.Buffer

Creating a buffer:

Buffer("first", "second", "third", "and a fourth")res96: Buffer[String] = ArrayBuffer(first, second, third, and a fourth)
val numbers = Buffer(12, 2, 4, 7, 4, 4, 10, 3)numbers: Buffer[Int] = ArrayBuffer(12, 2, 4, 7, 4, 4, 10, 3)

Even though buffers are objects, don’t write new when you create one (because we instantiate buffers with a factory method).

A buffer may be empty:

val youCanAddNumbersHere = Buffer[Double]()youCanAddNumbersHere: Buffer[Double] = ArrayBuffer()

We use a type parameter to indicate the type of the elements that the buffer may contain in the future (Chapter 1.5). An explicit type parameter is needed when the element type can’t be inferred from the context, like in this empty-buffer example.

A buffer contains zero or more elements, stored in order, each at its own index. Indices run from zero(!) upwards.

Here’s how to look up a single element, given its index:

numbers(0)res97: Int = 12
numbers(3)res98: Int = 7

The above are actually shorthand expressions for calling the buffer’s apply method (Chapter 5.3):

numbers.apply(0)res99: Int = 12
numbers.apply(3)res100: Int = 7

The lift method similarly accesses a buffer element. However, lift returns the result in an Option and doesn’t crash at runtime if the index is invalid:

numbers(10000)java.lang.IndexOutOfBoundsException: 10000
...
numbers.lift(10000)res101: Option[Int] = None
numbers.lift(-1)res102: Option[Int] = None
numbers.lift(3)res103: Option[Int] = Some(7)

You can replace a buffer element with another:

numbers(3) = 1val theFourthElementIsNow = numbers(3)theFourthElementIsNow: Int = 1

The operator += adds a single element at the end of the buffer, thus increasing the buffer’s size:

numbers += 11res104: Buffer[Int] = ArrayBuffer(12, 2, 4, 1, 4, 4, 10, 3, 11)
numbers += -50res105: Buffer[Int] = ArrayBuffer(12, 2, 4, 1, 4, 4, 10, 3, 11, -50)

The operator -= removes an element:

numbers -= 4res106: Buffer[Int] = ArrayBuffer(12, 2, 1, 4, 4, 10, 3, 11, -50)
numbers -= 4res107: Buffer[Int] = ArrayBuffer(12, 2, 1, 4, 10, 3, 11, -50)

Here are some additional commands for adding and removing elements:

numbers.append(100)numbers.prepend(1000)numbersres108: Buffer[Int] = ArrayBuffer(1000, 12, 2, 1, 4, 10, 3, 11, -50, 100)
numbers.insert(5, 50000)numbersres109: Buffer[Int] = ArrayBuffer(1000, 12, 2, 1, 4, 50000, 10, 3, 11, -50, 100)
val removedFourthElement = numbers.remove(3)removedFourthElement: Int = 1
numbersres110: Buffer[Int] = ArrayBuffer(1000, 12, 2, 4, 50000, 10, 3, 11, -50, 100)

Collection types: buffers, vectors, lazy-lists, etc.

There are many types of collections. In O1, we first use mostly buffers, then increasingly turn to vectors. We eventually run into several other collection types, too.

Both buffers and vectors store elements in a specific order, each at its own index. The most obvious differences between the types are these:

• A buffer is a mutable collection. You can add elements to a buffer, changing its size. You can also remove elements or replace them with new ones.
• A vector is an immutable collection. When you create a vector, you specify any and all elements that will ever be in that vector. A vector’s element is never replaced by another; a vector’s size never changes.

You use a Vector much like you use a Buffer (shown above), except that you can’t change what’s in an existing vector. You also don’t need to an import command; Vectors are always available.

Here are a couple of examples:

val myVector = Vector(12, 2, 4, 7, 4, 4, 10, 3)myVector: Vector[Int] = Vector(12, 2, 4, 7, 4, 4, 10, 3)
myVector(6)res111: Int = 10
myVector.lift(10000)res112: Option[Int] = None

More collections:

• Strings are collections of elements. More on that in the next section below.
• Ranges are collections that represent ranges of numbers. See below for examples.
• An Array is a basic, numerically indexed data structure. Each array has a fixed size (like a vector) but its elements can be replaced by new ones (like a buffer’s). In Scala, using Arrays is near-identical to using vectors and buffers (Chapter 11.1).
• Lists are collections that work particularly well when the elements are processed in order. See Chapter 11.2 for a brief introduction.
• LazyLists are similar to “regular” Lists. Their special power is that a lazy-list’s elements are constructed and stored in memory only when or if necessary. For more on lazy-lists, see the separate section further down on this page or Chapter 7.1.
• A Set may only ever contain a single copy of each element (Chapter 9.2). The elements of a set aren’t ordered in the same sense as the other collections listed above.
• A Map is not indexed numerically but by key. Maps have a dedicated section on this page.
• Stacks follow the LIFO principle: whichever element was added last is removed first (Chapter 11.2).
• An Option is a collection with no more than a single element.

Factors such as readability and efficiency influence the choice of collection type; different collections are popular in different programming paradigms.

The official Scala documentation contains a compact summary of the available collection classes.

Collections may be nested: one collection may store references to other collections. See Chapter 6.1 for a discussion.

Strings as collections

A string is a collection (Chapters 5.2 and 5.6). You can work on a string much like you work on a vector. The elements of a String are Chars.

val myString = "llama"myString: String = llama
myString(3)res113: Char = m
myString.lift(3)res114: Option[Char] = Some(m)

Regular strings of type String are immutable. For instance, concatenating two strings generates a new, combined string rather than changing either of the originals. (Mutable representations of strings are possible, too; see Chapter 10.2).

Ranges of numbers

A Range object is an immutable collection that represents numbers within a specified interval (Chapters 5.2 and 5.6).

val fourToTen = Range(4, 11)fourToTen: Range =  Range 4 until 11
fourToTen(0)res115: Int = 4
fourToTen(2)res116: Int = 6

The first number is included in the range; the range ends just before the second number, which is not included.

You can also construct a Range by calling until or to on an Int (Chapter 5.2). The latter method includes the given end point in the resulting Range. For example, these two commands produce a seven-number range identical to the one above:

val anIndenticalRange = 4 until 11anIndenticalRange: Range =  Range 4 until 11
val alsoIdentical = 4 to 10alsoIdentical: Range =  Range 4 to 10

You don’t have to include every consecutive integer in the Range; you can skip some systematically:

val everyOtherInt = 1 to 10 by 2everyOtherInt: Range = Range 1 to 10 by 2
val everyThirdInt = 1 to 10 by 3everyThirdInt: Range = Range 1 to 10 by 3

Common Methods on Collections

This section complements the above introduction to collections by listing various general-purpose methods on Scala collections. All the methods listed here are first-order methods; you’ll find more tools further down at Processing Collections with Higher-Order Methods.

The examples in this section use strings and vectors to exemplify collections. However, all the methods in the examples are similarly available on buffers, arrays, and various other collection types. Some of them are also available on collections that don’t have numerical indices, such as Maps.

Checking size: size, isEmpty, and nonEmpty

Methods for examining the number of elements in a collection (Chapter 4.2):

Vector(10, 100, 100, -20).sizeres117: Int = 4
Vector().sizeres118: Int = 0
Vector(10, 100, 100, -20).isEmptyres119: Boolean = false
Vector(10, 100, 100, -20).nonEmptyres120: Boolean = true
Vector().isEmptyres121: Boolean = true
Vector().nonEmptyres122: Boolean = false
"llama".isEmptyres123: Boolean = false
"".isEmptyres124: Boolean = true

Element lookup: contains and indexOf

Methods for determining if a given element exists in a collection and, if so, where (Chapter 4.2):

val containsElementM = "llama mmama".contains('m')containsElementM: Boolean = true
val containsElementZ = "llama mmama".contains('z')containsElementZ: Boolean = false
val indexOfFirstA = "llama mmama".indexOf('a')indexOfFirstA: Int = 2
val similarOperationOnVector = Vector(10, 100, 100, -20).indexOf(-20)similarOperationOnVector: Int = 3
val negativeMeansNotFound = "llama mmama".indexOf('z')negativeMeansNotFound: Int = -1
val searchFromGivenIndexOnward3 = "llama mmama".indexOf('a', 4)searchFromGivenIndexOnward3: Int = 8
val searchBackwards = "llama mmama".lastIndexOf('a')searchBackwards: Int = 10

Parts of a collection: head, tail, take, drop, slice, etc.

