LRU（末位淘汰）算法

/**
 * @Author: subd
 * @Date: 2019/8/28 8:22
 * LRU
 * 末尾淘汰算法 线程不安全
 * 手写linkedHashMap 进行书写
 */
public class LRU {
    private Node head;
    private Node end;
    //缓存上限设置
    private int limit;
    private HashMap<String, Node> hashMap;

    public LRU(int limit) {
        this.limit = limit;
        this.hashMap = new HashMap<>();
    }

    public String get(String key) {
        Node node = hashMap.get(key);
        if (node == null) {
            return null;
        }
        refreshNode(node);
        return node.value;
    }

    public void put(String key, String value) {
        Node node = hashMap.get(key);
        if (node == null) {
            //如果key不存在，则插入key—value
            if (hashMap.size() >= limit) {
                String oldKey = removeNode(head);
                hashMap.remove(oldKey);
            }
            node = new Node(key, value);
            addNode(node);
            hashMap.put(key, node);
        } else {
            //如果key存在，则刷新Key-Value
            node.value = value;
            refreshNode(node);
        }
    }

    private void remove(String key) {
        Node node = hashMap.get(key);
        removeNode(node);
        hashMap.remove(key);
    }

    /**
     * 刷新被访问节点的位置
     *
     * @param node
     * @return
     */
    private void refreshNode(Node node) {
        //如果访问的是尾节点，则无需移动节点
        if (node == end) {
            return;
        }
        //移除节点
        removeNode(node);
        //重新插入节点
        addNode(node);
    }


    /**
     * 删除节点
     *
     * @param node
     * @return
     */
    private String removeNode(Node node) {
        if (node == head && node == end) {
            //移除唯一节点
            head = null;
            end = null;
        } else if (node == end) {
            //移除尾节点
            end = end.pre;
            end.next = null;
        } else if (node == head) {
            //移除头节点
            head = head.next;
            head.pre = null;
        } else {
            //移除中间节点
            node.pre.next = node.next;
            node.next.pre = node.pre;
        }
        return node.key;
    }

    /**
     * 尾部插入节点
     *
     * @param node
     */
    private void addNode(Node node) {
        if (end != null) {
            end.next = node;
            node.pre = end;
            node.next = null;
        }
        end = node;
        if (head == null) {
            head = node;
        }
    }

    /**
     * 节点类
     */
    public class Node {
        public Node pre;
        public Node next;
        public String key;
        public String value;

        public Node(String key, String value) {
            this.key = key;
            this.value = value;
        }
    }

    public static void main(String[] args) {
        LRU lru = new LRU(5);
        lru.put("001","用户1信息");
        lru.put("002","用户2信息");
        lru.put("003","用户3信息");
        lru.put("004","用户4信息");
        lru.put("005","用户5信息");
        lru.get("002");
        lru.put("004","用户2信息更新");
        lru.put("006","用户6信息更新");
        System.out.println(lru.get("001"));
        System.out.println(lru.get("006"));
    }
}