基于HashMap和双向链表的LRU缓存

public class LRU implements Iterable {

    private Node head;
    private Node tail;
    private HashMap map;
    private int maxSize;

    private class Node {

        Node pre;
        Node next;
        K k;
        V v;

        public Node(K k, V v) {
            this.k = k;
            this.v = v;
        }
    }


    public LRU(int maxSize) {

        this.maxSize = maxSize;
        this.map = new HashMap<>(maxSize * 4 / 3);
		
        head = new Node(null, null);
        tail = new Node(null, null);

        head.next = tail;
        tail.pre = head;
    }


    public V get(K key) {

        if (!map.containsKey(key)) {
            return null;
        }
		//
        Node node = map.get(key);
        //将结点从链表中断除
        unlink(node);
        //将结点添加至链表首部
        appendHead(node);

        return node.v;
    }


    public void put(K key, V value) {

        if (map.containsKey(key)) {
            Node node = map.get(key);
            unlink(node);
        }

        Node node = new Node(key, value);
        map.put(key, node);
        appendHead(node);
        
		//超过约定缓存大小时将尾部键值对删除
        if (map.size() > maxSize) {
            Node toRemove = removeTail();
            map.remove(toRemove.k);
        }
    }

	//结点的断除操作
    private void unlink(Node node) {

        Node pre = node.pre;
        Node next = node.next;

        pre.next = next;
        next.pre = pre;

        node.pre = null;
        node.next = null;
    }

	//添加至链表首部
    private void appendHead(Node node) {
        Node next = head.next;
        node.next = next;
        next.pre = node;
        node.pre = head;
        head.next = node;
    }

	//最不常用的键值对从尾部删除
    private Node removeTail() {

        Node node = tail.pre;

        Node pre = node.pre;
        tail.pre = pre;
        pre.next = tail;

        node.pre = null;
        node.next = null;

        return node;
    }


    @Override
    public Iterator iterator() {

        return new Iterator() {
            private Node cur = head.next;

            @Override
            public boolean hasNext() {
                return cur != tail;
            }

            @Override
            public K next() {
                Node node = cur;
                cur = cur.next;
                return node.k;
            }
        };
    }
}

该LRU工具添加、查找缓存因为使用了HashMap，时间复杂度为O(1)。