There are many ways to select one or more of the first elements in a collection (Chapters 4.2 and 5.2):

val firstElem = "llama".headfirstElem: Char = l
val noFirstElementSoThisFails = "".headjava.util.NoSuchElementException: next on empty iterator
...
val firstWrapped = "llama".headOptionfirstWrapped: Option[Char] = Some(l)
val firstMissing = "".headOptionfirstMissing: Option[Char] = None
val firstThreeElems = "llama".take(3)firstThreeElems: String = lla
val tooMuchButNoProb = "llama".take(1000)tooMuchButNoProb: String = llama
val allButLast = "llama".initallButLast: String = llam
val allButLastThree = "llama".dropRight(3)allButLastThree: String = ll
val worksOnDifferentCollections = Vector(10, 100, 100, -20).dropRight(2)worksOnDifferentCollections: Vector[Int] = Vector(10, 100)

None of these methods modifies the original collection. They create new collections that contain some of the elements of the originals. The same goes for the commands below, which select elements from the rear end of a collection:

val allButFirst = "llama".tailallButFirst: String = lama
val allButFirstThree = "llama".drop(3)allButFirstThree: String = ma
val lastOnly = "llama".lastlastOnly: Char = a
val lastWrapped = "llama".lastOptionlastWrapped: Option[Char] = Some(a)
val lastThree = "llama".takeRight(3)lastThree: String = mma

Cutting a string in two with splitAt (Chapter 8.4):

val myText = "llama/mmama"myText: String = llama/mmama
val pairOfPieces = myText.splitAt(6)pairOfPieces: (String, String) = (llama,/mmama)
val sameButLonger = (myText.take(6), myText.drop(6))sameButLonger: (String, String) = (llama,/mmama)

Selecting a slice of a collection:

Vector("first/0", "second/1", "third/2", "fourth/3", "fifth/4").slice(1, 4)res125: Vector[String] = Vector(second/1, third/2, fourth/3)

The element at the start index is included. The element at the end index isn’t.

Adding elements and combining collections

You can form a new collection by adding elements:

val numbers = Vector(10, 20, 100, 10, 50, 20)numbers: Vector[Int] = Vector(10, 20, 100, 10, 50, 20)
val oneMoreAppended = numbers :+ 999999oneMoreAppended: Vector[Int] = Vector(10, 20, 100, 10, 50, 20, 999999)
val oneMorePrepended = 999999 +: numbersoneMorePrepended: Vector[Int] = Vector(999999, 10, 20, 100, 10, 50, 20)
val combinedCollection = numbers ++ Vector(999, 998, 997)combinedCollection: Vector[Int] = Vector(10, 20, 100, 10, 50, 20, 999, 998, 997)

Adding elements like this, by constructing new collections, is possible also when the collection is immutable (as above). For examples of modifying an existing mutable collection, see the earlier section Basic use of a buffer.

A mnemonic for collection operators (like +:)

At first I thought I was going out of my mind, as I kept on getting errors. Then I realised I was using the +: operator the wrong way around 🤦🏼‍♀️.

Here’s a Scala mnemonic:

The COLon goes on the COLlection side.

That is, these are fine:

myVector :+ newElem    // appends an element
newElem +: myVector    // prepends an element

But these aren’t:

myVector +: newElem    // error
newElem :+ myVector    // error

Copying elements in a new collection: to, toVector, toSet, etc.

You can switch between collection types by copying elements from one collection to a new one (Chapter 4.2):

val myVector = "llama".toVectormyVector: Vector[Char] = Vector(l, l, a, m, a)
val myBuffer = myVector.toBuffermyBuffer: Buffer[Char] = ArrayBuffer(l, l, a, m, a)
val myArray = myBuffer.toArraymyArray: Array[Char] = Array(l, l, a, m, a)
val mySet = "happy llama".toSetmySet: Set[Char] = Set(y, a, m,  , l, p, h)
val anotherVector = myArray.to(Vector))anotherVector: Vector[Char] = Vector(l, l, a, m, a)
val myLazyList = myArray.to(LazyList)myLazyList: LazyList[Char] = LazyList(<not computed>)

There is a method like toVector or toBuffer for many collection types (but not all).

The resulting collection obeys the rules of its type. For instance, copying elements into a set eliminates any duplicates. Moreover, a set doesn’t keep the elements in their original order.

The general method to takes a parameter that specifies the type of the target collection.

newBuilder and another way of initializing a collection

Sometimes it’s desirable to gather elements one or more at a time and, once done, fix the construct a collection (e.g., a vector) from the gathered elements. To gather the elements, it often helps to have a temporary, mutable helper object that stores the elements that have already been dealt with.

The Scala API provides objects called Builders that can be used for just such a purpose. Many collection types come with a newBuilder method that creates a suitable and efficient Builder object. Here’s an example of constructing a Vector:

val collectedSoFar = Vector.newBuilder[String]collectedSoFar: ReusableBuilder[String,Vector[String]] = VectorBuilder@5dcf0772
collectedSoFar += "first"collectedSoFar += "second"collectedSoFar += "third"val finalCollection = collectedSoFar.result()finalCollection: Vector[String] = Vector(first, second, third)

Miscellaneous methods: mkString, indices, zip, reverse, flatten, etc.

The mkString method formats elements as a string (Chapter 4.2):

val myVector = Vector(100, 20, 30)myVector: Vector[Int] = Vector(100, 20, 30)
println(myVector.toString)Vector(100, 20, 30)
println(myVector)Vector(100, 20, 30)
println(myVector.mkString)1002030
println(myVector.mkString("---"))100---20---30

It’s easy to get all the indices of a collection as a Range (Chapter 5.6):

"laama".indicesres126: Range = Range 0 until 5
Vector(100, 20, 30).indicesres127: Range = Range 0 until 3

You can zip two collections into a collection of pairs (Chapter 8.4):

val species = Vector("llama", "alpaca", "vicuña")species: Vector[String] = Vector(llama, alpaca, vicuña)
val heights = Vector(180, 80, 60)heights: Vector[Int] = Vector(180, 80, 60)
val heightsAndSpecies = heights.zip(species)heightsAndSpecies: Vector[(Int, String)] = Vector((180,llama), (80,alpaca), (60,vicuña))
val threePairsSinceOnlyThreeHeights = heights.zip(Vector("llama", "alpaca", "vicuña", "guanaco"))threePairsSinceOnlyThreeHeights: Vector[(Int, String)] = Vector((180,llama), (80,alpaca), (60,vicuña))
val vectorOfPairsIntoPairOfVectors = heightsAndSpecies.unzipvectorOfPairsIntoPairOfVectors: (Vector[Int], Vector[String]) = (Vector(180, 80, 60), Vector(llama, alpaca, vicuña))
val speciesAndIndices = species.zip(species.indices)speciesAndIndices: Vector[(String, Int)] = Vector((llama,0), (alpaca,1), (vicuña,2))
val theSameThing = species.zipWithIndextheSameThing: Vector[(String, Int)] = Vector((llama,0), (alpaca,1), (vicuña,2))

The reverse of a collection has the same elements backwards (Chapter 4.2):

"llama".reverseres128: String = amall
Vector(10, 20, 15).reverseres129: Vector[Int] = Vector(15, 20, 10)

A nested collection can be flattened (Chapter 6.1):

val twoDimensional = Vector(Vector(1, 2), Vector(100, 200), Vector(2000, 1000))twoDimensional: Vector[Vector[Int]] = Vector(Vector(1, 2), Vector(100, 200), Vector(2000, 1000))
val oneDimensional = twoDimensional.flattenoneDimensional: Vector[Int] = Vector(1, 2, 100, 200, 2000, 1000)

See the Scala API documentation for many more miscellaneous methods such as sum, product, grouped, sliding, transpose, etc. Collections also have various powerful higher-order methods; see Processing Collections with Higher-Order Methods, below.

More on Functions

Higher-order functions

You can pass a function as a parameter to another function; below is a summary of an example from Chapter 6.1.

twice is a higher-order function:

def twice(operation: Int => Int, target: Int) = operation(operation(target))

When calling twice, the first parameter must be a function that takes in an integer and also returns an integer. The variable operation will then store a reference to that function.

twice calls its parameter function, takes the return value, and then calls the parameter again on that value.

Here are a couple of ordinary functions that work in combination with twice:

def next(number: Int) = number + 1

def doubled(original: Int) = 2 * original

Usage examples:

twice(next, 1000)res130: Int = 1002
twice(doubled, 1000)res131: Int = 4000

Function literals and anonymous functions

Instead of defining a function with def, you can write the function as a literal. A function literal defines an anonymous function (Chapter 6.2).

As an example, let’s use this higher-order function:

def twice(operation: Int => Int, target: Int) = operation(operation(target))

twice(number => number + 1, 1000)res132: Int = 1002
twice(n => 2 * n, 1000)res133: Int = 4000

This function literal defines an anonymous function that returns a number slightly larger than the one it receives. We give twice a reference to this anonymous function.

The function literal is marked by a right-pointing arrow. To the left of the arrow is a parameter list (which here consists of just one parameter) and to the right is the function body.

