code: /***************************************************************************
   *   Copyright (C) 2011 by Fabrizio Montesi <famontesi@gmail.com>          *
   *                                                                         *
   *   This program is free software; you can redistribute it and/or modify  *
   *   it under the terms of the GNU Library General Public License as       *
   *   published by the Free Software Foundation; either version 2 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 Public License for more details.                          *
   *                                                                         *
   *   You should have received a copy of the GNU Library General Public     *
   *   License along with this program; if not, write to the                 *
   *   Free Software Foundation, Inc.,                                       *
   *   59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.             *
   *                                                                         *
   *   For details about the authors of this software, see the AUTHORS file. *
   ***************************************************************************/
  
  package jolie.util;
  
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.Map;
  import java.util.Set;
  
  /**
   *
   * @author Fabrizio Montesi
   */
  public abstract class MultiMap< K, V >
  {
  	private final Map< K, Collection< V > > map = new HashMap<>();
  
  	protected abstract Collection< V > createCollection();
  
  	public boolean containsKey( K key )
  	{
  		return map.containsKey( key );
  	}
  
  	public Collection< V > get( K key )
  	{
  		Collection< V > r = map.get( key );
  		if ( r == null ) {
  			return Collections.emptyList();
  		}
  		return r;
  	}
  
  	public V put( K key, V value )
  	{
  		Collection< V > r = map.get( key );
  		if ( r == null ) {
  			r = createCollection();
  			map.put( key, r );
  		}
  		r.add( value );
  		return value;
  	}
  
  	public void clear()
  	{
  		map.clear();
  	}
  
  	public Set< K > keySet()
  	{
  		return map.keySet();
  	}
  }
  