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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.javaewah32;

/*
 * 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.
 */

import java.util.Arrays;

/**
 * Int array wrapper.
 * Users should not be concerned by this class.
 *
 * @author Gregory Ssi-Yan-Kai
 */
class IntArray implements Buffer32, Cloneable {

    /**
     * Creates a buffer with default size
     */
    public IntArray() {
        this(DEFAULT_BUFFER_SIZE);
    }
 
    /**
     * Creates a buffer with explicit size
     * @param bufferSize
     */
    public IntArray(int bufferSize) {
        if(bufferSize < 1) {
            bufferSize = 1;
        }
        this.buffer = new int[bufferSize];
    }
    
    @Override
    public int sizeInWords() {
        return this.actualSizeInWords;
    }

    @Override
    public void ensureCapacity(int capacity) {
        resizeBuffer(capacity - this.actualSizeInWords);
    }

    @Override
    public int getWord(int position) {
        return this.buffer[position];
    }

    @Override
    public int getLastWord() {
        return getWord(this.actualSizeInWords - 1);
    }

    @Override
    public void clear() {
        this.actualSizeInWords = 1;
        this.buffer[0] = 0;
    }
    
    @Override
    public void trim() {
        this.buffer = Arrays.copyOf(this.buffer, this.actualSizeInWords);
    }
    
    @Override
    public void setWord(int position, int word) {
        this.buffer[position] = word;
    }
    
    @Override
    public void setLastWord(int word) {
        setWord(this.actualSizeInWords - 1, word);
    }
    
    @Override
    public void push_back(int word) {
        resizeBuffer(1);
        this.buffer[this.actualSizeInWords++] = word;
    }

    @Override
    public void push_back(Buffer32 buffer, int start, int number) {
        resizeBuffer(number);
        if(buffer instanceof IntArray) {
            int[] data = ((IntArray)buffer).buffer;
            System.arraycopy(data, start, this.buffer, this.actualSizeInWords, number);
        } else {
            for(int i = 0; i < number; ++i) {
                this.buffer[this.actualSizeInWords + i] = buffer.getWord(start + i);
            }
        }
        this.actualSizeInWords += number;
    }
    
    @Override
    public void negative_push_back(Buffer32 buffer, int start, int number) {
        resizeBuffer(number);
        for (int i = 0; i < number; ++i) {
            this.buffer[this.actualSizeInWords + i] = ~buffer.getWord(start + i);
        }
        this.actualSizeInWords += number;
    }
    
    @Override
    public void removeLastWord() {
        setWord(--this.actualSizeInWords, 0);
    }
    
    @Override
    public void negateWord(int position) {
        this.buffer[position] = ~this.buffer[position];
    }
    
    @Override
    public void andWord(int position, int mask) {
        this.buffer[position] &= mask;
    }
    
    @Override
    public void orWord(int position, int mask) {
        this.buffer[position] |= mask;
    }
    
    @Override
    public void andLastWord(int mask) {
        andWord(this.actualSizeInWords - 1, mask);
    }
    
    @Override
    public void orLastWord(int mask) {
        orWord(this.actualSizeInWords - 1, mask);
    }
    
    @Override
    public void expand(int position, int length) {
        resizeBuffer(length);
        System.arraycopy(this.buffer, position, this.buffer, position + length, this.actualSizeInWords - position);
        this.actualSizeInWords += length;
    }
    
    @Override
    public void collapse(int position, int length) {
        System.arraycopy(this.buffer, position + length, this.buffer, position, this.actualSizeInWords - position - length);
        for(int i = 0; i < length; ++i) {
            removeLastWord();
        }
    }
    
    @Override
    public IntArray clone() {
        IntArray clone = null;
        try {
            clone = (IntArray) super.clone();
            clone.buffer = this.buffer.clone();
            clone.actualSizeInWords = this.actualSizeInWords;
        } catch (CloneNotSupportedException e) {
            e.printStackTrace(); // cannot happen
        }
        return clone;
    }


   @Override
   public void swap(final Buffer32 other) {
       if(other instanceof IntArray) {
           int[] tmp = this.buffer;
           this.buffer = ((IntArray) other).buffer;
           ((IntArray) other).buffer = tmp;

           int tmp2 = this.actualSizeInWords;
           this.actualSizeInWords = ((IntArray) other).actualSizeInWords;
           ((IntArray) other).actualSizeInWords = tmp2;
       } else {
           int[] tmp = new int[other.sizeInWords()];
           for(int i = 0; i < other.sizeInWords(); ++i) {
               tmp[i] = other.getWord(i);
           }
           int tmp2 = other.sizeInWords();

           other.clear();
           other.removeLastWord();
           other.push_back(this, 0, this.sizeInWords());

           this.buffer = tmp;
           this.actualSizeInWords = tmp2;
       }
    }
    
    /**
     * Resizes the buffer if the number of words to add exceeds the buffer capacity.
     * @param number the number of words to add
     */
    private void resizeBuffer(int number) {
        int size = newSizeInWords(number);
        if (size >= this.buffer.length) {
            int oldBuffer[] = this.buffer;
            this.buffer = new int[size];
            System.arraycopy(oldBuffer, 0, this.buffer, 0, oldBuffer.length);
        }
    }

    /**
     * Returns the resulting buffer size in words given the number of words to add.
     * @param number the number of words to add
     */
    private int newSizeInWords(int number) {
        int size = this.actualSizeInWords + number;
        if (size >= this.buffer.length) {
            if (size < 32768)
                size = size * 2;
            else if (size * 3 / 2 < size) // overflow
                size = Integer.MAX_VALUE;
            else
                size = size * 3 / 2;
        }
        return size;
    }
    
    /**
     * The actual size in words.
     */
    private int actualSizeInWords = 1;
    
    /**
     * The buffer (array of 32-bit words)
     */
    private int buffer[] = null;
    
    /**
     * The Constant DEFAULT_BUFFER_SIZE: default memory allocation when the
     * object is constructed.
     */
    private static final int DEFAULT_BUFFER_SIZE = 4;
    
}