JavaEWAH is a word-aligned compressed variant of the Java bitset class.
It uses a 64-bit run-length encoding (RLE) compression scheme.

The goal of word-aligned compression is not to achieve the best
compression, but rather to improve query processing time. Hence, we try
to save CPU cycles, maybe at the expense of storage. However, the EWAH
scheme we implemented is always more efficient storage-wise than an
uncompressed bitmap (implemented in Java as the BitSet class). Unlike
some alternatives, javaewah does not rely on a patented scheme.