Accessing an uninitialised static field in Java

Question

I'm indebted to this answer for the idea.

The following code compiles, but certainly shouldn't. It uses the value of x before it's been initialised. If you remove the StaticAssign. qualifier then it no longer compiles.

public class StaticAssign {

    public static final int x;

    static {
        System.out.println(StaticAssign.x);
        x = 5;
    }

    public static void main(String[] args) {}

}

On my machine, this consistently prints 0. If I change the type of x to String, it consistently prints null.

The question is: will it definitely print 0 or null, or might it access uninitialised memory and print out something else?

And even if this code gets through the compiler, why isn't it picked up by the JVM?

Is there a way to do nasty things based on this?

Answer 1

It actually has been initialized. Variables in the global scope are initialized automatically. Variables of Object type will be initialized to null primitive like int will be initialized to 0. A variable declared not in the global scope must be initialized ie. declared in a method. Another problem is declaring it as final this is telling the compiler it must be explicitly initialized. So by adding the x=5 you are bypassing the compiler error saying it must be explicitly initialized. When you access it before this line at run-time it is initialized to 0 like any other primitive int type variable in the global scope.