We could have written (number: Int) => number + 1, but the longer form is unnecessary here, because the fact that the parameter is an Int can be automatically inferred from the context.

Here’s another example of a higher-order function (from Chapters 6.1 and 6.2):

def areSorted(first: String, second: String, third: String, compare: (String, String) => Int) =
  compare(first, second) <= 0 && compare(second, third) <= 0

areSorted’s last parameter is a function that takes in two strings and returns an integer.

That function defines the criterion for comparing the other parameters.

A couple of usage examples:

val areSortedByLength = areSorted("Java", "Scala", "Haskell", (j1, j2) => j1.length - j2.length)areSortedByLength: Boolean = true
val areSortedByUnicode = areSorted("Java", "Scala", "Haskell", (j1, j2) => j1.compare(j2))areSortedByUnicode: Boolean = false

The last parameter gets its value from a function literal that defines the sorting criterion.

The brackets are required when an anonymous function takes multiple parameters.

Shorter function literals: anonymous parameters

Instead of naming the parameters in a function literal and using the rightward arrow, you can often write a more compact literal by using an underscore to mark unnamed parameters (Chapter 6.2). These two code fragments are equivalent:

twice(number => number + 1, 1000)
twice(n => 2 * n, 1000)

twice( _ + 1 , 1000)
twice( 2 * _ , 1000)

As are these two:

areSorted("Java", "Scala", "Haskell", (j1, j2) => j1.length - j2.length )
areSorted("Java", "Scala", "Haskell", (j1, j2) => j1.compare(j2) )

areSorted("Java", "Scala", "Haskell", _.length - _.length )
areSorted("Java", "Scala", "Haskell", _.compare(_) )

The compact notation works only in cases that are sufficiently simple. One restriction is that each anonymous parameter (underscore) can be used only once in the function body. You may also need to use the longer notation if the function literal contains further function calls. For more details, please see Chapter 6.2.

Processing Collections with Higher-Order Methods

Collections have many powerful higher-order methods that take in a function an apply it to the collection’s elements (Chapters 6.3, 6.4, 9.2, and 9.3). This section lists some of them. The examples use strings and vectors, but the same methods are available on other collections as well.

Repeating an operation: foreach

The foreach method performs an effect on each element of the collection (Chapter 6.3):

Vector(10, 50, 20).foreach(println)10
50
20
"llama".foreach( letter => println(letter.toUpper + "!") )L!
L!
A!
M!
A!

Here we’ve defined the repeating operation with an anonymous function.

Turning elements into something else: map, flatMap

The map method generates a collection whose elements are computed from those in the original collection as per the given parameter function (Chapter 6.3):

val words = Vector("Witness", "Opener", "Candy")words: Vector[String] = Vector(Witness, Opener, Candy)
words.map( word => "i" + word )res134: Vector[String] = Vector(iWitness, iOpener, iCandy)
words.map( word => word.length )res135: Vector[Int] = Vector(7, 6, 5)

Here’s the same with compact function literals:

words.map( "i" + _  )res136: Vector[String] = Vector(iWitness, iOpener, iCandy)
words.map( _.length )res137: Vector[Int] = Vector(7, 6, 5)

If map’s parameter function returns a collection, you get a nested structure:

val numbers = Vector(100, 200, 150)numbers: Vector[Int] = Vector(100, 200, 150)
numbers.map( number => Vector(number, number + 1) )res138: Vector[Vector[Int]] = Vector(Vector(100, 101), Vector(200, 201), Vector(150, 151))

flatMap does the same as map and flatten combined. It produces a “flatter” collection than map does (Chapter 6.3):

numbers.flatMap( number => Vector(number, number + 1) )res139: Vector[Int] = Vector(100, 101, 200, 201, 150, 151)

The properties of collection elements: exists, forall, filter, takeWhile, etc.

exists finds out whether a given criterion is true for even a single element in the collection (Chapter 6.3); forall similarly works out whether a criterion is true for all the elements of the collection; count computes the number of elements that meet a criterion:

val numbers = Vector(10, 5, 4, 5, -20)numbers: Vector[Int] = Vector(10, 5, 4, 5, -20)
numbers.exists( _ < 0 )res140: Boolean = true
numbers.exists( _ < -100 )res141: Boolean = false
numbers.forall( _ > 0 )res142: Boolean = false
numbers.forall( _ > -100 )res143: Boolean = true
numbers.count( _ > 0 )res144: Int = 4

find locates the first element that meets a given criterion (Chapter 6.3); indexWhere does the same but returns an index rather than the element itself (Chapter 6.4):

val numbers = Vector(10, 5, 4, 5, -20)numbers: Vector[Int] = Vector(10, 5, 4, 5, -20)
numbers.find( _ < 5 )res145: Option[Int] = Some(4)
numbers.find( _ == 100 )res146: Option[Int] = None
numbers.indexWhere( _ < 5 )res147: Int = 2
numbers.indexWhere( _ == 100 )res148: Int = -1

filter returns all the elements that meet a criterion (Chapter 6.3); filterNot does the inverse of that; partition splits the elements in those that meet the criterion and those that don’t:

val numbers = Vector(10, 5, 4, 5, -20)numbers: Vector[Int] = Vector(10, 5, 4, 5, -20)
val atLeastFive = numbers.filter( _ >= 5 )atLeastFive: Vector[Int] = Vector(10, 5, 5)
val underFive = numbers.filterNot( _ >= 5 )underFive: Vector[Int] = Vector(4, -20)
val thoseTwoAsAPair = numbers.partition( _ >= 5 )thoseTwoAsAPair: (Vector[Int], Vector[Int]) = (Vector(10, 5, 5),Vector(4, -20))

takeWhile keeps taking elements until it finds an element that meets the given criterion (Chapter 6.3); dropWhile takes exactly the elements that takeWhile doesn’t; span does both things at once:

val numbers = Vector(10, 5, 4, 5, -20)numbers: Vector[Int] = Vector(10, 5, 4, 5, -20)
val untilSmallEnough = numbers.takeWhile( _ >= 5 )untilSmallEnough: Vector[Int] = Vector(10, 5)
val firstSmallOnwards = numbers.dropWhile( _ >= 5 )firstSmallOnwards: Vector[Int] = Vector(4, 5, -20)
val bothAsAPair = numbers.span( _ >= 5 )bothAsAPair: (Vector[Int], Vector[Int]) = (Vector(10, 5),Vector(4, 5, -20))

Relative order of elements: maxBy, minBy, sortBy

The methods maxBy and minBy search for the collection’s largest or smallest element, using a given criterion (Chapter 9.2); sortBy formes a fully sorted version of the collection:

import scala.math.absimport scala.math.abs
val numbers = Vector(10, 5, 4, 5, -20)numbers: Vector[Int] = Vector(10, 5, 4, 5, -20)
val largestAbs = numbers.maxBy(abs)largestAbs: Int = -20
val smallestAbs = numbers.minBy(abs)smallestAbs: Int = 4
val sortedByAbs = numbers.sortBy(abs)sortedByAbs: Vector[Int] = Vector(4, 5, 5, 10, -20)
val words = Vector("the longest of them all", "short", "middling-sized", "shortish")words: Vector[String] = Vector(the longest of them all, short, middling-sized, shortish)
val longest = words.maxBy( _.length )longest: String = the longest of them all
val sortedByLength = words.sortBy( _.length )sortedByLength: Vector[String] = Vector(short, shortish, middling-sized, the longest of them all)

Looking for the maximal or minimal element fails in case there are no elements at all. A convenient way to deal with that special case is to use the maxByOption or minByOption:

words.maxByOption( _.length )res149: Option[String] = Some(the longest of them all)
words.minByOption( _.length )res150: Option[String] = Some(short)
words.drop(100).minByOption( _.length )res151: Option[String] = None

The above methods have variants named max, min, sorted, maxOption, and minOption, respectively. These By-less methods require that the elements have a natural ordering and base their behavior on that (Chapter 9.2). Here are some examples of natural sorting:

val ascendingNumbers = numbers.sortedascendingNumbers: Vector[Int] = Vector(-20, 4, 5, 5, 10)
val sortedByUnicode = words.sortedsortedByUnicode: Vector[String] = Vector(middling-sized, short, shortish, the longest of them all)
val theSameThing = words.sortBy( sana => sana )theSameThing: Vector[String] = Vector(middling-sized, short, shortish, the longest of them all)
val alsoTheSame = words.sortBy(identity)alsoTheSame: Vector[String] = Vector(middling-sized, short, shortish, the longest of them all)
val sortedLetters = "Let's offroad!".sortedsortedLetters: String = " !'Ladeffoorst"

If the elements to be sorted or compared are Doubles, you need to spefify how to order them. There are two standard ways of doing that: TotalOrdering and IeeeOrdering, either of which works fine for most purposes. (For more details, see the API docs.)

import scala.Ordering.Double.TotalOrderingimport scala.Ordering.Double.TotalOrdering
Vector(1.1, 3.0, 0.0, 2.2).sortedres152: Vector[Double] = Vector(0.0, 1.1, 2.2, 3.0)
Vector(1.1, 3.0, 0.0, 2.2).maxres153: Double = 3.0
Vector(-10.0, 1.5, 9.5).maxBy( _.abs )res154: Double = -10.0

Generic processing of elements: foldLeft and reduceLeft

The methods foldLeft and reduceLeft work at a slightly lower level of abstraction: you define precisely how to process each element in turn in order to construct a return value (Chapter 6.4). First, here’s foldLeft:

val numbers = Vector(10, 5, 4, 5, -20)numbers: Vector[Int] = Vector(10, 5, 4, 5, -20)
val sum = numbers.foldLeft(0)( (sumSoFar, next) => sumSoFar + next )sum: Int = 4
val sameThing = numbers.foldLeft(0)( _ + _ )sameThing: Int = 4

The method has two parameter lists: in the first, you put the initial value that is also the end result if the collection is empty; and...

