DualCollection.java Source code

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Here is the source code for DualCollection.java

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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();
        }
    }
}