Question

When going through the JLS 8.3.2.3 I wasn’t able to understand the following code.

class Z {
static { i = j + 2; }
static int i, j;
static { j = 4; }
}

The code is resulting in the error Cannot reference a field before it is defined

But if I change the code to

class Z {
static { i = 2; }
static int i, j;
static { j = 4; }
}

The code is getting compiled. But in both the cases the variable definition is after the initialization block. What is the mystery behind this?

Answer 1

You can assign to a value earlier than its declaration – you just can’t read it. So this fails too:

static { System.out.println(j + 2); }
static int j;

Whereas this is fine:

static { j = 5; }
static int j;

One of the four conditions in section 8.3.2.3 for an invalid usage is:

• The usage is not on the left hand side of an assignment.

(The double-negatives in that section are making my head hurt, but I think it relevant!)

To be honest, that part of the spec is one of the worst I’ve seen – it’s really unclear. But the upshot is that you can assign but not read 🙂