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.

Chapter 7.5: Inheritance

About This Page

Questions Answered: Say I have a regular class; how can I define a subtype for it? How do the classes and traits of the Scala API form a type hierarchy? I heard someone talk about “abstract classes”; what are those?

Topics: Inheritance: extending a superclass with subclasses. Some type hierarchies in the Scala API. Abstract classes.

What Will I Do? There’s some reading, a good chunk of which is optional but potentially interesting. There’s one small programming assignment.

Rough Estimate of Workload:? An hour or so.

Points Available: B20.

Related Modules: Traits.

Introduction

In the preceding chapters, we used traits to define supertypes for multiple classes. This chapter complements the preceding ones: we’ll see how a regular class may also serve as a supertype. This traditional object-oriented technique is known as inheritance (periytyminen).

Let’s Continue with Shapes

In the previous chapter, we defined the trait Shape and an implementing class Rectangle:

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

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

Suppose we also want to represent squares — that is, rectangles with four sides of equal length? We’d like to be able to write Square(10) and the like.

Of course, we might simply extend Shape in a Square class:

class Square(val sideLength: Double) extends Shape:
  def area = this.sideLength * this.sideLength

The bothersome thing about this solution is that the code is quite redundant: the algorithm for computing the area of a square is the same as that for a rectangle; it just happens that the sides are equal in length. The code isn’t up to par in terms of conceptual modeling, either: it makes squares a subtype of Shape alongside rectangles, whereas we humans prefer to think of a squares as a special case of a rectangle; each square is a square, a shape, and a rectangle.

The fix is simple: we can define squares as a subtype of rectangles, as shown in the diagram below. We can do this even though Rectangle is a regular class, not a trait.

Subclasses and Superclasses

Let’s add something to class Rectangle:

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

This is a regular class, which can be instantiated directly, but it is open (avoin) for extension. By adding the keyword open, we indicate that this class is designed to be freely usable as a superclass (yliluokka) for any other class.

Square can then be defined as Rectangle’s subclass (aliluokka):

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

We use the familiar extends keyword, but this time we follow it with the name of a regular class rather than a trait. We say: the class Square inherits the class Rectangle. This gives all Square objects the additional type of Rectangle.

Square has only one 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. A subclass may pass constructor parameters to its superclass; the idea is the same as with traits (Chapter 7.3). 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.

How mandatory is the open keyword?

There are reasons why it’s not a good idea to write subclasses for any random class that isn’t designed to be inherited from. Those reasons tend to come to the fore in larger-scale programming projects and won’t be obvious from O1’s introductory-level materials. However, the optional material at the end of this chapter hints at some of the complications in class-based inheritance.

Anyhow, we explicitly write open to state that a Scala class has been designed for inheritance and programmers are free to inherit from it as they please. If you omit open, any subtypes that your class might have should be defined in the same file. Technically, omitting open does not utterly prevent inheriting from the class in other files, but the Scala compiler will issue warnings about the suspicious code, and it’s best to avoid such inheritance. (Cf. sealing a class with sealed, which completely prevents direct subtypes from being defined in other files; Chapter 7.4.)

In some exceptional circumstances, it is reasonable to write a subclass for a non-open class that is defined elsewhere. In that case, the programmer may disable the compiler warning with import scala.language.adhocExtensions. In O1, you should have no reason to do this.

Inheritance vs. traits

Extending a superclass with a subclass looks much like extending a trait with a class. The two techniques are closely related.

There are some differences, though; let’s compare. First, consider traits and how the Rectangle class extends the Shape trait:

When using a trait	Example
The supertype is represented by a trait.	`trait Shape`
Each subtype extends (“mixes in”) that trait.	`class Rectangle` `extends Shape`
A trait can define abstract methods and variables.	`def area: Double`
A trait cannot be instantiated directly.	One does not simply write `Shape()`.
A class (or trait) may directly extend multiple traits.	`class X extends Trait1, Trait2, Trait3` is fine.
A trait cannot pass constructor parameters to its supertype(s).	`trait Trait1 extends Trait2(someParams)` is invalid.

