java基于Hash表和双向链表简单实现LRU Cache

package lru;

import java.util.HashMap;

public class LRUCache2 {
    public final int capacity;
    class DNode{
        K key;
        V value;
        DNode next;
        DNode pre;
        public DNode(K key, V value, LRUCache2.DNode pre, LRUCache2.DNode next) {
            super();
            this.key = key;
            this.value = value;
            this.pre = pre;
            this.next = next;
        }
        
    }
    
　　//附加头结点
    DNode head=new DNode(null,null,null,null);
    DNode last=head;

    HashMap map=new HashMap();
    
    public LRUCache2(int capacity) {
        this.capacity=capacity;
    }
    
    public V get(K key) {
        DNode node=map.get(key);
        if(node!=null) {
            moveToHead(node);
        }
        return node==null?null:node.value;
    }

    public void put(K key, V value) {
        DNode node=map.get(key);
        if(node==null) {
            if(map.size()>=capacity) {
                removeLast();
            }
            node=new DNode(key, value,head,head.next);
            if(head.next!=null) 
                head.next.pre=node;
            head.next=node;
            map.put(node.key, node);
            if(last==head) last=node;
        }else {
            node.value=value;
            moveToHead(node);
        }
        
    }
    private void removeLast() {
        // TODO Auto-generated method stub
        if(last==null) {
            throw new IllegalArgumentException("cant remove before put");
        }
        map.remove(last.key);
        last.pre.next=null;
        last=last.pre;
    }

    private void moveToHead(LRUCache2.DNode node) {
        // TODO Auto-generated method stub
        if(head.next==node) return;
        if(last==node) {
            last=node.pre;
        }
        node.pre.next=node.next;
        if(node.next!=null) node.next.pre=node.pre;
        node.next=head.next;
        node.pre=head;
        head.next.pre=node;
        head.next=node;
    }
}