Creation date: 14 March, 2011

Tricky Polymorphism

Look at the following example Java class:

public class Main {

  static class A {
     private String title = "Title of A";

     public String getTitle() {
       return title;
    }

    String go() {
       return getTitle();
    }
  }

  static class B extends A {
    private String title = "Title of B";

    public String getTitle() {
       return title;
    }
  }

  public static void main(String... args) {
     A a = new A();
     B b = new B();
     System.out.println(a.go());
     System.out.println(b.go());
  }
}

If you run this class you'll get

Title of A
Title of B

Method a.go() calls a.getTitle() and it's OK. Method b.go() calls inherited method go() of class A. method go() in this case calls the method getTitle() of class B.

But if you change modifier of method getTitle() in class A from public to private (highlighted with bold font) the result will

Title of A
Title of A

In this case despite of polymorphism go() method of class A will call it's private getTitle() method. Watch out of this tricky question if you are goung to apply for SCJP exam. You may see the similar questions in exam too. The private method can not be inherited. Therefore polymorphizm does not take place here and ancestor of class calls its own method.

Author: Jafar N.Aliyev (Jsoft)

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