And here is a similar table about inheritance and the example of Rectangle as a superclass for Square:

When inheriting from a regular class	Example
The supertype is represented by a regular class (which is best to mark as open, if meant for inheritance).	`open class Rectangle`
Each subtype extends (“inherits”) that superclass.	`class Square` `extends Rectangle`
An ordinary class cannot define abstract methods or variables. (But you can. See below.)	All the methods on `Rectangle` have a method body.
A superclass can be instantiated directly.	`Rectangle(...)` works.
(In Scala and many other languages:) A class cannot have more than one immediate superclass.	`class X extends` `Super1, Super2` is invalid. (But `class X extends Super1,` `Trait1, Trait2` is fine.)
Any `class` may pass constructor parameters to its supertype(s).	`class Square(size: Int) extends Rectangle(size, size)` is fine.

There are many programs where it’s reasonable to use either of these techniques.

Assignment: Items within Items

Introduction

Imagine we’re working on an application that needs to represent items of various sorts. We have a simple class Item:

class Item(val name: String):
  override def toString = this.name

For this small programming assigment, let’s adopt the following goal: we wish our program to have not only “simple items”, as defined above, but also items that can contain other items. For instance, we might have a bag that contains a book and a box, and the box might further contain a ring.

Task description

Write a subclass for Item. Name it Container. This subclass will represent items that can contain other items, such as bags and boxes:

Containers each have a name, just like other items do.
They have an addContent method, which places an item within the container.
And a contents method that returns the previously added items in a Vector.
Their toString implementation differs from that of other items.

Your class should work as shown below.

val container1 = Container("box")container1: o1.items.Container = box containing 0 item(s)
container1.addContent(Item("ring"))
container1res0: o1.items.Container = box containing 1 item(s)
val container2 = Container("bag")container2: o1.items.Container = bag containing 0 item(s)
container2.addContent(Item("book"))
container2.addContent(container1)
container2res1: o1.items.Container = bag containing 2 item(s)
container1.contentsres2: Vector[o1.items.Item] = Vector(ring)
container2.contentsres3: Vector[o1.items.Item] = Vector(book, box containing 1 item(s))

The toString method’s return value should count only the container’s immediate contents. For instance, in the REPL session above, the bag is shown as containing only two items, even though the box in the bag further contains the ring.

(How could we count all the “contents of the contents”, too? You’ll see in Chapter 12.2.)

Instructions and hints

In o1.items within the Traits module, you’ll find that Item class. It’s missing the open keyword, though; type that in.
In the same package, you’ll find some starter code for Container.
Don’t forget to pass a constructor parameter to the superclass Item.
Don’t use val to (re)define the instance variable name within the Container subclass. That variable is already defined in the superclass. It’s perfectly fine to give Container’s constructor parameter the name name, though.
Note that what you override in this assignment isn’t the generic toString from AnyRef, as in many other programs, but the toString implementation in Item.
Can you implement the toString in Container so that you use the superclass’s implementation rather than directly accessing the container’s name? (This is optional.)
Do not add any public members in the class beyond what was requested. Private members you are free to add as you see fit.

A+ presents the exercise submission form here.

Abstract Classes

The distinction between traits and class-based inheritance is further complicated by the fact that it’s possible to define so-called abstract classes (abstract class). Abstract classes resemble traits in multiple ways but aren’t quite identical.

For our purposes in O1, abstract classes aren’t a central topic; we’ll generally prefer traits for our subtyping needs. As your programming experience grows, you’ll learn to choose between traits and abstract classes. For now, it is enough to be aware that abstract classes exist; that much is good to know because they aren’t a rarity in Scala programs or indeed in programs written in other languages. Not all programming languages have traits and abstract classes as separate concepts.

