Returning in a static initializer

Question

This isn't valid code:

public class MyClass
{
    private static boolean yesNo = false;

    static
    {
        if (yesNo)
        {
            System.out.println("Yes");
            return; // The return statement is the problem
        }
        System.exit(0);
    }
}

This is a stupid example, but in a static class constructor we can't return;. Why? Are there good reasons for this? Does someone know something more about this?

So the reason why I should do return is to end constructing there.

Thanks

Answer 1

I think the reason is that initializers are carried together with field initializations (and with constructors, in the case of instance initializers). In other words, the JVM only recognizes one place to initialize static fields, and thus all initializations - whether in blocks or not - must be done there.

So, for example, when you write a class:

class A {
    static int x = 3;
    static {
        y = x * x;
    }
    static int z = x * x;
}

Then it's actually as if you've written:

class A {
    static int x, y, z;
    static {
        x = 3;
        y = x * x;
        z = x * x;
    }
}

This is confirmed if you look at the disassembly:

static {};
  Code:
   0:   iconst_3
   1:   putstatic       #5; //Field x:I
   4:   getstatic       #5; //Field x:I
   7:   getstatic       #5; //Field x:I
   10:  imul
   11:  putstatic       #3; //Field y:I
   14:  getstatic       #5; //Field x:I
   17:  getstatic       #5; //Field x:I
   20:  imul
   21:  putstatic       #6; //Field z:I
   24:  return

So if you would have added a "return" somewhere in the middle of your static initializer it would also have prevented z from being calculated.

Answer 2

• the program flow can always be structured to go without the need for return. (In your example putting System.exit(0) in an else clause would achieve the desired result)

• in you really need it, you can move the code in a static method and call it from the initializer:

.

static {
    staticInit();
}

private static void staticInit() {
    if (yesNo) {
        System.out.println("Yes");
        return;
    }
    System.exit(0);
}

Note that this is not a static constructor, it's a static initializer. Nothing gets constructed.

Answer 3

From JSL regarding static initializers:

"It is a compile-time error for a static initializer to be able to complete abruptly (§14.1, §15.6) with a checked exception (§11.2). It is a compile-time error if a static initializer cannot complete normally (§14.21)."

Abrupt completion (among others): "return with no value", "return with a given value", etc.

So a return statement in a static initializer is an "abrupt completion" and produces a compile-time error.

Answer 4

What should you return to? In a static initializer there is no caller, so a return doesn't make sense as far as I see it. Static initializers are executed when the class is loaded for the first time.