[SOLVED] Why is there no compilation error on the line above the indicated line?

Why is there no compilation error on the line above the indicated line?

I'm reading up on Java and I am scratching my head as to why System.out.println("a: " + a); does not yield a compilation error. Where is a ever initialized?

public class localVariableEx {
    public static int a;
    public static void main(String[] args) {
        int b;
        System.out.println("a: " + a);
        System.out.println("b: " + b);  //Compilation error
    }
}

Solution

The relevant rules of this are described in the JLS § 4.12.5 Initial Values of Variables (emphasis mine):

Each class variable, instance variable, or array component is initialized with a default value when it is created (§15.9, §15.10): [...]

[...]

A local variable (§14.4, §14.14) must be explicitly given a value before it is used, by either initialization (§14.4) or assignment (§15.26), in a way that can be verified by the compiler using the rules for definite assignment (§16).

So while instance variables (such as a) automatically get a default value, local variables (such as b) don't get one and must not be used unless the compiler can verify that a value has been assigned to them.