This code just made me stare at my screen for a few minutes:
loop:
for (;;) {
// ...
}
I have never seen this before, and I had no idea Java has a "loop" keyword (NetBeans doesn't even color it like a keyword), and it does compile fine with JDK 6.
What is the explanation?
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;
}
}
}
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.
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).
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
}
}
}
It's not a keyword; it's a label.
It allows you to go a labeled break and labeled continue.
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.
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.
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.
You could write almost anything, as it is a label... You have an example here
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:
It is not a keyword, but a label. If inside the for loop you write break loop;, and you exit that loop.
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.