Let’s see a quick example. In a Chapter 7.3 program, we had this trait named Entity:

trait Entity(val name: String):
  def contact: NaturalPerson
  def kind: String
  override def toString = s"$name ($kind)"

Two of the methods are abstract. Such methods, whose implementation is left to subtypes to take care of, cannot appear in completely regular classes. However, the following alternative definition of Entity is valid.

abstract class Entity(val name: String):
  def contact: NaturalPerson
  def kind: String
  override def toString = s"$name ($kind)"

The word abstract turns this into an abstract class. Such a class may contain abstract methods just like a trait can. And just like a trait cannot be directly instantiated, neither can an abstract class. Abstract classes are open for extension by default, so there is no need to write open.

A class that is not abstract may be referred to as a concrete class.

An abstract class is similar to a trait in some ways and similar to a concrete superclass in other ways. Let’s compare:

	Trait	Abstract superclass	Concrete superclass
Can it have abstract methods?	Yes.	Yes.	No.
Can it be directly instantiated?	No.	No.	Yes.
Can it pass constructor parameters to its supertype(s)?	No.	Yes.	Yes.
Can you extend several of them (listed after `extends`)?	Yes.	No.	No.

Generally speaking, should I use a trait or an abstract class?

Rule of thumb: Unless you have a specific reason to use an abstract class, use a trait instead, since traits are more flexibly extendable.

It can be tricky to choose between a trait and an abstract superclass. However, in O1, such decisions are generally made for you and provided in the assignment specifications. You can worry about learning to make these decisions yourself later, in Programming Studio 2, for example. For some more information, you could also take a look Scala Cookbook or Programming in Scala, which are some of the recommendations on our Books and Other Resources page.

Class Hierarchies in the Scala API

Like traits, inheritance is useful in forming type hierarchies.

Some examples of hierarchies can be found in Scala’s standard API. Let’sconsider a few.

A hierarchy of GUI elements

The package scala.swing provides a selection of classes that represent GUI elements, building blocks for graphical user interfaces. The diagram below depicts a part of this hierarchy.

../_images/inheritance_swing.png — Some of these classes from package `scala.swing` are traits, some are regular classes. Each supertype defines generic properties common to all the extending types. At the top level, `UIElement` defines properties common to all kinds of GUI elements, such as background color and size.

About the Swing GUI library

Chapter 12.4 contains an introduction to the GUI library known as Swing. That optional chapter is at the rear end of O1 and this ebook, but if you’re itching to read more about GUIs, go ahead and read it. Now that you’ve been introduced to inheritance, you should have the sufficient prerequisite knowledge for the chapter.

The `Option` hierarchy (and sealed superclasses)

There’s a small class hierarchy associated with the Option type, too.

Since Chapter 4.3, you’ve known that there are two kinds of Option objects. Any object of type Option is either a Some object that contains a value or None. This is actually an example of inheritance: Option is an abstract class that the concrete class Some and the singleton object None extend.

Chapter 7.4 mentioned how to seal a trait: the sealed keyword makes it impossible to directly extend a trait except within the file that defines the trait itself. A superclass can also be sealed, and Option is just such a sealed class.

As we noted in Chapter 4.3, an Option can be either a Some or None (which are defined in the same file) but it can never, ever be anything else. Which is exactly what you want when you use Option. You can’t extend Option with a class of your own, and that’s good.

The mother of all classes: `Any`

Let’s examine some objects in the Scala REPL:

val miscellany = Vector(123, "llama", true, Vector(123, 456), Square(10), IArray(1, 2, 3))miscellany: Vector[Any] = Vector(123, llama, true, Vector(123, 456), o1.shapes.Square@114c3c7, Array(1, 2, 3))

We create a vector that contains some completely disparate objects: an Int, a String, a Boolean, a Vector of Int, a Square, and an immutable array (a sort of collection).

Scala infers that the type of the vector’s elements is Any. That is, what we have is a “vector of any sorts of objects”. This suggests that integers, vectors, squares, and what have you, are all of type Any.

