Inheritance:

Abstract classes

- -

What is an Abstract Class?

Defining Abstract Classes

Abstract Methods

Final Classes

-

What is an Abstract Class?

• A class which cannot be instantiated.
• Is used to couple subclasses to common construction & behaviors
• Is used by creating an inheriting subclass that can be instantiated.

-

How does it affect its children classes?

• An abstract class does a few things for the inheriting subclass:
  1. Define methods which can be used by the inheriting subclass.
  2. Define abstract methods which the inheriting subclass must implement.
  3. Provide a common interface which allows the subclass to be interchanged with all other subclasses.

-

Deciding on Declaring a class Abstract

• Should be created when several classes must be coupled to common construction of an entity whose definition is deferred.
  • When a class must be coupled to a behavior, we typically use an interface.
• Should be created when an entity should not be instantiable, but contains data-members which should be used by subclasses.
• Pet, Animal, Mammal, are examples of potential abstract classes.

-

Defining Abstract Classes

abstract public class Shape {
  private double width;
  private double height;
  public Shape(double width, double height) {
    this.width = width;
    this.height = height;
  }
  // getters & setters omitted
}

-

Deciding on declaring a method Abstract

• An abstract method is a deferral of implementation
• An abstract method should be created when an entity should be instantiable, but contains data-members which should be used by subclasses.

-

Declaring Abstract Method

• Methods which are declared, are abstract.

abstract public class Shape {
  abstract public Double getArea();
}

-

Declaring Abstract Method

• Interface method-declarations are implicitly abstract and public.

public interface Shape {
  Double getArea();
}

-

Providing implementation for Abstract Method

• Methods which are defined, are not abstract.

public class Square extends Shape {
  public Square(double length) {
    super(length, length)
  }

  @Override
  public Double getArea() {
    return Math.pow(length, 2);
  }
}

-

Declaring a Final Class

• Prevents class-hierarchy from deepening.
• Declare a class final when it should not be subclassed (i.e. - making it closed to modification)
• String is a final class.

-

Declaring a Final Class Examples

public final class Square extends Shape {
  public Square(double length) {
    super(length, length)
  }
  public Double getArea() {
    return Math.pow(length, 2);
  }
}

-

Final Methods

• Making a method final prevents other sub-classes from overriding the method.