Local variable scope and for loop

Question

I just have started to learn Java. I have some dummy question. I don't really get why in this situation:

int j = 5;
for (int j = 0; j < 10; j++) {
   // do sth
}

my compiler says : the variable j is already defined in the scope.
Why this second j is a problem ? I thought that it should simply shadow the first one.

Answer 1

The problem is that you're declaring the variable j twice: One out of the for loop and one inside. Just delete the line above the for and you'll be good to go.

Local variables aren't shadowed - perhaps you had fields in mind (but that's something different from what you have here).

Answer 2

A simpler yet similar scenario is:

int i = 0;
{
   int i = 2;
}

So you have two i variables. Which one do you mean when you reference i ?

The Java compiler doesn't allow 'shadowing' here. The definitions are ambiguous, and the compiler is working to warn you of this.