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C++ to Java → Inheritance →

Abstract classes and methods

In Java, an abstract method is declared with the keyword abstract and given without an implementation. If a class includes abstract methods, then the class itself must be declared abstract.

The speak method in this Animal class is abstract. Note how the method signature ends with a semicolon and there is no method body. This makes sense as the implementation of the speak method depends on the type of the animal and it is meaningless to provide a common implementation for all animal types.

public abstract class Animal {

    protected String name;

    public Animal(String name){
        this.name = name;
    }
    public abstract String speak();
}

As one method of the class is abstract, the class itself is abstract.

An abstract class is declared with the keyword abstract. Abstract classes can be used as reference type but cannot be instantiated.

This Account class has been declared as abstract although it does not have any abstract methods. Attempting to instantiate Account objects will result in a compile error.

public abstract class Account {

    int number;

    void close(){
        //...
    }
}

Account a; OK to use as a type
a = new Account(); Compile error!

In Java, even a class that does not have any abstract methods can be declared as an abstract class.

When an abstract class is subclassed, the subclass should provide implementations for all of the abstract methods in its superclass or else the subclass must also be declared abstract.

The Feline class below inherits from the abstract class Animal but it does not provide an implementation for the abstract method speak. As a result, the Feline class needs to be abstract too.

public abstract class Feline extends Animal {
    public Feline(String name) {
        super(name);
    }

}

The DomesticCat class inherits the abstract Feline class and provides the implementation for the abstract method speak. As a result, it need not be (but can be) declared as abstract.

public class DomesticCat extends Feline {
    public DomesticCat(String name) {
        super(name);
    }

    @Override
    public String speak() {
        return "Meow";
    }
}
  • Animal a = new Feline("Mittens");
    Compile error! Feline is abstract.
  • Animal a = new DomesticCat("Mittens");
    OK. DomesticCat can be instantiated and assigned to a variable of Animal type (the assignment is allowed by polymorphism).

Exercises:

[Key Exercise] print area with abstract Shape

The Main class below keeps a list of Circle and Rectangle objects and prints the area (as an int value) of each shape when requested.

public class Main {
    private static Shape[] shapes = new Shape[100];
    private static int shapeCount = 0;

    public static void addShape(Shape s){
        shapes[shapeCount] = s;
        shapeCount++;
    }

    public static void printAreas(){
        for (int i = 0; i < shapeCount; i++){
            shapes[i].print();
        }
    }

    public static void main(String[] args) {
        addShape(new Circle(5));
        addShape(new Rectangle(3, 4));
        addShape(new Circle(10));
        addShape(new Rectangle(4, 4));
        printAreas();
    }
}


Circle of area 78
Rectangle of area 12
Circle of area 314
Rectangle of area 16

Circle class and Rectangle class is given below:

public class Circle extends Shape {

    private int radius;

    public Circle(int radius) {
        this.radius = radius;
    }

    @Override
    public int area() {
        return (int)(Math.PI * radius * radius);
    }

    @Override
    public void print() {
        System.out.println("Circle of area " + area());
    }
}

public class Rectangle extends Shape {
    private int height;
    private int width;

    public Rectangle(int height, int width){
        this.height = height;
        this.width = width;
    }

    @Override
    public int area() {
        return height * width;
    }

    @Override
    public void print() {
        System.out.println("Rectangle of area " + area());
    }
}

Add the missing Shape class as an abstract class with two abstract methods.

Partial solution

Statements about abstract classes