Question

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Java static class initialization

Why is the string variable updated in the initialization block and not the integer(even though the block is written first)

class NewClass
{
    static 
    {
       System.out.println(NewClass.string+" "+NewClass.integer);
    }

    final static String string="static";
    final static Integer integer=1;

    public static void main(String [] args)//throws Exception
    {
    }
}

My output is

static null

P.S:Also noticed that string variable initialization happens before the block only when i insert the final modifier. why is that?why not for integer as well?I have declared it as final static too

Answer 1

From section 12.4.2 of the JLS, snipped appropriately:

The procedure for initializing C is then as follows:

• Then, initialize the final class variables and fields of interfaces whose values are compile-time constant expressions (§8.3.2.1, §9.3.1, §13.4.9, §15.28).

• Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block.

So for non-compile-time-constants, it’s not a case of "all variables" and then "all static initializers" or vice versa – it’s all of them together, in textual order. So if you had:

static int x = method("x");

static {
    System.out.println("init 1");
}

static int y = method("y");

static {
    System.out.println("init 2");
}

static int method(String name) {
    System.out.println(name);
    return 0;
}

Then the output would be:

x
init 1
y
init 2

Even making x or y final wouldn’t affect this here, as they still wouldn’t be compile-time constants.

P.S:Also noticed that string variable initialization happens before the block only when i insert the final modifier.

At that point, it’s a compile-time constant, and any uses of it basically inlined. Additionally, the variable value is assigned before the rest of the initializers, as above.

Section 15.28 of the JLS defines compile-time constants – it includes all primitive values and String, but not the wrapper types such as Integer.