Inheritance

Inheritance:

 

Inheritance is one of the cornerstones of object-oriented programming because it allows the creation of hierarchical classifications. Using inheritance, you can create a general class that defines traits common to a set of related items. This class can then be inherited by other, more specific classes, each adding those things that are unique to it. In the terminology of Java, a class that is inherited is called a superclass. The class that does the inheriting is called a subclass. Therefore, a subclass is a specialized version of a superclass. It inherits all of the instance variables and methods defined by the superclass and adds its own, unique elements.

 

Inheritance Basics

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, standalone 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.