Composition and Inheritance in Scala

Ruining (Ray) Li

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Apr 27, 2021

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Abstract class

abstract class Element {
    def contents: Array[String]
}

• A method that has no implementation = The method is an abstract member of the class.
• A class with abstract members must itself be declared abstract.
• The abstract modifier signifies that the class may have abstract members that do not have an implementation.
• You cannot instantiate an abstract class.
• Terminology: declare the abstract method, define no concrete methods.

Parameterless methods

abstract class Element {
    def contents: Array[String]
    def height: Int = contents.length 1
    def height(): Int = contents.length 2  
    val height = contents.length 3
}

• 1 is Parameterless Method: Recommended — if the method does not change mutable state (data field).
• 1 and 3 should be completely equivalent from a client’s point of view: uniform access principle.
• 3 faster — the field values are pre-computed; 1 takes less memory

Inheritance

class ArrayElement(conts: Array[String]) extends Element {
    def contents: Array[String] = conts
}

• Private members of the superclass are not inherited in a subclass.
• Overriding. N.B. Fields and methods belong to the same namespace in Scala, so it’s ok for a field to override a parameterless method, e.g., the following is fine:

class ArrayElement(conts: Array[String]) extends Element {
    val contents: Array[String] = conts
}

Parametric field

class ArrayElement(
    val contents: Array[String]
) extends Element

• The field is unreassignable (i.e., val).

Override modifier

• Required when overriding a concrete member in a parent class.
• Optional when overriding an abstract member.
• Forbidden when not overriding.

Polymorphism & Dynamic Binding

• The actual method implementation invoked is determined at run time based on the class of the object, not the type of the variable or expression.

Final Modifier

• Final before class: the entire class cannot be subclassed.
• Final before member (i.e., field/method): the member cannot be overridden.

Any Class

• Superclass of all.
• AnyVal: superclass of all value classes.
• AnyRef: superclass of all reference classes.