Constructors and instance variables

[Gina vernon]

Hi,
The study guide says that instance variables/methods cannot be accessed before a call to the class's constructor. But it seems there is no way one would be able to make that error, since you have to call a constructor in order to access instance variables/methods.

Right/wrong?

Thanks.

[rajendhar gangoni]

hi..


I think you are right.

[Ruben Soto]

Gina vernon wrote:Hi,
The study guide says that instance variables/methods cannot be accessed before a call to the class's constructor. But it seems there is no way one would be able to make that error, since you have to call a constructor in order to access instance variables/methods.

Right/wrong?

Thanks.

Well, actually you could try to access an instance variable or method defined in the superclass from the subclass' constructor super(...) call. For example:

class Super {
   int x = 1;

   Super(int y) {
      x = y;
   }
}

class Sub extends Super {
   int a = 10;

   Sub(int b) {
      super(x); // Illegal, trying to access superclass instance variable before super constructor has completed
      a = b;
   }
}

[Gina vernon]

Ruben Soto wrote:
Well, actually you could try to access an instance variable or method defined in the superclass from the subclass' constructor super(...) call. For example:

Ok,got it. thanks.

[Ruben Soto]

Gina vernon wrote:

Ruben Soto wrote:
Well, actually you could try to access an instance variable or method defined in the superclass from the subclass' constructor super(...) call. For example:

Ok,got it. thanks.

That's great, Gina!

Yes, the obvious scenario is accessing an instance variable or method through an instance of the class (which will never give you any problems, since in order to have an instance all the instantiation code must have run.) The more insidious scenario is when you are trying to access an instance variable or method from subclass code before the super constructor has run.

However, you can access instance variables in instance variable initializer expressions (provided you don't use illegal forward referencing.)

public class AccessBeforeConstructor {
   int x = 1;
   int y = x;

   AccessBeforeConstructor() {
      System.out.println("Instance variable x apparently (but not actually)" +
                         "accessed before this constructor runs.");
   }
}

The trick here is that the compiler will actually make instance initializers a part of the actual constructor code.