# 5.4: Every Class Has a Superclass

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Each class in Smalltalk inherits its behaviour and the description of its structure from a single superclass. This means that Smalltalk has single inheritance.

SmallInteger superclass        → Integer
Integer superclass             → Number
Number superclass              → Magnitude
Magnitude superclass           → Object
Object superclass              → ProtoObject
ProtoObject superclass         → nil


Traditionally the root of the Smalltalk inheritance hierarchy is the class Object (since everything is an object). In Squeak, the root is actually a class called ProtoObject, but you will normally not pay any attention to this class. ProtoObject encapsulates the minimal set of messages that all objects must have. However, most classes inherit from Object, which defines many additional messages that almost all objects ought to understand and respond to. Unless you have a very good reason to do otherwise, when creating application classes you should normally subclass Object, or one of its subclasses.

$$\bigstar$$ A new class is normally created by sending the message subclass: instanceVariableNames:... to an existing class. There are a few other methods to create classes. Have a look at the protocol Kernel-Classes ⊳ Class ⊳ subclass creation to see what they are.

Although Squeak does not provide multiple inheritance, since version 3.9 it has incorporated a mechanism called traits for sharing behaviour across unrelated classes. Traits are collections of methods that can be reused by multiple classes that are not related by inheritance. Using traits allows one to share code between different classes without duplicating code.

## Abstract methods and abstract classes

An abstract class is a class that exists to be subclassed, rather than to be instantiated. An abstract class is usually incomplete, in the sense that it does not define all of the methods that it uses. The “missing” methods — those that the other methods assume, but which are not themselves defined — are called abstract methods.

Smalltalk has no dedicated syntax to specify that a method or a class is abstract. By convention, the body of an abstract method consists of the expression self subclassResponsibility. This is known as a “marker method”, and indicates that subclasses have the responsibility to define a concrete version of the method. self subclassResponsibility methods should always be overridden, and thus should never be executed. If you forget to override one, and it is executed, an exception will be raised.

A class is considered abstract if one of its methods is abstract. Nothing actually prevents you from creating an instance of an abstract class; everything will work until an abstract method is invoked.

Example: the class Magnitude.

Magnitude is an abstract class that helps us to define objects that can be compared to each other. Subclasses of Magnitude should implement the methods <, = and hash. Using such messages Magnitude defines other methods such as >, >=, <=, max:, min: between:and: and others for comparing objects. Such methods are inherited by subclasses. The method < is abstract and defined as shown in Code $$\PageIndex{1}$$.

Code $$\PageIndex{1}$$ (Squeak): Magnitude»<

Magnitude»< aMagnitude
↑self subclassResponsibility


By contrast, the method >= is concrete; it is defined in terms of <:

Code $$\PageIndex{2}$$ (Squeak): Magnitude»=

>= aMagnitude
"Answer whether the receiver is greater than or equal to the argument."
↑(self < aMagnitude) not


The same is true of the other comparison methods.

Character is a subclass of Magnitude; it overrides the subclassResponsibility method for < with its own version of < (see Code $$\PageIndex{3}$$). Character also defines methods = and hash; it inherits from Magnitude the methods >=, <=, = and others.

Code $$\PageIndex{3}$$ (Squeak): Character»<

Character»< aCharacter
↑self asciiValue < aCharacter asciiValue


## Traits

A trait is a collection of methods that can be included in the behaviour of a class without the need for inheritance. This makes it easy for classes to have a unique superclass, yet still share useful methods with otherwise unrelated classes.

To define a new trait, simply replace the subclass creation template by a message to the class Trait.

Code $$\PageIndex{4}$$ (Squeak): Defining a New Trait

Trait named:
#TAuthor uses: { }
category: 'SBE--Quinto'


Here we define the trait TAuthor in the category SBE-Quinto. This trait does not use any other existing traits. In general we can specify a trait composition expression of other traits to use as part of the uses: keyword argument. Here we simply provide an empty array.

Traits may contain methods, but no instance variables. Suppose we would like to be able to add an author method to various classes, independent of where they occur in the hierarchy. We might do this as follows:

Code $$\PageIndex{5}$$ (Squeak): An author Method

TAuthor»author
"Returns author initials"
↑ 'on' "oscar nierstrasz"


Now we can use this trait in a class that already has its own superclass, for instance the SBEGame class that we defined in Chapter 2. We simply modify the class creation template for SBEGame to include a uses: keyword argument that specifies that TAuthor should be used.

Code $$\PageIndex{6}$$ (Python): Using a Trait

BorderedMorph subclass: #SBEGame
uses: TAuthor
instanceVariableNames: 'cells'
classVariableNames: ''
poolDictionaries: ''
category: 'SBE--Quinto'


If we now instantiate SBEGame, it will respond to the author message as expected.

SBEGame new author → 'on'


Trait composition expressions may combine multiple traits using the + operator. In case of conflicts (i.e., if multiple traits define methods with the same name), these conflicts can be resolved by explicitly removing these methods (with -), or by redefining these methods in the class or trait that you are defining. It is also possible to alias methods (with @), providing a new name for them.

Traits are used in the system kernel. One good example is the class Behavior.

Code $$\PageIndex{7}$$ (Squeak): Behavior Defined Using Traits

Object subclass: #Behavior

Here we see that the method addTraitSelector:withMethod: defined in the trait TPureBehavior has been aliased to basicAddTraitSelector:withMethod:. Support for traits is currently being added to the browsers.