A combination of two collections into a collection : Collections « Collections « Java Tutorial

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/* DualCollection.java

{{IS_NOTE
  Purpose:
    
  Description:
    
  History:
    Sun Sep  2 21:29:38     2007, Created by tomyeh
}}IS_NOTE

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

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

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);
      else
        return first;
    else
      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;
  }

  //Collection//
  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() {
      _it.remove();
    }
  }
}