“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 is a label. Generally a label used in Java to transfer the control flow at desired location while all keywords, like continue and break, have a specified choice of location.

Answer 2

It's not a keyword; it's a label.

It allows you to go a labeled break and labeled continue.

Answer 3

This is really a reply to seanizer's comment on org.life.java's answer, but I wanted to put in some code so I couldn't use the comment feature.

While it is very rare that I find a use for "break label", it does happen occassionally. The most common case is when I am searching for something that is in a structure requiring a nested loop to search, like:

search:
for (State state : stateList)
{
  for (City city : state.cityList)
  {
    if (city.zipcode.equals(wantZip))
    {
      doSomethingTo(city);
      break search;
    }
  }
}

Usually in such cases I push the whole thing into a subroutine so that on a hit I can return the found object, and if it falls out the bottom of the loop I can return null to indicate a not found, or maybe throw an exception. But this is occasionally useful.

Frankly, I think the inventors of Java included this feature because between this and exception handling, they eliminated the last two legitimate uses for GOTO.

Very late addendum:

I saw a great gag line of code once. The programmer wrote:

http://www.example.com/xyz.jsp
for (Foo foo1 : foolist)

He didn't actually say "example.com" but our company's web site.

It gives the impression that there's a URL in the code. It compiles successfully, like it does something. But ... what does it do?

In reality it does nothing. "http:" is a label that he never references. Then the "//" makes the rest of the line a comment.

Answer 4

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.

Answer 5

As other posters have said, it is a label, not a keyword. Using labels allows you to do things like:

outer: for(;;) {
   inner: for(;;) {
     break outer;
   }
}

This allows for breaking of the outer loop.

Link to documentation.

Answer 6

It's a break point label, to allow you to break out of a specified loop, rather than simply the innermost one you happen to be in.

It's used on line 148.