All Scala classes and singleton objects — including those you write — automatically inherit from a class called Any, even though we don’t usually record this inheritance explicitly in code. All the objects in a Scala program always have the Any type in addition to any other types they may have.

It’s possible to use this top-level class as a type for a variable, as illustrated below.

var someObject: Any = "kumquat"someObject: Any = kumquat
someObject.isInstanceOf[Any]res4: Boolean = true
someObject.isInstanceOf[String]res5: Boolean = true
someObject.isInstanceOf[Square]res6: Boolean = false
someObject = Square(10)someObject: Any = o1.shapes.Square@ecfb83
someObject.isInstanceOf[Any]res7: Boolean = true
someObject.isInstanceOf[String]res8: Boolean = false
someObject.area-- Error: ... value area is not a member of Any

As the name suggests, you can use a variable of type Any to refer to any object. The object can be a string or a square, for instance.

So, the static type of someObject is Any, and the expression someObject.isInstanceOf is valid because (and only because) the isInstanceOf method is defined in the Any class and is therefore available on any Scala object.

The attempted method call someObject.area fails even though the variable happens to store a Square object which has an area method. The variable’s static type limits what you can do with it.

In most cases, it’s not sensible to use variables of type Any, since the type’s genericity so constricts what you can do with such variables. With Any as the static type, you can use the variable only for the operations that are defined on Any. Those extremely generic methods that are common to all Scala objects include isInstanceOf, toString, ==, !=, and a handful of others. Generally, it makes sense to use variables with more specific types, which is of course what we’ve been doing already. (See the optional material below for a further discussion.)

The mother of most classes: `AnyRef` a.k.a. `Object`

Scala’s root type Any divides in two main “branches”. It has two immediate subclasses, AnyVal and AnyRef:

This division to AnyRef and AnyVal has to do with the way the Scala language is implemented and isn’t too significant to the Scala beginner or for learning the basics of programming more generally. Nevertheless, it’s good to be aware that these types exist so you know why they sometimes crop up in Scaladocs, REPL outputs, and error messages.

AnyVal is a superclass for some API classes that define certain data types that are simple but operate more efficiently than other classes do. The familiar data types Int, Double, Boolean, Char, and Unit descend from AnyVal, as do a few others. It’s relatively uncommon to extend AnyVal in an application; in O1, we don’t do that at all.

AnyRef, on the other hand, is a superclass for all other classes and singleton objects. The classes String and Vector, for instance, derive from AnyRef, as does the Item class we just wrote.

When using the common JVM-based implementation of Scala (as we do; Chapter 5.4), AnyRef is sometimes (and somewhat confusingly) also known by the name Object for JVM-specific technical reasons.

We can spot AnyVal and AnyRef a.k.a. Object in the REPL, too:

val miscellany2 = Vector(123, true)miscellany2: Vector[AnyVal] = Vector(123, true)
val miscellany3 = Vector("llama", Vector(123, 456), Square(10))miscellany3: Vector[Object] = Vector(llama, Vector(123, 456), o1.shapes.Square@667113)

Int and Boolean descend from AnyVal.

String, Vector, and Square descend from Object, which is effectively synonymous with AnyRef.

One more: Matchable

In addition to the aforementioned very generic supertypes at the top of Scala’s hierarchy, you may run into one more: the Matchable trait is a supertype for all Scala types on which pattern matching (match) works. Both AnyRef and AnyVal extend Matchable, so Matchable is very nearly as all-encompassing as Any. (Pattern matching works on almost all types in Scala, but there are some very exceptional types that don’t have the Matchable trait. For more information, you could see online, but for course purposes, there’s no need to.)

Tailoring Instances of a Supertype

In Chapter 2.4, you learned to create an object that has an instance-specific method in addition to the methods defined in a class:

object superman extends Person("Clark"):
  def lenna = "WOOSH!"
end superman// defined object superman

Since then, we’ve used this technique in combination with Views: we have defined singleton objects that extend View and have custom methods.

