Question

for the following code:

class A
{
    public static int X;
    static { X = B.Y + 1;}
}
public class B
{
    public static int Y = A.X + 1;
    static {}
    public static void main(String[] args) {
        System.out.println("X = "+A.X+", Y = "+B.Y);
    }
}

the output is:

X = 1, Y = 2

Why? And How?

P.S: Code snippet taken from JavaCamp.org

Answer 1

Here is what happens in chronological order:

1. Class B contains the main-method so it is loaded by the class loader.

2. Initialization of B references A, so class A is loaded.

3. A has a static variable X initialized to B.Y + 1.

   The initialization of B.Y hasn’t been executed yet, so B.Y evaluates to 0, and thus 1 is assigned to A.X

4. Now A has finished loading, and the initialization of B.Y can take place.

   The value of A.X + 1 (1 + 1) is assigned to B.Y.

5. The values of A.X and B.Y are printed as 1 and 2 respectively.

Further reading:

Java Language Specification, §12.4.1 When Initialization Occurs