I think that statement labels are one of the most underused features of Java. It's elegant, and a very simple solution to a problem which people often solve using boolean variables and complex additional conditions, incurring considerable runtime overhead. Most people probably know that this is valid Java:
LABEL: {
...
if (cond) break LABEL;
...
if (cond) break LABEL;
...
}
Or this:
LABEL:
while (...) {
while (...) {
.... break LABEL;
}
}
But did you know that all of the following are also valid Java
boolean c0 = ...;
L0: if (c0) { }
if (c0) L1: { }
if (c0) { } else L2: { }
if (c0) { } else L3: if (c0) { }
So, having accepted that, what does the following code fragment print?
if (true) L4:
break L4;
else
System.out.println("First else clause");
L5: if (true)
break L5;
else
System.out.println("Second else clause");
For bonus marks:
1) What is the bytecode generated by a recent Sun javac? This might surprise you, given the design policies of Java, but it makes a lot of sense.
2) Under what circumstances will javac not do that? (This, every Java programmer ought to know.)
From jcpp, a pure Java implementation of the C preprocessor, digraph handling. This is a second pass, perhaps there's an even better way to do it. The first pass was the obvious block of conditionals, but I think this reads better.
case '%':
d = read();
if (d == '=')
tok = new Token(MOD_EQ);
else if (d == '>')
tok = new Token('}'); // digraph
else if (d == ':') PASTE: {
d = read();
if (d != '%') {
unread(d);
tok = new Token('#'); // digraph
break PASTE;
}
d = read();
if (d != ':') {
unread(d);
unread('%');
tok = new Token('#'); // digraph
break PASTE;
}
tok = new Token(PASTE); // double-digraph
}
else
unread(d);
break;
