Iterator Union of Iterators : Iterator « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. Iterator

/*
    GNU LESSER GENERAL PUBLIC LICENSE
    Copyright (C) 2006 The Lobo Project

    This library 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 library 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
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

    Contact info: lobochief@users.sourceforge.net
*/
/*
 * Created on Jun 9, 2005
 */

import java.util.*;

/**
 * @author J. H. S.
 */
 class CollectionUtilities {
  /**
   * 
   */
  private CollectionUtilities() {
    super();
  }
  
  public static Enumeration getIteratorEnumeration(final Iterator i) {
    return new Enumeration() {
      public boolean hasMoreElements() {
        return i.hasNext();
      }
      
      public Object nextElement() {
        return i.next();
      }
    };
  }

  public static Iterator iteratorUnion(final Iterator[] iterators) {
    return new Iterator() {
      private int iteratorIndex = 0;
      private Iterator current = iterators.length > 0 ? iterators[0] : null; 
      
      public boolean hasNext() {  
        for(;;) {
          if(current == null) {
            return false;
          }
          if(current.hasNext()) {
            return true;
          }
          iteratorIndex++;
          current = iteratorIndex >= iterators.length ? null : iterators[iteratorIndex];
        }       
      }
      
      public Object next() {
        for(;;) {
          if(this.current == null) {
            throw new NoSuchElementException();
          }
          try {
            return this.current.next();
          } catch(NoSuchElementException nse) {
            this.iteratorIndex++;
            this.current = this.iteratorIndex >= iterators.length ? null : iterators[this.iteratorIndex];           
          }
        }
      }
      
      public void remove() {
        if(this.current == null) {
          throw new NoSuchElementException();
        }
        this.current.remove();
      }
    };
  }
  
  public static Collection reverse(Collection collection) {
    LinkedList newCollection = new LinkedList();
    Iterator i = collection.iterator();
    while(i.hasNext()) {
      newCollection.addFirst(i.next());
    }
    return newCollection;
  }
  
  public static Iterator singletonIterator(final Object item) {
    return new Iterator() {
      private boolean gotItem = false;
      
      public boolean hasNext() {
        return !this.gotItem;
      }

      public Object next() {
        if(this.gotItem) {
          throw new NoSuchElementException();
        }
        this.gotItem = true;
        return item;
      }

      public void remove() {
        if(!this.gotItem) {
          this.gotItem = true;
        }
        else {
          throw new NoSuchElementException();
        }
      }
    };
  }
}