A combination of two collections into a collection : Collection « Collections Data Structure « Java

A combination of two collections into a collection


/* DualCollection.java

    Sun Sep  2 21:29:38     2007, Created by tomyeh

Copyright (C) 2007 Potix Corporation. All Rights Reserved.

  This program is distributed under GPL Version 3.0 in the hope that
  it will be useful, but WITHOUT ANY WARRANTY.

import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.AbstractCollection;

 * A combination of two collections into a collection.
 * @author tomyeh
 * @since 3.0.0
public class DualCollection extends AbstractCollection
implements java.io.Serializable {
  private final Collection _first, _second;

  /** Returns a collection by combining two collections.
   * It checks whether any of them is null, or equals. And, returns
   * the non-null one if another is null.
   * If both null, it returns null.
  public static final
  Collection combine(Collection first, Collection second) {
    if (first == second) //we don't use equals to have better performance
      return first;

    if (first != null)
      if (second != null)
        return new DualCollection(first, second);
        return first;
      return second;
  /** Constructor.
   * It is better to use {@link #combine} instead of this method
   * since it checks whether any of them is null or equals.
  public DualCollection(Collection first, Collection second) {
    _first = first != null ? first: Collections.EMPTY_LIST;
    _second = second != null ? second: Collections.EMPTY_LIST;

  public int size() {
    return _first.size() + _second.size();
  public Iterator iterator() {
    return new Iter();
  private class Iter implements Iterator {
    private Iterator _it;
    private boolean _bSecond;

    private Iter() {
      _it = _first.iterator();

    public boolean hasNext() {
      return _it.hasNext() || (!_bSecond && !_second.isEmpty());
    public Object next() {
      if (!_bSecond && !_it.hasNext()) {
        _it = _second.iterator();
        _bSecond = true;
      return _it.next();
    public void remove() {


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