Dynamic Method Dispatch in Java | Overcome Overriding at Runtime | Java for Beginner | Java Basic.

Dynamic method dispatch

Method overriding forms the basis for one of Java’s most powerful concepts: dynamic method dispatch. Dynamic method dispatch is the mechanism by which a call to an overridden method is resolved at run time, rather than compile time. Dynamic method dispatch is important because this is how Java implements run-time polymorphism.

Java uses this fact to resolve calls to overridden methods at run time. Here is how. When an overridden method is called through a superclass reference, Java determines which version of that method to execute based upon the type of the object being referred to at the time the call occurs.

Thus, this determination is made at run time. When different types of objects are referred to, different versions of an overridden method will be called.



Program for Dynamic method dispatch


// Dynamic Method Dispatch class A
{
   void callme()
   {
      System.out.println("Inside A's callme method");
   }
}

class B extends A
{
   // override callme()
   void callme()
   {
      System.out.println("Inside B's callme method");
   }
}

class C extends A
{
   // override callme()
   void callme()
   {
      System.out.println("Inside C's callme method");
   }
}

class Dispatch
{
   public static void main(String args[])
   {
      A a = new A(); // object of type A

      B b = new B(); // object of type B

      C c = new C(); // object of type C

      A r; // obtain a reference of type A
      r = a; // r refers to an A object
      r.callme();
      // calls A's version of callme
      r = b;
      // r refers to a B object
      r.callme();
      // calls B's version of callme
      r = c;
      // r refers to a C object
      r.callme();
      // calls C's version of callme
   }
}

Output:

Inside A’s callme method
Inside B’s callme method 
Inside C’s callme method

This program creates one superclass called A and two subclasses of it, called B and C. Subclasses B and C override callme( ) declared in A. Inside the main( ) method, objects of type A, B, and C are declared. Also, a reference of type A, called r, is declared. 

The program then in turn assigns a reference to each type of object to r and uses that reference to invoke callme( ). As the output shows, the version of callme( ) executed is determined by the type of object being referred to at the time of the call. 

Had it been determined by the type of the reference variable, r, you would see three calls to A’s callme( ) method.