“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

The question is answered, but as a side note:

I have heard of interview questions a la "Why is this Java code valid?" (stripped the simpler example; here's the meaner one, thx Tim Büthe):

url: http://www.myserver.com/myfile.mp3
downLoad(url);

Would you all know what this code is (apart from awful)?

Solution: two labels, url and http, a comment www.myserver.com/myfile.mp3 and a method call with a parameter that has the same name (url) as the label. Yup, this compiles (if you define the method call and the local variable elsewhere).

Answer 2

That's not a keyword, it's a label. It's meant to be used with the break and continue keywords inside nested loops:

outer:
for(;;){
    inner:
    for(;;){
        if(){
            break inner; // ends inner loop
        } else {
            break outer; // ends outer loop
        }
    }
}

Answer 3

It is not a keyword it is a label.

Usage:

    label1:
    for (; ; ) {
        label2:
        for (; ; ) {
            if (condition1) {
                // break outer loop
                break label1;
            }
            if (condition2) {
                // break inner loop
                break label2;
            }
            if (condition3) {
                // break inner loop
                break;
            }
        }
    }

Documentation.

Answer 4

It is not a keyword, but a label. If inside the for loop you write break loop;, and you exit that loop.

Answer 5

It is a label, and labels in Java can be used with the break and continue key words for additional control over loops.

Here it is explained in a rather good way:

Thinking in Java, break and continue

Answer 6

You could write almost anything, as it is a label... You have an example here