java.util.BitSet -- set() doesn't work as expected

Question

Am I missing something painfully obvious? Or does just nobody in the world actually use java.util.BitSet?

The following test fails:

@Test
public void testBitSet() throws Exception {
    BitSet b = new BitSet();
    b.set(0, true);
    b.set(1, false);
    assertEquals(2, b.length());
}

It's really unclear to me why I don't end up with a BitSet of length 2 and the value 10. I peeked at the source for java.util.BitSet, and on casual inspection it seems to fail to make sufficient distinction between a bit that's been set false and a bit that has never been set to any value...

(Note that explicitly setting the size of the BitSet in the constructor has no effect, e.g.:

BitSet b = new BitSet(2);

Answer 1

People do use BitSet; however, they use it for something other than what you intend. It's probably best to think of BitSet as a very compact, memory-efficient form of Set<Integer> that has the peculiar property that you can't put negative numbers into it.

It's very common with BitSets to use them in the pattern of

for (int id = set.nextSetBit(0); id >= 0; id = set.nextSetBit(id + 1)) {
  // do stuff to a set index
}

after you do something to fill them up. This is equivalent to iterating over the elements of the Set.