import java.util.concurrent.ConcurrentHashMap;public class LRUCache {    private ConcurrentHashMap
    
      hashMap = new ConcurrentHashMap<>();    private int capacity;//容量    private LRUNode head;//头    private LRUNode tail;//尾    /**     * 设置缓存     *     * @param key   键     * @param value 值     */    public void set(String key, Object value) {        LRUNode node = hashMap.get(key);        if (node != null) {            node.value = value;            remove(node, false);        } else {            node = new LRUNode(key, value);            if (hashMap.size() >= capacity) {                // 每次容量不足时先删除最久未使用的元素                remove(tail, true);            }            hashMap.put(key, node);        }        // 将刚添加的元素设置为head        setHead(node);    }    /**     * 取缓存     *     * @param key 键     * @return 值     */    public Object get(String key) {        LRUNode node = hashMap.get(key);        if (node != null) {            // 将刚操作的元素放到head            remove(node, false);            setHead(node);            return node.value;        }        return null;    }    private void setHead(LRUNode node) {        // 先从链表中删除该元素        if (head != null) {            node.next = head;            head.prev = node;        }        head = node;        if (tail == null) {            tail = node;        }    }    // 从链表中删除此Node，此时要注意该Node是head或者是tail的情形    private void remove(LRUNode node, boolean flag) {        if (node.prev != null) {            node.prev.next = node.next;        } else {            head = node.next;        }        if (node.next != null) {            node.next.prev = node.prev;        } else {            tail = node.prev;        }        node.next = null;        node.prev = null;        if (flag) {            hashMap.remove(node.key);        }    }    public LRUCache(int capacity) {        this.capacity = capacity;    }    private class LRUNode {        String key;        Object value;        LRUNode prev;        LRUNode next;        private LRUNode(String key, Object value) {            this.key = key;            this.value = value;        }    }}复制代码