Slide 12.4: Polymorphism (cont.): Data members

Polymorphism (Cont.)

Runtime Polymorphism with Data Members
We can override methods only, not the variables (data members), so runtime polymorphism cannot be achieved by data members:

DataMember.java (runtime polymorphism with data members)

// Runtime polymorphism cannot be achieved by data members.
public class DataMember {
  public static void main( String args[ ] ) {
    A a = new B( );     // Object of type B
    System.out.print( a.x );
  }
}
// class A 
class A { int x = 10; } 
// class B 
class B extends A { int x = 20; }

shell> java DataMember

Output: Result:

In above program, both the class A (superclass) and B (subclass) have a common variable x. Now we make object of class B, referred by a which is of type of class A. Since variables are not overridden, so the statement a.x will always refer to data member of superclass.

Static vs Dynamic Binding

Static binding is done during compile-time while dynamic binding is done during run-time.
The private, final, and static methods and variables use static binding and are bonded by compiler while overridden methods are bonded during runtime based upon type of runtime object.