Java - Overriding with code and explanation.

In the previous chapter we talked about super classes and sub classes. If a class inherits a method from its super class, then there is a chance to override the method provided that it is not marked final.

The benefit of overriding is: ability to define a behavior that's specific to the sub class type. Which means a subclass can implement a parent calss method based on its requirement.

In object oriented terms, overriding means to override the functionality of any existing method.

Example:

Let us look at an example.

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package com.kota.core; class Exp { public int x = 3; public void abc() { x += 5; System.out.println("the method of exp"); } } public class Exp1 extends Exp { public int x = 8; public Exp1(int y) { x = y; } public void abc() { x += 5; System.out.println("the method of exp1"); } public static void main(String[] rr) { Exp b = new Exp1(10); b.abc(); System.out.println("the value of=" + b.x); } } /*Out put of the program: * the method of exp1 * the value of=3 * * * */

Rules for method overriding:

The argument list should be exactly the same as that of the overridden method.

The return type should be the same or a subtype of the return type declared in the original overridden method in the super class.

The access level cannot be more restrictive than the overridden method's access level. For example: if the super class method is declared public then the overridding method in the sub class cannot be either private or public. However the access level can be less restrictive than the overridden method's access level.

Instance methods can be overridden only if they are inherited by the subclass.

A method declared final cannot be overridden.

A method declared static cannot be overridden but can be re-declared.

If a method cannot be inherited then it cannot be overridden.

A subclass within the same package as the instance's superclass can override any superclass method that is not declared private or final.

A subclass in a different package can only override the non-final methods declared public or protected.

An overriding method can throw any uncheck exceptions, regardless of whether the overridden method throws exceptions or not. However the overriding method should not throw checked exceptions that are new or broader than the ones declared by the overridden method. The overriding method can throw narrower or fewer exceptions than the overridden method.

Constructors cannot be overridden.