Static field accessed before being written JAVA-W1060

Static members should not be accessed before being written.

As per the Java Language Specification, every variable in a program must have a value before it is used. If a field (static or not) in some class is not initialized explicitly, the compiler will initialize it with a default value. With static initializers, this could become a problem.

A class is allowed to have multiple static initialization block. All static initialization blocks execute in the order they are declared in a source file. If a static initializer block accesses (reads) a static field that is assigned a value further down in some other static initializer block, such an access would evaluate to the default value that is assigned by the Java compiler. This might not be desired.

Bad Practice

class Klass {
    private static int value;

    static {
        if (value > 40) { // `value` is 0 here.
            // ...
        }
    }

    static {
        value = 10;
    }

}

The following snippet fails to compile; accessing value in the static block is an incorrect forward reference.

class A {
    static {
        System.out.println(value); // This is a compile error.
    }

    private static int value = 10;
}

Recommended

Consider initializing all the static fields.

class Klass {
    private static int value = 10;

    static {
        access(value); // `value` is 10, as expected.
    }
}

Alternatively, rearrange your static blocks so that all static fields have a value before they are accessed.

class Klass {
    private static int value;

    static {
        value = 10;
    }

    static {
        access(value); // `value` is 10 here.
    }
}

References

• StackOverflow - The order of execution of multiple static initialization blocks