softcache.java

老外的在线考试

/*
 * Core - Library of useful classes that are used in many CyberDemia projects.
 * Copyright (C) 2003 CyberDemia Research and Services
 *
 * This library 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 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
 * Library General Public License for more details.
 * 
 * You should have received a copy of the GNU Library General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA  02111-1307, USA.
 *
 * See the COPYING file located in the top-level-directory of
 * the archive of this library for complete text of license.
 */
package com.cyberdemia.util;

import java.util.*;
import java.lang.ref.SoftReference;

/**
* SoftCache is a cache of key-value pairs that is held entirely
* by soft references. It is meant to be used as a memory sensitive cache,
* in which any stored value may be removed by the garbage collector
* if the memory runs low. The is no ordering or preference as to which
* object might be removed. Because values may be removed by the 
* garbage collector thread, it is quite expensive to query the size of
* the cache at any one time, because it will not be the same as the
* number of keys.
*
* @author Alexander Yap
* @version $Revision: 1.2 $ at $Date: 2004/02/18 08:35:17 $ by $Author: alexycyap $
*/
public class SoftCache
{
	public SoftCache()
	{
		m_cache = new HashMap();
	}
	
	public SoftCache(int initCapacity)
	{
		m_cache = new HashMap(initCapacity);
	}
	
	public Object put(Object key, Object val)
	{
		SoftReference ref = new SoftReference(val);
		SoftReference oldRef = (SoftReference)m_cache.put(key, ref);
		if (oldRef!=null)
		{
			return oldRef.get();
		}
		return null;
	}
	
	public Object remove(Object key)
	{
		SoftReference oldRef = (SoftReference)m_cache.remove(key);
		if (oldRef!=null)
		{
			return oldRef.get();
		}
		return null;
	}
	
	public Object get(Object key)
	{
		SoftReference ref = (SoftReference)m_cache.get(key);
		if (ref!=null)
		{
			return ref.get();
		}
		return null;
	}
	
	public void clear()
	{
		m_cache.clear();
	}
	
	private Map m_cache = null;
}