... in the second, you put a function that combines each intermediate result with the next element. In this example, we’ve used a simple summing function.

reduceLeft is similar, but it uses the first element as the initial value and thus needs only the function as a parameter:

import scala.math.minimport scala.math.min
val numbers = Vector(10, 5, 4, 5, -20)numbers: Vector[Int] = Vector(10, 5, 4, 5, -20)
val sum = numbers.reduceLeft( _ + _ )sum: Int = 4
val smallest = numbers.reduceLeft(min)smallest: Int = -20

The return value of reduceLeft shares its type with the elements of the collection, but foldLeft can generate a result of a different type:

val bigNumberExists = numbers.foldLeft(false)( (foundYet, next) => foundYet || next > 10000 )bigNumberExists: Boolean = false

Since reduceLeft assumes that the collection has at least one element, it crashes at runtime is the assumption is not met:

val empty = Vector[Int]()empty: Vector[Int] = Vector()
empty.foldLeft(0)( _ + _ )res155: Int = 0
empty.reduceLeft( _ + _ )java.lang.UnsupportedOperationException: empty.reduceLeft
...

reduceLeftOption is like reduceLeft but doesn’t crash on an empty input. It returns the result in an Option wrapper:

empty.reduceLeftOption( _ + _ )res156: Option[Int] = None

For more collection methods, see the Scala API documentation.

Option as a collection

Option is a kind of collection: every Option has either a single element (Some) or zero elements (None). See Chapter 8.2 for a discussion. Below is a list of examples of collection methods applied to Options.

The examples use these two variables:

val something: Option[Int] = Some(100)something: Option[Int] = Some(100)
val nothing: Option[Int] = Nonenothing: Option[Int] = None

size:

something.sizeres157: Int = 1
nothing.sizeres158: Int = 0

foreach:

something.foreach(println)100
nothing.foreach(println) // Doesn't print anything.

contains:

something.contains(100)res159: Boolean = true
something.contains(50)res160: Boolean = false
nothing.contains(100)res161: Boolean = false

exists:

something.exists( _ > 0 )res162: Boolean = true
something.exists( _ < 0 )res163: Boolean = false
nothing.exists( _ > 0 )res164: Boolean = false

forall:

something.forall( _ > 0 )res165: Boolean = true
something.forall( _ < 0 )res166: Boolean = false
nothing.forall( _ > 0 )res167: Boolean = true

filter:

something.filter( _ > 0 )res168: Option[Int] = Some(100)
something.filter( _ < 0 )res169: Option[Int] = None
nothing.filter( _ > 0 )res170: Option[Int] = None

map:

something.map( 2 * scala.math.Pi * _ )res171: Option[Double] = Some(628.3185307179587)
nothing.map( 2 * scala.math.Pi * _ )res172: Option[Double] = None

flatten:

Some(something)res173: Some[Option[Int]] = Some(Some(100))
Some(nothing)res174: Some[Option[Int]] = Some(None)
Some(something).flattenres175: Option[Int] = Some(100)
Some(nothing).flattenres176: Option[Int] = None

flatMap:

def myFunc(number: Int) = if (number != 0) Some(1000 / number) else NonemyFunc: (number: Int)Option[Int]
something.flatMap(myFunc)res177: Option[Int] = Some(10)
Some(0).flatMap(myFunc)res178: Option[Int] = None
nothing.flatMap(myFunc)res179: Option[Int] = None

Creating elements with a function: tabulate

Scala’s collection types come with a factory method named tabulate that creates collections by using a given “formula” to initialize each element (Chapters 6.1 and 6.2).

This method takes two parameter lists. The first indicates the number of elements — the size of the collection to be created. The second supplies a function that is called on each index to create the corresponding element:

Vector.tabulate(10)( index => index * 2 )res180: Vector[Int] = Vector(0, 2, 4, 6, 8, 10, 12, 14, 16, 18)

tabulate repeatedly calls the function it receives, passing in each index in turn. Here, a doubling function has been called on each of the numbers from 0 to 9.

You can do the same in more than one dimension:

Vector.tabulate(3, 4)( (first, second) => first * 100 + second )res181: Vector[Vector[Int]] = Vector(Vector(0, 1, 2, 3), Vector(100, 101, 102, 103), Vector(200, 201, 202, 203))

Lazy-Lists and Related Topics

The LazyList class

A lazy-list is a collection whose elements are generated and stored only when needed, lazily (Chapter 7.1). It’s designed for processing the needed elements in order. You can operate on a lazy-list’s elements one by one without storing them all in memory simultaneously.

In many respects, a lazy-list is just like the other collection types described above. For instance, it’s possible to create a lazy-list by typing in all its elements or by copying the contents of an existing collection:

val myLazyData = LazyList(10.2, 32.1, 3.14159)myLazyData: LazyList[Double] = LazyList(<not computed>)
myLazyData.mkString(" ")res182: String = 10.2 32.1 3.14159
val vectorOfWords = Vector("first", "second", "third", "fourth")vectorOfWords: Vector[String] = Vector(first, second, third, fourth)
val lazyWords = vectorOfWords.to(LazyList)lazyWords: LazyList[String] = LazyList(<not computed>)

LazyLists have many familiar methods. Here are just a few:

lazyWords.drop(2).headres183: String = third
lazyWords.filter( _.length > 4 ).map( _ + "!" ).foreach(println)third!
fourth!

In the examples above, the lazy-lists were finite. However, unlike the other collections, a lazy-lists may also be infinite. One way to create an infinite lazy-list is the continually factory method:

val myLazyStrings = LazyList.continually("Oliver")myLazyStrings: LazyList[String] = LazyList(<not computed>)
myLazyStrings.take(5).foreach(println)Oliver
Oliver
Oliver
Oliver
Oliver

The lazy-list has an endless supply of elements that it generates by evaluating the expression "Oliver" (repeating that evaluation whenever it must). Since, in this example, that expression is a literal, all of this collection’s elements are identical.

The original lazy-list is infinite, but take returns a sublist of a specified size.

An infinite lazy-list may also have elements that differ from each other:

LazyList.continually( Random.nextInt(100) ).takeWhile( _ <= 90 ).mkString(",")res184: String = 0,65,83,38,75,33,11,18,75,51,3

This lazy-list consists of random numbers. It generates new numbers only when it needs to.

We form a truncated version of the original lazy-list. It ends where it happens to generate a sufficiently large number. Calling the takeWhile method is still not enough to make the lazy-list generate the random numbers; it just makes a LazyList object that is capable of generating them up to a point.

mkString uses all the elements of the list to construct a string. This forces the LazyList object to evaluate the number-generating expression repeatedly.

One way to structure an interactive program is to use a lazy-list. The following example from Chapter 7.1 prompts the user for input until they say "please" and reports the length of each input as shown in this example run:

Enter some text: hello
The input is 5 characters long.
Enter some text: stop
The input is 4 characters long.
Enter some text: please

object SayPlease extends App {
  def report(input: String) = "The input is " + input.length + " characters long."
  def inputs = LazyList.continually( readLine("Enter some text: ") )
  inputs.takeWhile( _ != "please" ).map(report).foreach(println)
}

The lazy-list “brings” inputs for the program to process. This is an infinite list of strings that are generated by asking the user to provide them. However, this command only defines the lazy-list whose elements come from calling readLine whenever a new element is needed.

takeWhile returns a partial lazy-list that has been cut at the magic word "please".

map generates a lazy-list of reports whose each element is formed (as needed) by calling readLine and applying report to the resulting string. This command also doesn’t prompt the user for the inputs yet, nor does it call report on them; it simply prepares a lazy-list that does that if and when we later access the elements.

foreach orders the lazy-list to print out the elements of the report list. Before it can process an elemenrt, the lazy-list object is forced to determine what that element is by prompting the user for input and a applying report. In practice, what we get is a program that repeatedly receives keyboard input and reports its length.

