No, it doesn't mean it's taking 0ms - it shows it's taking a smaller amount of time than you can measure with currentTimeMillis(). That may well be 10ms or 15ms. It's not a good method to call for timing; it's more appropriate for getting the current time.
To measure how long something takes, consider using System.nanoTime instead. The important point here isn't that the precision is greater, but that the resolution will be greater... but only when used to measure the time between two calls. It must not be used as a "wall clock".
Note that even System.nanoTime just uses "the most accurate timer on your system" - it's worth measuring how fine-grained that is. You can do that like this:
public class Test {
public static void main(String[] args) throws Exception {
long[] differences = new long[5];
long previous = System.nanoTime();
for (int i = 0; i < 5; i++) {
long current;
while ((current = System.nanoTime()) == previous) {
// Do nothing...
}
differences[i] = current - previous;
previous = current;
}
for (long difference : differences) {
System.out.println(difference);
}
}
}
On my machine that shows differences of about 466 nanoseconds... so I can't possibly expect to measure the time taken for something quicker than that. (And other times may well be roughly multiples of that amount of time.)