static variable load process

Question

Recently, I research how virtual machine load static variable, but I encounter a problem as follows:

public class Example{

    static{
        num = 3;
        System.out.print(num);
    }
    public static int num;
}

The compiler reports an error. I think the num has memory area and value. Why can't I access num variable?

Answer 1

static blocks and declarations are executed in the order they are coded. This means that at the time the static block executes, the num field is not yet defined.

The simple fix is moving the declaration above the block.

The Java Language Specification Section 8.3.2.3 (among other things) says that a variable may assigned a value before being declared, but the value of the variable may not be accessed before the variable is declared unless its name is fully qualified, so this compiles:

public class Example {
    static{
        num = 3; // assignment OK without qualifying the name
        System.out.print(Example.num); // name must be qualified to access value
    }
    public static int num;
}

---

Still more interesting is that if a variable is accessed before being declared, its default value is used, and further the initialization on declaration still occurs, so this code:

public class Example {
    static{
        System.out.println(Example.num); // uses default value
        num = 3;                         // assignment OK
        System.out.println(Example.num); // assigned value (3) is visible
    }

    public static int num = 1;           // initialization to 1 occurs

    static{
        System.out.print(Example.num);   // initialized value (1) is visible 
    }
}

Produces this output:

0
3
1

Wow!