Tailoring instances in this way is actually a form of inheritance. For example, what the above command actually does is inherit a single object from the Person superclass. Along the same lines, we have written diverse implementations for the abstract makePic method of the supertype View.

When creating a single object, you can combine superclasses and traits flexibly in various other ways as well. Read on if you wish.

Defining a subtype “on the fly”

You can also extend a trait “on the fly” as you create an object.

To set up the next example, let’s define a couple of traits and one concrete class. These three types are entirely distinct from each other.

class Animal(val species: String):
  override def toString = "an animal, more specifically a " + this.species
trait Spitting:
  def spit = "ptui!"
trait Humped:
  def numberOfHumps: Int// defined class Animal
// defined trait Spitting
// defined trait Humped

Let’s try a new way of creating an object:

val pet = new Animal("llama") with Spittingpet: Animal & Spitting = an animal, more specifically a llama

We define, “on the fly”, a new subclass of Animal that has no specified name and that has the trait Spitting. We immediately instantiate this new type to produce a single instance of it.

The word new is required in command such as this, which not only create a new instance but also define a new type for it. Note the use of with as well.

This object is of the combined type Animal & Spitting and has all the properties of Animals as well as all the properties defined in the Spitting trait:

pet.speciesres9: String = llama
pet.spitres10: String = ptui!

Of course, we could have also defined a named class, as shown below, and instantiated it as per usual.

class Llama extends Animal("llama"), Spitting// defined class Llama

Adding methods on a type defined “on the fly”

Let’s continue our toy example. We’ll create an object of a type that:

is a subclass of Animal;
additionally extends the traits Humped and Spitting; and
implements the abstract method numberOfHumps in the Humped trait in a specific way.

val shipOfTheDesert = new Animal("dromedary") with Humped with Spitting:
  def numberOfHumps = 1shipOfTheDesert: Animal & Humped & Spitting = an animal, more specifically a dromedary

Finally, here’s how you could use the same technique to produce an object of type Spitting with its own particular implementation of toString:

val fisherman = new Spitting:
  val name = "Eemeli"
  override def toString = this.namefisherman: Spitting = Eemeli

Here we followed new with the name of a trait rather than a concrete class. However, this command does not directly instantiate the trait (which isn’t even possible). Instead, it defines a new anonymous type that extends the trait and additionally has the properties listed below. It is that new type that is then instantiated.

Terms for googling: scala trait mixin, scala anonymous subclass.

Rule of thumb: static types in general and the static types of parameters in particular should be as “wide” as possible. That is, where possible, we’d like our variables to have a superclass or a trait as their type rather than a more specific class. That way, our methods and classes will be more generally useful and easier to modify.

Compare:

def doSomething(circle: Circle) =
  // ...

vs.

def doSomething(shape: Shape) =
  // ...

The first definition is sensible in case doSomething should work only on circles (say, because it depends on being able to access the given object’s radius, which other shapes don’t have). Otherwise, the second definition is probably better, since it works on different shapes, including future subclasses of Shape that haven’t even been written yet.

You can’t always generalize; otherwise we’d just give everything the Any type. But do generalize when you can.

Between public and private: protected

A subclass’s instances also have the superclasses’ private variables as part of their state. Inherited methods commonly use those private variables. However, you can’t directly refer to a private member of a superclass from the subclass’s program code. The same goes for private members on traits.

However, Scala, like some other languages, features another access modifier, protected. This modifier allows precisely what we just said private disallows: a protected variable or method is accessible not only to the class or trait itself but also within the code of its subtypes. (But not just anywhere, like a public member is.) An internet search should bring up many examples.

Multiple inheritance

As noted above, a class can extend only a single superclass directly. Why?

Search online for multiple inheritance (moniperintä). You may also wish to look up the “deadly diamond of death” that is sometimes associated with multiple inheritance and that is held to be a serious problem by some but not all programmers.

