Method Overriding in Java

In a class hierarchy, when a method in a subclass has the same name and type signature as a method in its super class, then the method in the subclass is said to override the method in the super class. When an overridden method is called from within a subclass, it will always refer to the version of that method defined by the subclass. The version of the method defined by the super class will be hidden.

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.

Program Override.java

class A
 {
  int i, j;
  A(int a, int b)
   {
    i = a;
    j = b;
   }
  void show()
   {
    System.out.println("i and j: " + i + " " + j);
   }
 }

// Create a subclass by extending class A.


class B extends A
 {
    int k;
    B(int a, int b, int c)
     {
      super(a, b);
      k = c;
     }
// overload show()

    void show(String msg)
     {
      System.out.println(msg + k);
     }
 }

class Override
 {
   public static void main(String args[])
    {
     B subOb = new B(10, 20, 30);
     subOb.show("This is k: ");

// this calls show() in B

     subOb.show();

// this calls show() in A
    }
}

Here, super.show( ) calls the superclass version of show( ). Method overriding occurs only when the names and the type signatures of the two methods are identical. If they are not, then the two methods are simply overloaded

Output 

G:\>javac Override.java
G:\>java Override
This is k: 3
i and j: 1 2