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      Overriding

      Java Tutorial

      This Java tutorial is tailored for newcomers, offering a journey from basic principles to complex Java programming techniques. Completing this tutorial equips you with a solid understanding of Java, preparing you for advanced learning. You'll emerge ready to tackle the challenges of becoming a top-tier software engineer, with the skills to innovate and excel in the vast world of software development.

      Overriding

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

      Benefit of Overriding in Java

      The benefit of overriding is: ability to define a behavior that's specific to the subclass type, which means a subclass can implement a parent class method based on its requirement.
      In object-oriented terms, overriding means to override the functionality of an existing method.

      Java Method Overriding

      Method overriding allows us to achieve run-time polymorphism and is used for writing specific definitions of a subclass method that is already defined in the superclass.
      The method is superclass and overridden method in the subclass should have the same declaration signature such as parameters list, type, and return type.

      Usage of Java Method Overriding

      Following are the two important usages of method overriding in Java:
      • Method overriding is used for achieving run-time polymorphism.
      • Method overriding is used for writing specific definition of a subclass method (this method is known as the overridden method).

      Example of Method Overriding in Java

      Let us look at an example.
      class Animal {
         public void move() {
            System.out.println("Animals can move");
         }
      }
      
      class Dog extends Animal {
         public void move() {
            System.out.println("Dogs can walk and run");
         }
      }
      
      public class TestDog {
      
         public 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
         }
      }

      Output

      Animals can move
      Dogs can walk and run
      In the above example, you can see that even though b is a type of Animal it runs the move method in the Dog class. The reason for this is: In compile time, the check is made on the reference type. However, in the runtime, JVM figures out the object type and would run the method that belongs to that particular object.
      Therefore, in the above example, the program will compile properly since Animal class has the method move. Then, at the runtime, it runs the method specific for that object.
      Consider the following example

      Example

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

      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 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.

      Java Method and Constructor Overriding

      In Java, each class has a different name and the constructor's name is the same as the class name. Thus, we cannot override a constructor as they cannot have the same name.

      Java Method Overriding: Using the super Keyword

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

      Example: Using the super Keyword

      class Animal {
         public void move() {
            System.out.println("Animals can move");
         }
      }
      
      class Dog extends Animal {
         public void move() {
            super.move();   // invokes the super class method
            System.out.println("Dogs can walk and run");
         }
      }
      
      public class TestDog {
      
         public static void main(String args[]) {
            Animal b = new Dog();   // Animal reference but Dog object
            b.move();   // runs the method in Dog class
         }
      }

      Output

      Animals can move
      Dogs can walk and run