Enum - static initialization order [duplicate]

Question

Accessing static variables in constructor of enum class is forbidden in java. Why in enumerations the order of static initialization is not swapped?

Example of correct code:

enum Test
{
    TEST1, TEST2;

    static int x;
}

Why do developers of java did not created such code as correct:

enum Test
{
    static int x;

    TEST1, TEST2;
}

Then static variables could be used in constructor. Now it is forbidden.

Does it have any reason or it is just their design?

Answer 1

Well, it's to ensure safe instance initialization - the enum instances are very similar to static final instances of the enum class, and the language has defined that they be initialized first.

But if you know a trick or two you can effectively use static variables in an enum constructor:

enum Test {
    TEST1, TEST2;

    static class Holder {
        static int x;
    }

    Test() {
        Holder.x++; // no compiler error
    }
}

For more info see Initialization-on-demand holder idiom

Answer 2

From JLS (§8.9):

An enum declaration specifies a new enum type

---

EnumDeclaration: ClassModifiersopt enum Identifier Interfacesopt EnumBody

EnumBody: { EnumConstants-opt ,opt EnumBodyDeclarations-opt }

---

As you can see, the body should be defined with enum-constants declared first and other body declarations may follow - not the other way around!

Further, you don't have to use lazy initialization like bohemian suggested you can do it in a simpler manner. According to JLS you can't do:

enum Test {
    TEST1, TEST2;
    static int x;

    Test(){
        x = 1; // <-- compilation error!
    }
}

but you can use static initializer block:

enum Test {
    TEST1, TEST2;
    static int x;

    static{
        x = 1; // works!
    }
}

The reason that you CAN use the latter is that static declarations are executed in the same order they were declared - assigning x=1 will occur only after x was declared, unlike using a constructor. If you want to verify it - you can add System.out.println() calls to the constructor and static block - you'll see that the constructor is being executed first.