Overloading and Overriding

[Srinivas Palam]

Can some one please explain with simple class example, the meaning of object type and reference type. I am little confused. I was reading scjp6 book and found following description.

To summarize, which overridden version of the method to call (in other words, from which class in the inheritance tree) is decided at runtime based on object type, but which overloaded version of the method to call is based on the reference type of the argument passed at compile time. If you invoke a method passing it an Animal reference to a Horse object, the compiler knows only about the Animal, so it chooses the overloaded version of the method that takes an Animal. It does not matter that at runtime there's actually a Horse being passed.

[Jelle Klap]

public abstract class Animal {
  public void makeNoise() {
    System.out.println("Animal making generic animal noise");
  }
}

public class Horse extends Animal {
  
  @Override
  public void makeNoise() {
    System.out.println("Horse neighing");
  }

  public void buck() {
    System.out.println("Horse bucking");
  }
}

public class AnimalFarm {
  public static void main(String[] args) {
    Animal animal = new Horse();
    animal.makeNoise(); // 1
    animal.buck();  // 2 
    Horse horse = (Horse) animal;
    horse.buck();  // 3
  }
}

1. What would the output be?
2. Will this compile?
3. Will this compile?

[Seetharaman Venkatasamy]

Srinivas Palam wrote: object type and reference type.

there is no object type, i guess it is reference in java. (as far i know ;) )

[gurpeet singh]

Consider the expression

Animal a = new Horse();

here a is a reference variable of the type Animal. so i guess reference type means type of the reference

Object type means type of the object on the other end of a which is Horse. now new Horse() is a Horse, is a Animal(assume Horse subclass from Animal), is a Object.

[Rakesh K. Cherukuri]

just to add to gurupreet,

         List l = new ArrayList();

Here List is the reference type
ArrayList is the object type

FWIW,

when it comes to production ready code, the object type wont be so straight forwardly clear. It depends on the object passed at runtime. For ex.

        Animal animal = listOfAnimals.get(1);

where listOfAnimals is a list of some animals.

Hope that helps.

- Rakesh K.

[Nikhil Sagar]

Seetharaman Venkatasamy wrote:

Srinivas Palam wrote: object type and reference type.

there is no object type, i guess it is reference in java. (as far i know ;) )

In addition to this, there is no object type, there is reference variable type and the actual object on the heap of a particular class referred by the reference variable at run-time.

[Srinivas Palam]

Thanks for your responses. Now I understood.

One more question, as we cannot override final and staic methods, Is it applicable to overloading methods also? My answer is Yes, but want to confirm.

[gurpeet singh]

Srinivas Palam wrote:Thanks for your responses. Now I understood.

One more question, as we cannot override final and static methods, Is it applicable to overloading methods also? My answer is Yes, but want to confirm.

do you want to say that we cannot overload final and/or static methods ?

the answer is no we can. we can overload final methods and/or static methods. overloading means same method name but with different number or type of arguments passed. for e.g

public final void SomeMethod(int a , int b) { }

public final String SomeMethod(int a , int b, int c) { }

public final static String SomeMethod(Animal a ) { }

[Himai Minh]

Hi, Srinivas Palam buddy,

class Animal{
    protected void eat () {
          ....
     }

}

class Horse extends Animal{
 
     //overriding method
      protected void eat(){
           ....
        }

     //overloading method
     protected void eat (String food){
             .......

     }
}

  class Main{

         public static void main (String[] args){

            Animal a = new Horse();
               a.eat() ;          // calls the eat version of Horse

               
               a.eat("Grass") ;   
             //compilation error, in Animal class, it does not have eat (String) method. 
            //The compiler knows a is an Animal, but it does not know a is a Horse.


              //calls the eat (String) method of Horse. The compiler now knows a is a Horse.
              ((Horse)a).eat("Grass");
          }       
   } 

[Seetharaman Venkatasamy]

Hi Srinivas Palam,

Please, UseOneThreadPerQuestion