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A Java Optimization Framework
There is a newer version: 4.6.0
package soot.util;

/*-
 * #%L
 * Soot - a J*va Optimization Framework
 * %%
 * Copyright (C) 2002 Ondrej Lhotak
 * %%
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation, either version 2.1 of the
 * License, or (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Lesser Public License for more details.
 * 
 * You should have received a copy of the GNU General Lesser Public
 * License along with this program.  If not, see
 * .
 * #L%
 */

import java.util.Iterator;

/**
 * A java.util.Map-like map with Numberable objects as the keys.
 *
 * @author Ondrej Lhotak
 */

public final class SmallNumberedMap implements INumberedMap {

  public SmallNumberedMap() {
    //
  }

  @Override
  public boolean put(Numberable key, T value) {
    int pos = findPosition(key);
    if (array[pos] == key) {
      if (values[pos] == value) {
        return false;
      }
      values[pos] = value;
      return true;
    }
    size++;
    if (size * 3 > array.length * 2) {
      doubleSize();
      pos = findPosition(key);
    }
    array[pos] = key;
    values[pos] = value;
    return true;
  }

  @Override
  public T get(Numberable key) {
    return (T) values[findPosition(key)];
  }

  @Override
  public void remove(Numberable key) {
    int pos = findPosition(key);
    if (array[pos] == key) {
      array[pos] = null;
      values[pos] = null;
      size--;
    }
  }

  /** Returns the number of non-null values in this map. */
  public int nonNullSize() {
    int ret = 0;
    for (Object element : values) {
      if (element != null) {
        ret++;
      }
    }
    return ret;
  }

  @Override
  public Iterator keyIterator() {
    return new KeyIterator(this);
  }

  /** Returns an iterator over the non-null values. */
  public Iterator iterator() {
    return new ValueIterator(this);
  }

  abstract class SmallNumberedMapIterator implements Iterator {
    SmallNumberedMap map;
    int cur = 0;

    SmallNumberedMapIterator(SmallNumberedMap map) {
      this.map = map;
      seekNext();
    }

    protected final void seekNext() {
      try {
        while (map.values[cur] == null) {
          cur++;
        }
      } catch (ArrayIndexOutOfBoundsException e) {
        cur = -1;
      }
    }

    public final boolean hasNext() {
      return cur != -1;
    }

    public abstract C next();
  }

  class KeyIterator extends SmallNumberedMapIterator {
    KeyIterator(SmallNumberedMap map) {
      super(map);
    }

    public final Numberable next() {
      Numberable ret = array[cur];
      cur++;
      seekNext();
      return ret;
    }

    @Override
    public void remove() {
      array[cur - 1] = null;
      values[cur - 1] = null;
    }
  }

  class ValueIterator extends SmallNumberedMapIterator {
    ValueIterator(SmallNumberedMap map) {
      super(map);
    }

    public final T next() {
      Object ret = values[cur];
      cur++;
      seekNext();
      return (T) ret;
    }

    @Override
    public void remove() {
      array[cur - 1] = null;
      values[cur - 1] = null;
    }
  }

  /* Private stuff. */

  private final int findPosition(Numberable o) {
    int number = o.getNumber();
    if (number == 0) {
      throw new RuntimeException("unnumbered");
    }
    number = number & (array.length - 1);
    while (true) {
      if (array[number] == o) {
        return number;
      }
      if (array[number] == null) {
        return number;
      }
      number = (number + 1) & (array.length - 1);
    }
  }

  private final void doubleSize() {
    Numberable[] oldArray = array;
    Object[] oldValues = values;
    int newLength = array.length * 2;
    values = new Object[newLength];
    array = new Numberable[newLength];
    for (int i = 0; i < oldArray.length; i++) {
      Numberable element = oldArray[i];
      if (element != null) {
        int pos = findPosition(element);
        array[pos] = element;
        values[pos] = oldValues[i];
      }
    }
  }

  private Numberable[] array = new Numberable[8];
  private Object[] values = new Object[8];
  private int size = 0;

}