The convenient factory method LazyList.from creates an infinite list of numbers:

val positiveNumbers = LazyList.from(1)positiveNumbers: LazyList[Int] = LazyList(<not computed>)
positiveNumbers.take(3).foreach(println)1
2
3
LazyList.from(0, 10).take(3).foreach(println)0
10
20
val firstBigSquare = LazyList.from(0).map( n => n * n ).dropWhile( _ <= 1234567 ).headfirstBigSquare: Int = 1236544

More ways to create a LazyList

The iterate method creates a lazy-list that generates each element by re-applying a function to the previous element:

def alternating = LazyList.iterate(1)( x => -2 * x )alternating: LazyList[Int]
alternating.take(4).foreach(println)1
-2
4
-8

You can use a recursive definition to define any kind of lazy-list. This simple example does the same as LazyList.from(1):

def positiveNumbers(first: Int): LazyList[Int] = first #:: positiveNumbers(first + 1)positiveNumbers: (first: Int)LazyList[Int]
positiveNumbers(1).take(3).foreach(println)1
2
3

The operator #:: combines a single value and a lazy-list, yielding another lazy-list. The value to the left of the operator becomes the first element; it’s followed by the elements of the lazy-list on the right-hand side.

The definition is recursive; it refers to itself. We form a sequence of positive integers by starting at the given integer and following it with a sequence of positive integers that starts at the next integer.

Passing unevaluated parameters “by name”

Lazy-lists are based on the idea that a method may receive a parameter that holds an unevaluated expression rather than the value of that expression. Such an unevaluated parameter — a by-name parameter — is evaluated only when (or if) the method reaches a point that actually uses that parameter.

Below is a small example of a by-name parameter.

def printAndReturn(number: Int) = {
  println("I'll return my parameter " + number)
  number
}printAndReturn: (number: Int)Int
def test(number: Int, numberGeneratingExpr: =>Int) = if (number >= 0) numberGeneratingExpr else -1test: (number: Int, numberGeneratingExpr: => Int)Int

Our first function simply reports when it’s being called.

The second function’s second parameter is a by-name parameter, as indicated by the arrow =>. This parameter is evaluated only when or if it’s used during an invocation of test.

The output demonstrates how the parameter works:

test(printAndReturn(10), printAndReturn(100))I'll return my parameter 10
I'll return my parameter 100
res185: Int = 100
test(printAndReturn(-10), printAndReturn(100))I'll return my parameter -10
res186: Int = -1

There’s nothing unusual about the first parameter. The corresponding expression is evaluated first no matter what; then its value (10 or -10) is passed to test.

When the first parameter is positive, we end up in the branch that evaluates the second parameter and thus calls printAndReturn a second time.

When the first parameter is negative, we end up in the branch that returns -1. The second parameter is never needed and never evaluated.

Repeating Commands in a Loop

for loops

A for loops repeats an operation on each element in a collection (Chapter 5.5):

val myBuffer = Buffer(100, 20, 5, 50)myBuffer: Buffer[Int] = Buffer(100, 20, 5, 50)
for (elem <- myBuffer) {
  println("Current element: " + elem)
  println("That plus one: " + (elem + 1))
}Current element: 100
That plus one: 101
Current element: 20
That plus one: 21
Current element: 5
That plus one: 6
Current element: 50
That plus one: 51

The loop body is executed for each element in turn.

The round brackets are mandatory.

The left-pointing arrow <- is followed by an expression that determines the source of the elements.

The name on the left defines a new variable, which the programmer can name freely. This variable is available within the loop and always refers to the element currently being processed (here: the current number from the buffer).

The curly brackets delimit the loop body. Use them if the body consists of multiple commands in sequence. It’s customary to always use the curlies if the loop is effectful (see the style guide), which is the case in nearly all the loops you’ll see in O1.

You’re free to use a combination of instructions in the loop body. You can put in an if, for example:

for (currentElem <- myBuffer) {
  if (currentElem > 10) {
    println("This element is greater than ten: " + currentElem)
  } else {
    println("Here we have a small number.")
  }
}This element is greater than ten: 100
This element is greater than ten: 20
Here we have a small number.
This element is greater than ten: 50

A for loop can iterate over other kinds of collections, too (Chapter 5.6). Here are some of examples, two with a Range and one with a String:

for (number <- 10 to 15) {
  println(number)
}10
11
12
13
14
15
for (index <- myBuffer.indices) {
  println("Index " + index + " stores the number " + myBuffer(index))
}The index 0 stores the number 100
The index 1 stores the number 20
The index 2 stores the number 5
The index 3 stores the number 50
for (letter <- "test") {
  println(letter)
}t
e
s
t

Here’s one more loop. It iterates over a collection of pairs (see Pairs and Other Tuples above) that have been generated by zipWithIndex (see Common Methods on Collections, above):

for ((element, index) <- myBuffer.zipWithIndex) {
  println("Index " + index + " stores the number " + element)
}Index 0 stores the number 100
Index 1 stores the number 20
Index 2 stores the number 5
Index 3 stores the number 50

You’ll find many more examples of for loops in Chapters 5.5 and 5.6.

More features of Scala’s for expressions

Scala’s for expression is capable of various things that aren’t much discussed, or needed, in O1. You can, for instance, use a for to generate a new collection rather than performing effectful operations. That requires the additional keyword yield:

val myVector = Vector(100, 0, 20, 5, 0, 50)myVector: Vector[Int] = Vector(100, 0, 20, 5, 0, 50)
for (number <- myVector) yield number + 100res187: Vector[Int] = Vector(200, 100, 120, 105, 100, 150)
for (word <- Vector("llama", "alpaca", "vicuña")) yield word.lengthres188: Vector[Int] = Vector(5, 6, 6)

You can also add a filter:

for (number <- myVector; if number != 0) yield 100 / numberres189: Vector[Int] = Vector(1, 5, 20, 2)

In Scala, for loops are just a different notation for writing higher-order method calls that invoke foreach, map, flatMap, and filter; cf. Processing Collections with Higher-Order Methods, above.

Nested loops

A loop body can contain another loop. This means that the entire inner loop will run every time the body of the outer loop is executed (Chapter 5.6).

Here’s one example:

val numbers = Vector(5, 3)numbers: Vector[Int] = Vector(5, 3)
val letters = "abcd"letters: String = abcd
for (number <- numbers) {
  println("Cycle of outer loop begins.")
  for (letter <- letters) {
    println(s"the number is $number and the letter is $letter")
  }
  println("Cycle of outer loop is over.")
}Cycle of outer loop begins.
the number is 5 and the letter is a
the number is 5 and the letter is b
the number is 5 and the letter is c
the number is 5 and the letter is d
Cycle of outer loop is over.
Cycle of outer loop begins.
the number is 3 and the letter is a
the number is 3 and the letter is b
the number is 3 and the letter is c
the number is 3 and the letter is d
Cycle of outer loop is over.

Nesting and for

You can combine nested traversals in a single for. The two programs below do the same thing.

for (number <- numbers) {
  for (letter <- letters) {
    println(number + "," + letter)
  }
}

for (number <- numbers; letter <- letters) {
  println(number + "," + letter)
}

do loops

A do loop ends in a condition that defines how long the computer will keep repeating the contents of the loop. Here’s an example from Chapter 8.3:

var number = 1number: Int = 1
do {
  println(number)
  number += 4
  println(number)
} while (number < 10)1
5
5
9
9
13

The first command initializes a variable that we’ll need later. This initializer isn’t a part of the actual loop.

A do loop has the do keyword at the top and the while keyword at the bottom. The loop body goes in curly brackets between the two. Don’t omit the brackets.

After while comes the looping condition, a Boolean expression. This expression is evaluated once every time the loop body has been executed. If it evaluates to false, the loop terminates, otherwise it starts another cycle at the top. The round brackets around the condition are mandatory.

The body of a do loop repeats one or more times. In this example, there are three iterations. The first ends with number storing 5, the second with 9, and the third with 13, at which point the looping condition is not met.

while loops

A while loop is very similar to a do. The difference is that its program code starts with the looping condition and that condition is checked at the start of each loop cycle, rather than the end:

var number = 1number: Int = 1
while (number < 10) {
  println(number)
  number += 4
  println(number)
}1
5
5
9
9
13

The word while starts the loop. It’s followed by the looping condition in round brackets. This condition is checked for the first time before the loop body has been executed even once.

Since number equals 1, the looping condition number < 10 is true at the start of the loop. Since that is the case, this while produces the same output as the do loop above.

In constrast to the body of a do loop, which runs one or more times, the body of a while loop may not run even once:

var number = 20number: Int = 20
while (number < 10) {
  println(number)
  number += 4
  println(number)
}

Now that the looping condition doesn’t hold to begin with, the loop does nothing.

For more examples, see Chapter 8.3.

