作了一个有缓存功能的Map, 大小固定,采用LRU,只保留最近使用的.

import java.util.AbstractMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Map;
import java.util.Queue;
import java.util.Set;


/**
 * The PooledMap is used to cache some Objects in a certain count.
 *
 * @param <Key>
 * @param <Value>
 */
public class PooledMap<Key, Value> extends AbstractMap<Key, Value>{
	private int maxCount = 10;
	private Queue<Entry> queue = new LinkedList<Entry>();
	
	/**
	 * The Entry for this Map.
	 * @author Andy Han
	 *
	 */
	private class Entry implements Map.Entry<Key, Value>{
		private Key key;
		private Value value;
		
		public Entry(Key key, Value value){
			this.key = key;
			this.value = value;
		}

		@Override
		public String toString() {
			return key + "=" + value;
		}
		
		public Key getKey() {
			return key;
		}

		public Value getValue() {
			return value;
		}

		public Value setValue(Value value) {
			return this.value = value;
		}
	}
	
	/**
	 * Default Constructor.
	 */
	public PooledMap() {
	}
	
	/**
	 * Constructor.
	 * @param size the size of the pooled map;
	 */
	public PooledMap(int size) {
		maxCount = size;
	}

	@Override
	public Value put(Key key, Value value) {
		while(queue.size() >= maxCount){
			queue.remove();
		}
		queue.add(new Entry(key, value));
		return value;
	}
	
	@Override
	public Value get(Object key){
		for(Iterator<Entry> iter = queue.iterator();iter.hasNext();){
			Entry type = iter.next();
			if(type.key.equals(key)){
				queue.remove(type);
				queue.add(type);
				return type.value;
			}
		}
		return null;
	}
	
	@Override
	public Set<Map.Entry<Key, Value>> entrySet() {
		Set<Map.Entry<Key, Value>> set = new HashSet<Map.Entry<Key, Value>>();
		set.addAll(queue);
		return set;
	}

	@Override
	public void clear() {
		queue.clear();
	}
	
	@Override
	public Set<Key> keySet() {
		Set<Key> set = new HashSet<Key>();
		for(Entry e : queue){
			set.add(e.getKey());
		}
		return set;
	}

	@Override
	public Value remove(Object obj) {
		for(Entry e : queue){
			if(e.getKey().equals(obj)){
				queue.remove(e);
				return e.getValue();
			}
		}
		return null;
	}
	
	public static void main(String[] args) {
		PooledMap<String, String> map = new PooledMap<String, String>(3);
		map.put("1", "11111");
		map.put("2", "22222");
		map.put("3", "33333");
		System.out.println(map.size() == 3);
		System.out.println(map.values().size() == 3);
		System.out.println(map.remove("3") == "33333");
		System.out.println(map.size() == 2);
		
		System.out.println(map.get("1") == "11111");
		map.put("4", "44444");
		
		System.out.println(map.get("6") == null);
		System.out.println(map.get("4") == "44444");
		System.out.println(map.size() == 3);
		System.out.println(map.containsKey("4"));
		System.out.println(map.containsValue("44444"));
		System.out.println(map.containsKey("1"));
		System.out.println(map.containsKey("2"));
		
		System.out.println(map.isEmpty() == false);
		
		map.remove("1");
		map.remove("4");
		map.remove("2");
		System.out.println(map.isEmpty() == true);
		map.clear();
	}
}