Programming Tutorials

Basics of Inheritance in Java

By: Baski in Java Tutorials on 2007-09-08  

To inherit a class, you simply incorporate the definition of one class into another by using the extends keyword. To see how, let's begin with a short example. The following program creates a superclass called A and a subclass called B. Notice how the keyword extends is used to create a subclass of A.

// A simple example of inheritance.
// Create a superclass.
class A {
    int i, j;

    void showij() {
        System.out.println("i and j: " + i + " " + j);
    }
}

// Create a subclass by extending class A.
class B extends A {
    int k;

    void showk() {
        System.out.println("k: " + k);
    }

    void sum() {
        System.out.println("i+j+k: " + (i + j + k));
    }
}

class SimpleInheritance {
    public static void main(String args[]) {
        A superOb = new A();
        B subOb = new B();
        // The superclass may be used by itself.
        superOb.i = 10;
        superOb.j = 20;
        System.out.println("Contents of superOb: ");
        superOb.showij();
        System.out.println();
        /*
         * The subclass has access to all public members of
         * its superclass.
         */
        subOb.i = 7;
        subOb.j = 8;
        subOb.k = 9;
        System.out.println("Contents of subOb: ");
        subOb.showij();
        subOb.showk();
        System.out.println();
        System.out.println("Sum of i, j and k in subOb:");
        subOb.sum();
    }
}

The output from this program is shown here:

Contents of superOb:
i and j: 10 20
Contents of subOb:
i and j: 7 8
k: 9
Sum of i, j and k in subOb:
i+j+k: 24

As you can see, the subclass B includes all of the members of its superclass, A. This is why subOb can access i and j and call showij(). Also, inside sum(), i and j can be referred to directly, as if they were part of B. Even though A is a superclass for B, it is also a completely independent, stand-alone class. Being a superclass for a subclass does not mean that the superclass cannot be used by itself. Further, a subclass can be a superclass for another subclass. The general form of a class declaration that inherits a superclass is shown here:

class subclass-name extends superclass-name {
  // body of class
}

You can only specify one superclass for any subclass that you create. Java does not support the inheritance of multiple superclasses into a single subclass. (This differs from C++, in which you can inherit multiple base classes.) You can, as stated, create a hierarchy of inheritance in which a subclass becomes a superclass of another subclass. However, no class can be a superclass of itself.