Maps

A map is a collection whose elements are key–value-pairs (Chapter 8.4). It doesn’t rely on numerical indices; instead, it uses keys for looking up the corresponding values. Key–value pairs are represented as orginary tuples (see Pairs and Other Tuples). The same value may appear multiple times in a map, but the keys must be unique.

Here’s one way to create a Map:

val finnishToEnglish = Map("kissa" -> "cat", "laama" -> "llama", "tapiiri" -> "tapir", "koira" -> "puppy")finnishToEnglish: Map[String,String] = Map(koira -> puppy, tapiiri -> tapir, kissa -> cat, laama -> llama)

The elements of any Map are key–value pairs.

A Map has two type parameters: the type of the keys and the type of the values. In this example, we have a Map whose keys and values are both strings.

Accessing values: get, contains, apply

The contains method tells us if a given key is present in the map:

finnishToEnglish.contains("tapiiri")res190: Boolean = true
finnishToEnglish.contains("Mikki-Hiiri")res191: Boolean = false

You can use get to fetch the value that matches a given key. The result comes as an Option:

finnishToEnglish.get("kissa")res192: Option[String] = Some(cat)
finnishToEnglish.get("Mikki-Hiiri")res193: Option[String] = None

You can also access a value as shown below, but then you’ll cause a runtime error if there is no matching key in the map:

finnishToEnglish("kissa")res194: String = cat
finnishToEnglish("Mikki-Hiiri")java.util.NoSuchElementException: key not found: Mikki-Hiiri
...

Modifying a Map

Scala’s standard API comes with two different Map classes, one for mutable maps and one for immutable ones; immutable Maps are always available without an import. The examples on this page use immutable Maps unless otherwise specified, but here are a few examples of effects on a mutable Map.

import scala.collection.mutable.Mapimport scala.collection.mutable.Map
val finnishToEnglish = Map("kissa" -> "cat", "laama" -> "llama", "tapiiri" -> "tapir", "koira" -> "puppy")finnishToEnglish: Map[String,String] = Map(koira -> puppy, tapiiri -> tapir, kissa -> cat, laama -> llama)

Here are two different ways to add a key–value pair to a mutable map (Chapter 8.4):

finnishToEnglish("hiiri") = "mouse"finnishToEnglish += "sika" -> "pig"res195: Map[String, String] = Map(koira -> puppy, tapiiri -> tapir, kissa -> cat, sika -> pig, hiiri -> mouse,
laama -> llama)

The same commands work for replacing an existing key-value pair: if the key already exists, the new pair will replace the old one.

Here are two different ways to remove a pair from a mutable map:

finnishToEnglish.remove("tapiiri")res196: Option[String] = Some(tapir)
finnishToEnglish -= "laama"res197: Map[String, String] = Map(koira -> puppy, kissa -> cat, sika -> pig, hiiri -> mouse)

Missed lookups and default values: getOrElse, withDefault, etc.

When you call getOrElse, you pass in an expression that specifies a “default value” (Chapter 8.4):

val finnishToEnglish = Map("kissa" -> "cat", "laama" -> "llama", "tapiiri" -> "tapir", "koira" -> "puppy")finnishToEnglish: Map[String,String] = Map(koira -> puppy, tapiiri -> tapir, kissa -> cat, laama -> llama)
finnishToEnglish.getOrElse("kissa", "unknown word")res198: String = cat
finnishToEnglish.getOrElse("Mikki-Hiiri", "unknown word")res199: String = unknown word

The return type of getOrElse is String, whereas for get it was Option[String].

If the Map is mutable, you can also use getOrElseUpdate. When this method fails to find the given key, it adds the given value to the Map, which means that the lookup will always succeed in the end:

import scala.collection.mutable.Mapimport scala.collection.mutable.Map
val finnishToEnglish = Map("kissa" -> "cat", "laama" -> "llama", "tapiiri" -> "tapir", "koira" -> "puppy")finnishToEnglish: Map[String,String] = Map(koira -> puppy, tapiiri -> tapir, kissa -> cat, laama -> llama)
finnishToEnglish.getOrElseUpdate("lude", "bug")res200: String = bug
finnishToEnglishres201: Map[String,String] = Map(lude -> bug, koira -> puppy, tapiiri -> tapir, kissa -> cat, laama -> llama)

As an alternative to the above methods, you can give the entire Map a generic default value (Chapter 8.4):

val finToEng = Map("kissa" -> "cat", "tapiiri" -> "tapir", "koira" -> "dog").withDefaultValue("not found")finToEng: Map[String,String] = Map(koira -> dog, tapiiri -> tapir, kissa -> cat)
finToEng("kissa")res202: String = cat
finToEng("Mikki-Hiiri")res203: String = not found

We use withDefaultValue to let the Map know what it should default to on a failed lookup.

When we then look for a nonexistent key, we don’t get an error but the default value.

withDefault

The previous example used a fixed default value. If you want to customize the default values, you can use withDefault to set a “default function” instead:

def report(missingKey: String) = "you looked up " + missingKey + " but to no avail"report: (missingKey: String)String
val finToEng = Map("kissa" -> "cat", "tapiiri" -> "tapir", "koira" -> "dog").withDefault(report)finToEng: Map[String,String] = Map(koira -> dog, tapiiri -> tapir, kissa -> cat)
finToEng("kissa")res204: String = cat
finToEng("Mikki-Hiiri")res205: String = you looked up Mikki-Hiiri but to no avail

Making a Map from an existing collection: toMap, groupBy

You can call toMap to form a Map from any collection of pairs (Chapter 9.2):

val animals = Vector("dog", "cat", "platypus", "otter", "llama", "pig")animals: Vector[String] = Vector(dog, cat, platypus, otter, llama, pig)
val animalCounts = Vector(2, 12, 35, 5, 7, 5)animalCounts: Vector[Int] = Vector(2, 12, 35, 5, 7, 5)
val vectorOfPairs = animals.zip(animalCounts)vectorOfPairs: Vector[(String, Int)] = Vector((dog,2), (cat,12), (platypus,35), (otter,5), (llama,7), (pig,5))
val animalMap = vectorOfPairs.toMapanimalMap: Map[String,Int] = Map(dog -> 2, otter -> 5, platypus -> 35, llama -> 7, cat -> 12, pig -> 5)
animalMap("llama")res206: Int = 7

In this example, we first create a couple of distinct collections and zip them together to produce a vector that contains pairs.

toMap takes such a collection of pairs and generates a Map.

The groupBy method constructs a Map that contains the elements of an existing collection grouped on the basis of what a given function returns on each of those elements:

val animalCounts = Vector(2, 12, 35, 5, 7, 5)animalCounts: Vector[Int] = Vector(2, 12, 35, 5, 7, 5)
val groupedByParity = animalCounts.groupBy( _ % 2 == 0 )groupedByParity: Map[Boolean,Vector[Int]] = Map(false -> Vector(35, 5, 7, 5), true -> Vector(2, 12))
val animals = Vector("dog", "cat", "platypus", "otter", "llama", "pig")animals: Vector[String] = Vector(dog, cat, platypus, otter, llama, pig)
val groupedByLength = animals.groupBy( _.length )groupedByLength: Map[Int,Vector[String]] = Map(8 -> Vector(platypus), 5 -> Vector(otter, llama), 3 -> Vector(dog, cat, pig))

Both toMap and groupBy return immutable maps.

For more examples, see Chapter 9.2.

Other methods on Maps: keys, values, map, etc.

Maps are collections and, as such, provide many of the same methods as other collections do (see Collection Basics, Common Methods on Collections, and Processing Collections with Higher-Order Methods). They don’t have methods that rely on numerical indices, but methods such as isEmpty, size, and foreach work fine, as do many others:

val finToEng = Map("kissa" -> "cat", "tapiiri" -> "tapir", "koira" -> "dog")finToEng: Map[String,String] = Map(koira -> dog, tapiiri -> tapir, kissa -> cat)
finToEng.isEmptyres207: Boolean = false
finToEng.sizeres208: Int = 3
finToEng.foreach(println)(koira,dog)
(tapiiri,tapir)
(kissa,cat)

The keys and values methods (Chapter 8.4) are specific to Maps. They return collections that contain just the keys or just the values of the Map:

finToEng.keys.foreach(println)koira
tapiiri
kissa
finToEng.values.foreach(println)dog
tapir
cat

The map method (Chapter 8.4) of a Map operates on key–value pairs:

finToEng.map( pair => pair._1 -> pair._2.length )res209: Map[String,Int] = Map(kissa -> 3, tapiiri -> 5, koira -> 3)

As shown above, the method generates a new Map in which the original key–value pairs have been replaced by the given function’s return values.

For the full list of methods, go to the official documentation.

Supertypes and Subtypes

To represent a supertype and its subtypes, you can either define a trait (Chapter 7.2) and have the subtypes extend it or define a superclass (Chapter 7.3) and inherit from it.

