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4.23 Lecture 8

Introduction to Systems Programming - Lectures - Class 8.pdf

Extending classes

In Java, as in most OOP languages, any class will be a subclass of another. In Java we have a root class which is the exception of this rule. Object is not a subclass of any other class, but all other classes are a subclass of Object.
Any class A that is a subclass of B will have access to:

  • Public fields, methods, and constructors.
  • Package fields, methods, and constructors.
  • Protected fields, methods, and constructors.

Any class A that is a subclass of B can redefine/overwrite:

  • Methods, but access permissions must remain equal or greater.
    E.g.: a protected method in B can be made protected or public in A.

If class B is a subclass of class A, then B can be used in place of A. The opposite is not possible.
Continuing from the previous example, any constructor of B must call a constructor of A as its first statement. This rule might not always be visible because of empty constructors, if A has an empty constructor, then it will be used by default if a constructor in B doesn’t call it explicitly.

Superclass constructor call

  • Subclass constructors must always contain a 'super' call.
  • If none is written, the compiler inserts one (without parameters).
    works only, if the superclass has a constructor without parameters
  • Must be the first statement in the subclass constructor

Extending classes

“B extends A” can be seen as “B is an A”.
Inheritance also applies to interfaces:
​public class LinkedList<T> implements List<T>​
Also means “LinkedList is a List”

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Extending classes in Java-What can we extend?

  • Interfaces (extending an interface is not the same as implementing one!).
  • Classes.
  • Abstract classes.

An abstract class in a partially implemented class, it cannot have instances, we have seen an example before in class Figure.

Subclasses and subtyping

  • Classes define types.
  • Subclasses define subtypes.
  • Objects of subclasses can be used where objects of supertypes are required.
    This is called substitution

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Subclasses and parameter passing

public void addArea(Figure figure)
{
    this.area += figure.area();
}
Figure circle = new Circle(3.5f);
Figure rectangle = new Rectangle(4.0f, 2.0f);
Square square = new Square(2.0f);
addArea(circle);
addArea(rectangle);
addArea(square);

A question

If a variable of class/type A can be assigned any value of type A or a subclass of A; what types (non-primitive) can be assigned to an Object variable?

Can be assigned a value of any non-primitive type.

Inheritance hierarchy

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Possible solutions

  • Redefine method display on each Post subtype/subclass.
  • Make display abstract in Post, so it must be defined in each subclass.
  • Define method display as:

public void display()
{
//show author
//call displayBody()
//show timestamp and votes
}
* And make displayBody an abstract method

To discuss

Given class A with methods m1, m2, and m3; classes B and C as subclasses of A overriding methods m1, and m2; and class D as a subclass of B overriding method m2.

Given the statements

A x = randomlnstanceOfA();
x.m2();
x.m3();

With method randomlnstanceOfA​ giving an instance of A, B, C, or D. How do we know which method "m2" and "m3" is called?

  • For x.m2(), you cannot know which specific method is called without knowing the runtime type of the object. It depends on whether x holds an instance of A, B, C, or D.
  • For x.m3(), you know exactly which method is called. Since m3() is not overridden in any subclass, the original implementation from class A is always executed, regardless of the runtime type of x