Java Overriding


When a method in a sub class has same name and type signature as a method in its super class, then the method is known as overridden method.

Method overriding is also referred to as runtime polymorphism.

The key benefit of overriding is the abitility to define method that's specific to a particular subclass type.


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

Example of Method Overriding

class Facility
{
 public void Student()
 {
  System.out.println("Hello");
 }
}

class Collage extends Facility
{
 public void Student()   //Student() method overriden by Collage class.
 {
  System.out.println("Hello World !");
 }
}

As you can see here Collage class gives it own implementation of Student() method. Method must have same name and same type signature.

NOTE : Static methods cannot be overridden because, a static method is bounded with class where as instance method is bounded with object.


Difference between Overloading and Overriding

Method Overloading
Method Overriding
Parameter must be different and name must be same. Both name and parameter must be same.
Compile time polymorphism. Runtime polymorphism.
Increase readability of code. Increase reusability of code.
Access specifier can be changed. Access specifier most not be more restrictive than original method(can be less restrictive).

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