Traits

You define a trait much like you define a class. This trait from Chapter 7.2 represents the abstract concept of a shape:

trait Shape {

  def isBiggerThan(another: Shape) = this.area > another.area

  def area: Double    

}

All shapes have an isBiggerThan method that compares the areas of two shapes.

All shapes also have an area method for computing the size of the shape. This method is abstract: it has no body and you can’t invoke it as such. We’ll define the algorithms for computing areas differently for the different subtypes that extend Shape (see below).

isBiggerThan takes a reference to any Shape object. All such objects will have some kind of implementation for area, so we can call that method on the parameter.

Classes often take constructor parameters. Traits never do that (although that is expected to change in future versions of Scala).

Extending a trait

The two classes below mix in the Shape trait (Chapter 7.2). They define subtypes of the more general Shape supertype:

class Circle(val radius: Double) extends Shape {
  def area = scala.math.Pi * this.radius * this.radius
}

class Rectangle(val sideLength: Double, val anotherSideLength: Double) extends Shape {
  def area = this.sideLength * this.anotherSideLength
}

The keyword extends marks Circle as a subtype of Shape. This implies that all objects of type Circle are not only circles but shapes, too. They have, for instance, the isBiggerThan method defined in the Shape trait.

A class can implement the abstract methods of a trait. For instance, here we define that a circle is a shape whose area comes from the formula π * r² and a rectangle is a shape whose area is the product of its two sides.

A class can extend multiple traits. The extends keyword should appear only once though; use with for the other supertypes:

class X extends A with B with C with D with Etc

Static and dynamic types

Chapter 7.2 draws a distinction between static types and dynamic types:

var myShape: Shape = new Circle(1)myShape: o1.shapes.Shape = o1.shapes.Circle@1a1a02e
myShape = new Rectangle(10, 5)myShape: o1.shapes.Shape = o1.shapes.Rectangle@7b519d

The variable myShape has the static type Shape. It may refer to any object of type Shape, which might be a Circle or a Rectangle or an instance of some other subtype of Shape. The static type of a variable or expression can always be determined from the program code.

In this example, the variable myShape is first assigned a value whose dynamic type is Circle. That value is then replaced with another whose dynamic type is Rectangle. The value’s dynamic type is required to be compatible with the variable’s static type.

All Scala objects have the isInstanceOf method, which examines the dynamic type of an object. The code below determines that myShape currently stores a reference to an object that is both a Rectangle and a Shape:

myShape.isInstanceOf[Rectangle]res210: Boolean = true
myShape.isInstanceOf[Shape]res211: Boolean = true

In the example above, we had explicitly set the static type of myShape as Shape. Below, we don’t, which is why the attempted assignment fails:

var experiment = new Circle(1)experiment: o1.shapes.Circle = o1.shapes.Circle@1c4207e
experiment = new Rectangle(10, 5)<console>:11: error: type mismatch;
 found   : o1.shapes.Rectangle
 required: o1.shapes.Circle
       experiment = new Rectangle(10, 5)
                    ^

The variable’s static type is inferred as Circle, so the variable can only store references to Circle objects, no other shapes.

Static types restrict what we can do with a value (Chapter 7.2):

var test: Shape = new Circle(10)test: o1.shapes.Shape = o1.shapes.Circle@9c8b50
test.radius<console>:12: error: value radius is not a member of o1.shapes.Shape
             test.radius
                  ^

The static type of test is Shape. An arbitrary Shape object doesn’t have a radius even though circles do.

You can use match to make a decision based on a value’s dynamic type:

test match {
  case actualCircle: Circle =>
    println("It's a circle and its radius is " + 1«actualCircle.radius»)
  case _ =>
    println("It's not a circle.")
}It's a circle and its radius is 10.0

Inheritance

A class can inherit another class. In this example from Chapter 7.3, we have a class Square that inherits class Rectangle:

class Rectangle(val sideLength: Double, val anotherSideLength: Double) extends Shape {
  def area = this.sideLength * this.anotherSideLength
}

class Square(size: Double) extends Rectangle(size, size)

We follow extends with the superclass’s name: the subclass Square inherits from Rectangle. This gives all Square objects the additional type of Rectangle (and Shape, since Rectangle extends the Shape trait).

Square takes a constructor parameter that determines the length of each side.

To create an instance of a subclass, any initialization steps that the superclass demands must also be performed. In this example, we state that whenever a Square object is created, we initialize a Rectangle so that each of its two constructor parameters (each side length) gets the value of the new Square’s single constructor parameter.

In a concrete class, all methods have an implementation. You can also define an abstract class that, like a trait, may have abstract methods. Here’s an example from Chapter 7.3:

abstract class Transaction(val vatAdded: Boolean) {

  def totalPrice: Double

  def priceWithoutTax = if (this.vatAdded) this.totalPrice / 1.24 else this.totalPrice

}

The abstract keyword turns the class into an abstract class. This class can’t be directly instantiated.

The totalPrice method is abstract. Any concrete subclasses need to have an implementation for this method so that all Transaction objects can actually run this method.

This table from Chapter 7.3 juxtaposes traits, abstract classes, and ordinary superclasses:

	Trait	Abstract superclass	Concrete superclass
Can it have abstract methods?	Yes.	Yes.	No.
Can it be directly instantiated using new?	No.	No.	Yes.
Can it take constructor parameters?	No.	Yes.	Yes.
Can a class extend several of them (using extends and with)?	Yes.	No.	No.

You can combine the techniques. For instance, you can have a class inherit from a superclass and extend a number of traits. Or you can have a class extend a trait.

Singleton objects can also extend classes and traits.

Reimplementing a method with override

A subtype can override a supertype method (Chapters 2.4 and 7.3). The toString method is overridden particularly often, but here’s a different example:

class Super {
  def one() = {
    println("superclass: one")
  }
  def two() = {
    println("superclass: two")
  }
}

class Sub extends Super {
  override def one() = {
    println("subclass: one")
  }
  override def two() = {
    println("subclass: two")
    super.two()
  }
}

val experiment = new Subexperiment: Sub = Sub@1bd9da5
experiment.one()subclass: one
experiment.two()subclass: two
superclass: two

We have chosen to override both of the two methods in the superclass.

The one method of a Sub object works independently of anything that the superclass does: this implementation replaces the one in the superclass.

As a part of the subclass’s two method, we’ve chosen to call the superclass’s version of the same method, so...

... a Sub object first generates the output specified in the subclsses, then does whatever the superclass method does.

The super keyword (note the lower case) refers to the supertype’s definition. It’s available not just in overridden methods but throughout the body of any subtype.

Scala’s class hierarchy

All Scala objects are of type Any. Any has two direct subclasses, AnyVal and AnyRef:

• AnyVal is a superclass of the common data types Int, Double, Boolean, Char, Unit, and a few others. It’s relatively uncommon to extend AnyVal in an application.
• AnyRef, also known as Object, is a superclass for all the classes and singleton objects that don’t derive from AnyVal, such as String and Vector. Any classes that you define automatically inherit AnyRef unless you specify otherwise.

For a further discussion, see Chapter 7.3.

Constraints on subtyping: sealed and final

The word sealed at the top of a trait or class means that you are allowed to directly extend that trait or class only within the same file (Chapter 7.2). For instance, the API class Option is defined like this:

sealed abstract class Option /* Etc. */

Nothing can extend Option except the singleton None and the subclass Some, which are defined in the same file. This guarantees that every last Option object is either None or a Some.

The word final (Chapter 7.3) similarly prevents extending a class altogether. You can also write final in a method definition (before def): this prevents subtypes from overriding the method.

Random Numbers

The singleton object Random has methods that generate (pseudo)random numbers:

import scala.util.Randomimport scala.util.Random
Random.nextInt(10)res212: Int = 8
Random.nextInt(10)res213: Int = 6
Random.nextInt(10)res214: Int = 2

The integers we generate here fall between zero and nine. That is, they are smaller than the parameter value 10 that we pass in.

The singleton Random accesses the computer’s clock to obtain a random seed for its algorithm. Alternatively, you can create a Random object that uses a custom seed:

val myGenerator1 = new Random(74534161)myGenerator1: Random = scala.util.Random@75fbc2df
val myGenerator2 = new Random(74534161)myGenerator2: Random = scala.util.Random@3f92984e

We create two random-number generators. Each one uses an arbitrary integer as its seed.

myGenerator1.nextInt(100)res215: Int = 53
myGenerator1.nextInt(100)res216: Int = 38
myGenerator1.nextInt(100)res217: Int = 97
myGenerator2.nextInt(100)res218: Int = 53
myGenerator2.nextInt(100)res219: Int = 38
myGenerator2.nextInt(100)res220: Int = 97

The generators rely on the same algorithm. When the seed numbers are identical and we call nextInt the same way on both generators, we get two identical sequences of “random” numbers.

