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Bitsets are sets of non-negative integer elements that are implemented in one or more words of packed bits. The internal representation of a BitSet uses an array of Longs. The first Long covers elements from 0 to 63, the second from 64 to 127, and so on. 1 For every Long, each of its 64 bits is set to 1 if the corresponding element is contained in the set, and is unset otherwise. It follows that the size of a bitset depends on the largest integer that's stored in it. If N is that largest integer, then the size of the set is N/64 Long words, or N/8 bytes, plus a small number of extra bytes for status information.
Bitsets are hence more compact than other sets if they contain many small elements. Another advantage of bitsets is that operations such as membership test with contains, or element addition and removal with += and -= are all extremely efficient.
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