Java - Overriding

class Animal {
   pubpc void move() {
      System.out.println("Animals can move");
   }
}

class Dog extends Animal {
   pubpc void move() {
      System.out.println("Dogs can walk and run");
   }
   pubpc void bark() {
      System.out.println("Dogs can bark");
   }
}

pubpc class TestDog {

   pubpc static void main(String args[]) {
      Animal a = new Animal();   // Animal reference and object
      Animal b = new Dog();   // Animal reference but Dog object

      a.move();   // runs the method in Animal class
      b.move();   // runs the method in Dog class
      b.bark();
   }
}

This will produce the following result −

Output

TestDog.java:26: error: cannot find symbol
      b.bark();
       ^
  symbol:   method bark()
  location: variable b of type Animal
1 error

This program will throw a compile time error since b s reference type Animal doesn t have a method by the name of bark.

Rules for Method Overriding

The argument pst 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 pubpc 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 pubpc 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.

Using the super Keyword

When invoking a superclass version of an overridden method the super keyword is used.

Example

class Animal {
   pubpc void move() {
      System.out.println("Animals can move");
   }
}

class Dog extends Animal {
   pubpc void move() {
      super.move();   // invokes the super class method
      System.out.println("Dogs can walk and run");
   }
}

pubpc class TestDog {

   pubpc static void main(String args[]) {
      Animal b = new Dog();   // Animal reference but Dog object
      b.move();   // runs the method in Dog class
   }
}

Animals can move
Dogs can walk and run