“loop:” in Java code. What is this, and why does it compile?

Question

This code just made me stare at my screen for a few minutes:

loop:
for (;;) {
    // ...
}

(line 137 here)

I have never seen this b

Answer 1

It's a label, though look at the following example:

int a = 0;
int b = 0
while (a<10){
    firstLoop:
    a++;
    while(true){
        b++
        if(b>10){
            break firstLoop;
        }
    }
 }

When b>10 the execution flow goes to the outer loop.