Random objects have other randomizing methods beyond nextInt. One worth mentioning here is shuffle (Chapter 7.4):

val numbers = (1 to 10).toVectornumbers: Vector[Int] = Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
Random.shuffle(numbers)res221: Vector[Int] = Vector(8, 9, 7, 4, 6, 1, 10, 2, 5, 3)
Random.shuffle(numbers)res222: Vector[Int] = Vector(8, 6, 4, 5, 9, 1, 3, 7, 2, 10)

For more on randomness, see Chapter 3.6.

Working with Files

The example program below reads in a text file (Chapter 12.1). It prints out the lines from example.txt, prefixing each one with a line number:

import scala.io.Source

object NumberedPrintout extends App {

  val file = Source.fromFile("subfolder/example.txt")

  try {

    var lineNumber = 1
    for (line <- file.getLines) {
      println(lineNumber + ": " + line)
      lineNumber += 1
    }

  } finally {
    file.close()
  }

}

fromFile takes in a file path and returns an object of type Source that is capable of accessing the file. The path may be relative (as shown here) or absolute.

We use a loop to iterate over each of the lines, which we access through the getLines method. (There are alternative ways to iterate over the contents of a file; see Chapter 12.1.)

The try–finally construct ensures that the file-closing code in the finally block will be executed even if the attempt to read the data from the file fails for some reason.

The example program below writes text into a file:

import java.io.PrintWriter
import scala.util.Random

object WritingExample extends App {

  val fileName = "examplefolder/random.txt"
  val file = new PrintWriter(fileName)
  try {
    for (n <- 1 to 10000) {
      file.println(Random.nextInt(100))
    }
    println("Created a file " + fileName + " that contains pseudorandom numbers.")
    println("In case the file already existed, its old contents were replaced with new numbers.")
  } finally {
    file.close()
  }
}

You can create a PrintWriter object as shown. Pass in the name of the file you intend to create or rewrite.

The println method writes a single line of text into the file.

Closing the file connection is particularly important when writing. This ensures that all the data scheduled for writing actually finds its way into the file on disk.

Graphical User Interfaces

Programmers use different libraries for writing graphical user interfaces. O1’s ebook features two libraries: the GUI tools in O1Library and Scala’s more generic GUI library, Swing.

O1Library’s GUI tools

The key component of O1’s GUI toolkit is the class o1.View. Below is an example that summarizes some of its main features.

The basic idea is this: a View is a window that graphically displays an object that serves as an application’s domain model (Chapter 2.7). In the example below, the model is an instance of this toy class:

// Each "Thing" is a mutable object. It has a location and a color.
class Thing(var color: Color) {
  var location = new Pos(10, 10)

  def move() = {
    this.location = this.location.add(1, 1)
  }

  def returnToStart() = {
    this.location = new Pos(10, 10)
  }
}

Let’s write a GUI that looks like this and displays a Thing as a circle against a two-color background:

In this example app, the “thing” keeps moving rightwards and downwards as the clock ticks. It returns back to its starting location near the top-left corner when the user double-clicks the mouse. It changes color depending on the position of the mouse cursor.

Here’s the code that implements the GUI:

object ViewExample extends App {
  val thing = new Thing(Blue)
  val background = rectangle(200, 400, Red).leftOf(rectangle(200, 400, Blue))

  val gui = new View(thing, 10, "A Diagonally Moving Thing") {

    def makePic = {
      val picOfThing = circle(20, thing.color)
      background.place(picOfThing, thing.location)
    }

    override def onTick() = {
      thing.move()
    }

    override def onMouseMove(mousePos: Pos) = {
      thing.color = if (mousePos.x < 200) Red else Blue
    }

    override def onClick(click: MouseClicked) = {
      if (click.clicks > 1) {
        thing.returnToStart()
      }
    }

    override def isDone = thing.pos.x > 400
  }

  gui.start()
}

When you create a View, you need to specify which object it is a view to (here: a Thing object). You can use optional parameters to set the tick rate of the app’s clock (here: 10) and the title of the GUI window.

Any View object needs a makePic method that determines which image to display onscreen. Here, we form the image by placing a small circle against a rectangular background image.

Event-handler methods (Chapter 3.1) react to the passing of time and the user’s actions in the GUI. A few examples are shown here: the “thing” moves on each clock time, changes color when the mouse cursor enters a different region, and returns to the top-left corner on a double click.

We capture the MouseClicked object that describes the event and ask it to provide the number of consecutive clicks (Chapter 3.6).

The isDone method defines when the GUI should stop responding to events. In this app, that happens if the “thing” reaches a location far enough on the right.

Creating a View object isn’t enough to display the window and start the clock. You do that by calling start.

For more information, see Chapters 3.1, 3.6, and the Scaladocs.

The Swing GUI library

Chapter 12.3 is an introduction to the GUI library Swing. This example from that chapter demonstrates several of the library’s key features:

import scala.swing._
import scala.swing.event._

object EventTestApp extends SimpleSwingApplication {
  val firstButton = new Button("Press me, please")
  val secondButton = new Button("No, press ME!")
  val prompt = new Label("Press one of the buttons.")

  val allPartsTogether = new BoxPanel(Orientation.Vertical)
  allPartsTogether.contents += prompt
  allPartsTogether.contents += firstButton
  allPartsTogether.contents += secondButton
  val buttonWindow = new MainFrame
  buttonWindow.contents = allPartsTogether
  buttonWindow.title = "Swing Test App"
  this.listenTo(firstButton)
  this.listenTo(secondButton)
  this.reactions += {
    case clickEvent: ButtonClicked =>
      val clickedButton = clickEvent.source
      val textOnButton = clickedButton.text
      Dialog.showMessage(allPartsTogether, "You pressed the button that says: " + textOnButton, "Info")
  }

  def top = this.buttonWindow
}

The app object inherits a class that represents Swing-based applications.

We create some objects that represent GUI elements.

We lay out the elements vertically in a panel.

We set the panel as the contents of the window and adjust additional window properties.

We register an object (here: the app object itself) as an event listener for the buttons.

We define how to react to the events that the listening object is informed of.

When an event occurs, we run some code that displays an auxiliary window (a dialog). This code has access to the variable clickEvent, which stores a ButtonClicked object that represents the GUI event that occurred.

A SimpleSwingApplication needs a top window that gets displayed as soon as the application is launched.

Reserved Words

The following are reserved words in Scala and therefore cannot be used as identifiers:

abstract case     catch    class    def       do      else    extends  false  final
finally  for      forSome  if       implicit  import  lazy    match    new    null
object   override package  private  protected return  sealed  super    then   this
throw    trait    try      true     type      val     var     while    with   yield
_        :        =        =>       <<:       <%      >:      #        @      ⇒     ←

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Credits

Thousands of students have given feedback that has contributed to this ebook’s design. Thank you!

The ebook’s chapters, programming assignments, and weekly bulletins have been written in Finnish and translated into English by Juha Sorva.

The appendices (glossary, Scala reference, FAQ, etc.) are by Juha Sorva unless otherwise specified on the page.

The automatic assessment of the assignments has been developed by: (in alphabetical order) Riku Autio, Nikolas Drosdek, Joonatan Honkamaa, Jaakko Kantojärvi, Niklas Kröger, Teemu Lehtinen, Strasdosky Otewa, Timi Seppälä, Teemu Sirkiä, and Aleksi Vartiainen.

The illustrations at the top of each chapter, and the similar drawings elsewhere in the ebook, are the work of Christina Lassheikki.

The animations that detail the execution Scala programs have been designed by Juha Sorva and Teemu Sirkiä. Teemu Sirkiä and Riku Autio did the technical implementation, relying on Teemu’s Jsvee and Kelmu toolkits.

The other diagrams and interactive presentations in the ebook are by Juha Sorva.

The O1Library software has been developed by Aleksi Lukkarinen and Juha Sorva. Several of its key components are built upon Aleksi’s SMCL library.

The pedagogy of using O1Library for simple graphical programming (such as Pic) is inspired by the textbooks How to Design Programs by Flatt, Felleisen, Findler, and Krishnamurthi and Picturing Programs by Stephen Bloch.

The course platform A+ was originally created at Aalto’s LeTech research group as a student project. The open-source project is now shepherded by the Computer Science department’s edu-tech team and hosted by the department’s IT services. Markku Riekkinen is the current lead developer; dozens of Aalto students and others have also contributed.

The A+ Courses plugin, which supports A+ and O1 in IntelliJ IDEA, is another open-source project. It was created by Nikolai Denissov, Olli Kiljunen, Nikolas Drosdek, Styliani Tsovou, Jaakko Närhi, and Paweł Stróżański with input from Juha Sorva, Otto Seppälä, Arto Hellas, and others.

For O1’s current teaching staff, please see Chapter 1.1.

Additional credits appear at the ends of some chapters.

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