基于LinkedHashMap的LRU缓存实现（FIFO亦可）

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Set;

/**
 * @author wh445306
 * @version 1.0
 * @Desciption:LRUCache 参考：https://www.cnblogs.com/lzrabbit/p/3734850.html
 * @date 2021-01-06 18:06
 */

public class LRUCache <K, V>{
    private final int MAX_CACHE_SIZE;
    private final float DEFAULT_LOAD_FACTORY = 0.75f; 

    LinkedHashMap<K, V> map;

    public LRUCache(int cacheSize) {
        MAX_CACHE_SIZE = cacheSize;
        int capacity = (int)Math.ceil(MAX_CACHE_SIZE / DEFAULT_LOAD_FACTORY) + 1;
        /*
         * HashMap负载因子默认即为0.75，是一个折衷的取值
         * 第三个参数设置为true，代表linkedlist按访问顺序排序，可作为LRU缓存
         * 第三个参数设置为false，代表按插入顺序排序，可作为FIFO缓存
         */
        map = new LinkedHashMap<K, V>(capacity, DEFAULT_LOAD_FACTORY, true) {
            @Override
            protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
                return size() > MAX_CACHE_SIZE;
            }
        };
    }

    public synchronized void put(K key, V value) {
        map.put(key, value);
    }

    public synchronized V get(K key) {
        return map.get(key);
    }

    public synchronized void remove(K key) {
        map.remove(key);
    }

    public synchronized Set<Map.Entry<K, V>> getAll() {
        return map.entrySet();
    }

    public synchronized int size() {
        return map.size();
    }

    public synchronized void clear() {
        map.clear();
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<K, V> entry : map.entrySet()) {
            sb.append(String.format("%s: %s  ", entry.getKey(), entry.getValue()));
        }
        return sb.toString();
    }

    //测试
    public static void main(String[] args) {
        LRUCache<Integer, Integer> lru = new LRUCache<>(5);
        lru.put(1, 1);
        lru.put(2, 2);
        lru.put(3, 3);
        System.out.println(lru);
        lru.get(1);
        System.out.println(lru);
        lru.put(4, 4);
        lru.put(5, 5);
        lru.put(6, 6);
        System.out.println(lru);
    }
}

效果：