calling instance methods

[K Anshul]

Cannot call instance methods until after super constructor runs.

Then why does this code give the output 0,4

 class A
{
   A() {  print();   }
   void print() { System.out.println("A"); }
}
class B extends A
{
   int i =   Math.round(3.5f);
   public static void main(String[] args)
   {
      A a = new B();
      a.print();
   }
   void print() { System.out.println(i); }
}
 
 

[arnel nicolas]

From the point where you create object (a), the constructor for class A
invoked the print() method.We should now that methods are resolve at runtime and instance variable during compilation,
so the print() method of class B is invoked accsessing variable i, and at that time, variable i
was not yet initialized so the default value was 0 (it is an instance variable).

After object (a) was created, its print() method was invoked (because methods are resolve at runtime)
thus the value was rounded and prints 4.

arnel

[K Anshul]

Originally posted by arnel nicolas:
From the point where you create object (a), the constructor for class A
invoked the print() method.We should now that methods are resolve at runtime and instance variable during compilation,
so the print() method of class B is invoked accsessing variable i, and at that time, variable i
was not yet initialized so the default value was 0 (it is an instance variable).

After object (a) was created, its print() method was invoked (because methods are resolve at runtime)
thus the value was rounded and prints 4.

arnel

Thanks arnel
What i wanted to know was why is the print method called when super constructor has not completed.
Page 316 K&B

You cannot make a call to n instance method or access an instance variable until after the super consructor has run

But in this case when B's print method is called from within A's constructor, super constructor of B that is A has not yet compeleted. Why doesn't the above statement apply here?

[Barry Gaunt]

I think the statement read be "should not" rather than "cannot". It's clearly possible and it works as you can see by running the example.

You "should not" do it because it makes the construction of the super class rather unpredictable. When you write such a base class and write a constructor for it you should restrict the member methods that it calls to being private. In that way you prevent subclasses overriding the method and producing unpredictable (for you, the base class provider) results.