randomcache.java

OR Mapping工具

package org.ephman.utils;

import java.util.*;

/** A class to cache any set of objects

 * @version 1.0
 * @author Paul M. Bethe
 */

public class RandomCache {
	
	protected HashMap hm;
	
	private Vector els;
	
	private int maxElements;
	
	private int numElements;
	
	private Random randomGen;
	
	private final int MINIMUM_SIZE = 17;

	/** constructs a cache of default elements (17)
	 */
	public RandomCache () {
		this (0);
	}
	
	
	/**
	 *  a routine to flush out the cache
	 */
	public synchronized void flush () {
		numElements = 0;
    	els = new Vector (maxElements);
    	hm = new HashMap (maxElements);
    }
	
	/** if cacheSize is greater than 17, initializes the cache with size =
	 * the first prime >= cacheSize
	 * @param cacheSize the desired cache size
	 */
	
	public RandomCache (int cacheSize) {
		if (cacheSize < MINIMUM_SIZE)
			maxElements = MINIMUM_SIZE;
		else
			maxElements = Prime.primeAfter(cacheSize);
		numElements = 0;
		els = new Vector (maxElements);
		hm = new HashMap (maxElements);
		randomGen = new Random ();
	}
	
	/** Determine the number of elements in the cache never > than maxElements
	 * @return the number of elements currently in the cache
	 */
	public int getSize () { return numElements; }
	
	/** Unsynchronized find of an Object which was hashed on key
	 * @param key the Object whose hashcode will be used in lookup
	 * @return the Object last hashed on that code or null if none is found
	 */
	public Object get (Object key) {
		return hm.get (key);
	}
	
	/** A synchronized placement of an element into the cache - to guarantee
	 * that no more than 1 copy of an item is cached
	 * @param key the Object whose hashcode should be used to store <b>obj</b>
	 * @param obj the Object to store in cache
	 */
	public synchronized void putForceUpdate (Object key, Object obj)
		// pre:  object to be added to cache - may or may not be in already
		// post: object in Vector and index in HashMap
	{
		Object o = hm.remove (key);
		if (o == null)
			put (key, obj);
		else if (obj != null) {
			hm.put(key, obj);
		} /* not adding null by decision */
	}
	
	/** A synchronized placement of an element into the cache - to guarantee
	 * that no more than 1 copy of an item is cached
	 * @param key the Object whose hashcode should be used to store <b>obj</b>
	 * @param obj the Object to store in cache
	 */
	public synchronized void put (Object key, Object obj)
		// pre:  object to be added to cache - may or may not be in already
		// post: object in Vector and index in HashMap
	{
		Object o = this.get (key);
		if (o == null && obj != null) {
			if (numElements < maxElements) {
				els.add (numElements, key);
				hm.put (key, obj);
				numElements++;
			}
			else { /* need to free space for new element */
				int newLoc = randomGen.nextInt (maxElements);
				Object byeKey = els.elementAt(newLoc);
				hm.remove (byeKey);
				// old obj removed now add new one
				els.add (newLoc, key);
				hm.put (key,obj);
			}
		}
	}
	
	
}