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The bit array data structure is implemented in Java as the BitSet class. Unfortunately, this fails to scale without compression. 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.

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package com.googlecode.javaewah;

/*
 * Copyright 2009-2016, Daniel Lemire, Cliff Moon, David McIntosh, Robert Becho, Google Inc., Veronika Zenz, Owen Kaser, Gregory Ssi-Yan-Kai, Rory Graves
 * Licensed under the Apache License, Version 2.0.
 */

/**
 * The ChunkIterator interface is used to iterate over chunks of ones or zeros.
 *
 * @author Gregory Ssi-Yan-Kai
 */
public interface ChunkIterator {

    /**
     * Is there more?
     *
     * @return true, if there is more, false otherwise
     */
    boolean hasNext();

    /**
     * Return the next bit
     *
     * @return the bit
     */
    boolean nextBit();

    /**
     * Return the length of the next bit
     *
     * @return the length
     */
    int nextLength();

    /**
     * Move the iterator at the next different bit
     */
    void move();

    /**
     * Move the iterator at the next ith bit
     *
     * @param bits  the number of bits to skip
     */
    void move(int bits);

}