Why is the order of declarations important for static initializers?

Question

I have this code

private static Set<String> myField;

static {
    myField = new HashSet<String>();
    myField.add("test");
}

and it works. But when I flip the order, I get an illegal forward reference error.

static {
    myField = new HashSet<String>();
    myField.add("test"); // illegal forward reference
}

private static Set<String> myField;

I'm a little bit shocked, I didn't expect something like this from Java. :)

What happens here? Why is the order of declarations important? Why does the assignment work but not the method call?

Answer 1

First of all, let's discuss what a "forward reference" is and why it is bad. A forward reference is a reference to a variable that has not yet been initialized, and it is not confined only to static initalizers. These are bad simply because, if allowed, they'd give us unexpected results. Take a look at this bit of code:

public class ForwardRef {
    int a = b; // <--- Illegal forward reference
    int b = 10;
}

What should j be when this class is initialized? When a class is initialized, initializations are executed in order the first to the last encountered. Therefore, you'd expect the line

a = b; 

to execute prior to:

b = 10; 

In order to avoid this kind of problems, Java designers completely disallowed such uses of forward references.

EDIT

this behaviour is specified by section 8.3.2.3 of Java Language Specifications:

The declaration of a member needs to appear before it is used only if the member is an instance (respectively static) field of a class or interface C and all of the following conditions hold:

• The usage occurs in an instance (respectively static) variable initializer of C or in an instance (respectively static) initializer of C.

• The usage is not on the left hand side of an assignment.

• C is the innermost class or interface enclosing the usage.

A compile-time error occurs if any of the three requirements above are not met.