And here is some slightly provocative but interesting reading (mostly to those students who already know something about Java and its interface construct):'Interface' Considered Harmful.

If you extend multiple traits, which method implementations do you get?

Here’s a simple example:

trait X:
  def method: String
trait A extends X:
  override def method = "a’s method"
trait B extends X:
  override def method = "b’s method"// defined trait X
// defined trait A
// defined trait B
class AB extends A, B
class BA extends B, A// defined class AB
// defined class BA
AB().methodres11: String = b’s method
BA().methodres12: String = a’s method

For more information, look up scala linearization or see Books and Other Resources page for further reading.

The Liskov substitution principle

“Square as a subclass of rectangle” is a classic example when discussing the Liskov substitution principle, which we’ll only paraphrase here. This well-known design guideline is often held up as a criterion of object-oriented program quality.

For a class S to follow this principle, the following must be true:

If S is a subclass of T, all operations that are available and meaningful on instances of T are equally available and meaningful on instances of S.

In other words: where you have an instance of T, you can also use an instance of S.

Let’s return to the shape example and these two classes that we defined earlier:

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

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

Does this code follow the Liskov substitution principle? What if we used var instead of val in class Rectangle? Recall: our goal when writing Square was to model the fact that a square is a rectangle whose sides are equal in length.

Summary of Key Points

Inheritance is an object-oriented technique in which subclasses extend a superclass. The subclasses, which represent more specific concepts, inherit the generic properties defined in the superclass.
You may define a class as abstract, in which case it can contain abstract methods and variables just like a trait can. An abstract class can’t be instantiated directly but only via its subclasses.
Traits and superclasses have much in common, but there are differences, too. The exact differences depend on the programming language. (Some languages don’t have both concepts.)
- In O1, we’ll be using traits. (For purposes of the course, pretty much all you need to know about abstract classes is that they’re similar to traits.)
In Scala, all classes are part of a hierarchy whose base is the API class Any.
Links to the glossary: inheritance, subclass, superclass, type hierarchy, Any; abstract class; static type, dynamic type; open class, sealed class; final.

Credits

Thousands of students have given feedback and so 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, Antti Immonen, Jaakko Kantojärvi, Niklas Kröger, Kalle Laitinen, Teemu Lehtinen, Jaakko Nakaza, Strasdosky Otewa, Timi Seppälä, Teemu Sirkiä, Anna Valldeoriola Cardó, 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 has been designed and implemented by various students in collaboration with O1’s teachers.

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

Additional credits for this page

Chapter 7.5: Inheritance

Introduction

Let’s Continue with Shapes

Subclasses and Superclasses

Inheritance vs. traits

Assignment: Items within Items

Introduction

Task description

Instructions and hints

Abstract Classes

Class Hierarchies in the Scala API

A hierarchy of GUI elements

The `Option` hierarchy (and sealed superclasses)

The mother of all classes: `Any`

The mother of most classes: `AnyRef` a.k.a. `Object`

Tailoring Instances of a Supertype

Further Reading

Summary of Key Points

Feedback

End-of-Week Questions

Credits

Additional credits for this page

Chapter 7.5: Inheritance

Introduction

Let’s Continue with Shapes

Subclasses and Superclasses

Inheritance vs. traits

Assignment: Items within Items

Introduction

Task description

Instructions and hints

Abstract Classes

Class Hierarchies in the Scala API

A hierarchy of GUI elements

The Option hierarchy (and sealed superclasses)

The mother of all classes: Any

The mother of most classes: AnyRef a.k.a. Object

Tailoring Instances of a Supertype

Further Reading

Summary of Key Points

Feedback

End-of-Week Questions

Credits

Additional credits for this page

The `Option` hierarchy (and sealed superclasses)

The mother of all classes: `Any`

The mother of most classes: `AnyRef` a.k.a. `Object`