private variables and inheritance.

[paddy Mahadeva Iyer]

Hi guys, I am having a small problem while inheriting. Let me put my problem here with an example. Lets say i have 3 classes BaseClass, ChildClass and objectClass and the following is my code.

  
    public class BaseClass {
      private ObjectClass ob1;
 
      public method1 () {
       System.out.println( ob1);
      }
    }
 
     
    public class ChildClass extends  BaseClass {
      private ObjectClass ob1;
      
      public ChildClass () {
         this.ob1 = new ObjectClass();
      }
    }
 

Now lets say i am creating the object of the child class and called

   
      ChlidClass c1 = new ChildClass();
      c1.method1 ();
  

Now i get null printed out. How is that?

When i am calling the method on the child class's object the method is availble via inheritence. and here when it tries to access the variable the object in the variable obj1 of the child class should be accessible since the instance is that of child class.


This is not happening. Can somebody correct me?

[Henry Wong]

When i am calling the method on the child class's object the method is availble via inheritence. and here when it tries to access the variable the object in the variable obj1 of the child class should be accessible since the instance is that of child class.

Overriding (polymorphism) applies to methods -- not to instance variables. Since the method you are calling is a method of the base class, it uses the instance variables of the base class -- which has not been assigned in your example.

Henry

[Rajshekhar Paul]

method1() is not overridden in the ChildClass. So when you invoke method1() in ChildClass instance, actually BaseClass version is executed. In the BaseClass version obj1 is initialized as null. Hence the output is null.

[Punit Singh]

Inherited instance method will cannot access any thing declared in your derived class.
here method1() can access only variable declared in your superclass, so it is accessing obj1 declared in superclass and printing null.

[nav katoch]

[ December 10, 2008: Message edited by: nav katoch ]

[Rajshekhar Paul]

Hahaha. The return type is missing. It should be void in this case. I think it'S a typo.