instanceof Doubt again??

[Joe Harry]

Guys,

Consider the code below from Whizlabs,

class Test{
 final Object obj;
 Test(){
  obj = null; //Line 1
  System.out.println(obj instanceof Object);//Line 2
  Integer i = new Integer("1");//Line 3
  obj = i;//Line 4
 }
 
public static void main(String args[]){
 Test test = new Test();
 System.out.println(test.obj);
 }
}

The question was, On removing which line will the code compile??
The answer was Line 4. I'm confused.

[Barry Gaunt]

What does the compiler error message mean to you?

[ram gaurav]

It will give compile time error because obj is defined as FINAL and when you already assigned a value to obj in Line 1, then it will give compile time error when you again want to change its value in Line 4

[Sanjeev Singh]

Jothi,
Once a value to a final reference variable is assigned it can not be changed so //line 4 is invalid.

[Vinayagar Karpagam]

I believe the key in this question is the Line 2.
Line 4 creates problem because obj is declared as final. A final variable should be assigned a value before the constructor ends.
So, Line 1 can be commented to allow compilation if Line 2 is not there.
That means, Line 2 tries to access an uninitialized final variable (Line 4 assigns value in this case).

Better explanations from other ranchers, please.

[Joe Harry]

My doubt is, is assigning a value of null also considered??

[NDP Prasad]

If i Comment the Line4 and Run the program, I get the value for Line2 as false.

Iam expecting it should be true.

[Vinayagar Karpagam]

Yes Jothi. Even null is a valid final value.

Prasad: (null instanceof Object) == false. Because instanceof operator compares the run-time type of the obj.

[Carlos G�mez]

Jothi

try it:

public static void main(String[] args) {
        final Object o;
        o = null; // line 1
        // o = null; // line 2
}

it will get a compiler error because obj is defined as final and when you already assigned a value to obj in Line 1, you don't reasign a value in line 2.