Java static variables that depend upon each other

Question

There are two classes, each with it's own static field. What will happen when one field depends on another? For example:

public class A {
    public final static Something something = new Something(B.needed);
}

public class B {
    public final static Needed needed = new Needed();
}

Test I made suggest that in that case needed will be initialized before something. In test I was asking Java for variable of class A and B wasn't loaded before. However, result may be just coincidence. Is there any mechanism in Java that will guarantee me that needed will always initialize to be used as argument?

I made another test. Results also show that in extreme cases final primitive types have default value (so they actually have different values in different places of code, despite being final):

public class A {
    public final static int test = 3 - B.test;
}

public class B {
    public final static int test = 2 - A.test;
}

If you run test in a way that loads class A first, values will be:

A.test = 1
B.test = 2

Then if you run test with B loading first, values will be:

A.test = 3
B.test = -1

It seems that Java simply uses default int value 0. I was surprised that this code compiles.

Results of second test also suggest that Java tries to initialize static field it's planning to use when initializing variable from other class. Is this correct? Is it documented, predictable behavior?

Answer 1

As long as you have tree-like dependencies of your classes, you can rely on class X having been initialized when class Y needs it. As soon as you introduce loops (class X needs Y and Y needs X, or even A needs B which needs C which needs D which needs A), behaviour is undefined, and may change whenever you change your java version, JVM vendor, hardware platform or whatever. Don't do that - circular dependencies are almost always the result of bad planning.