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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.
 */

import java.nio.LongBuffer;

/**
 * java.nio.LongBuffer wrapper.
 * Users should not be concerned by this class.
 *
 * @author Gregory Ssi-Yan-Kai
 */
final class LongBufferWrapper implements Buffer {

    public LongBufferWrapper(LongBuffer buffer) {
        this.buffer = buffer;
    }

    public LongBufferWrapper(LongBuffer slice, int sizeInWords) {
    	  this.buffer = slice;
    	  this.actualSizeInWords = sizeInWords;
		}
    
    @Override
    public int sizeInWords() {
        return this.actualSizeInWords;
    }

    @Override
    public void ensureCapacity(int capacity) {
        if(capacity > buffer.capacity()) {
            throw new RuntimeException("Cannot increase buffer capacity. Current capacity: " + buffer.capacity() + ". New capacity: " + capacity);
        }
    }

    @Override
    public long getWord(int position) {
        return this.buffer.get(position);
    }

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

    @Override
    public void clear() {
        this.actualSizeInWords = 1;
        setWord(0, 0);
    }

    @Override
    public void trim() {
    }

    @Override
    public void setWord(int position, long word) {
        this.buffer.put(position, word);
    }

    @Override
    public void setLastWord(long word) {
        setWord(this.actualSizeInWords - 1, word);
    }

    @Override
    public void push_back(long word) {
        setWord(this.actualSizeInWords++, word);
    }

    @Override
    public void push_back(Buffer buffer, int start, int number) {
        for(int i = 0; i < number; ++i) {
            push_back(buffer.getWord(start + i));
        }
    }

    @Override
    public void negative_push_back(Buffer buffer, int start, int number) {
        for(int i = 0; i < number; ++i) {
            push_back(~buffer.getWord(start + i));
        }
    }

    @Override
    public void removeLastWord() {
        setWord(--this.actualSizeInWords, 0l);
    }

    @Override
    public void negateWord(int position) {
        setWord(position, ~getWord(position));
    }

    @Override
    public void andWord(int position, long mask) {
        setWord(position, getWord(position) & mask);
    }

    @Override
    public void orWord(int position, long mask) {
        setWord(position, getWord(position) | mask);
    }

    @Override
    public void andLastWord(long mask) {
        andWord(this.actualSizeInWords - 1, mask);
    }

    @Override
    public void orLastWord(long mask) {
        orWord(this.actualSizeInWords - 1, mask);
    }

    @Override
    public void expand(int position, int length) {
        for(int i = this.actualSizeInWords - position - 1; i >= 0; --i) {
            setWord(position + length + i, getWord(position + i));
        }
        this.actualSizeInWords += length;
    }

    @Override
    public void collapse(int position, int length) {
        for(int i = 0; i < this.actualSizeInWords - position - length; ++i) {
            setWord(position + i, getWord(position + length + i));
        }
        for(int i = 0; i < length; ++i) {
            removeLastWord();
        }
    }

    @Override
    public LongBufferWrapper clone() throws CloneNotSupportedException {
       return new LongBufferWrapper(this.buffer, this.actualSizeInWords);
    }

    @Override
    public void swap(final Buffer other) {
        if (other instanceof LongBufferWrapper) {// optimized version
            LongBufferWrapper o = (LongBufferWrapper) other;
            LongBuffer tmp = this.buffer;
            int tmp2 = this.actualSizeInWords;
            this.actualSizeInWords = o.actualSizeInWords;
            this.buffer = o.buffer;
            o.actualSizeInWords = tmp2;
            o.buffer = tmp;
        } else {
            other.swap(this);
        }
    }
    
    /**
     * The actual size in words.
     */
    private int actualSizeInWords = 1;
    
    /**
     * The buffer
     */
    private LongBuffer buffer;
    
}