注意
想法及时记录，实现可以待做。
Map
Map常用有哪些呢？如图所示：
简介
LinkedHashMap 继承自 HashMap，在 HashMap 基础上，通过维护一条双向链表，解决了 HashMap 不能随时保持遍历顺序和插入顺序一致的问题。
如果不熟悉LinkedHashMap，可以看Java8之LinkedHashMap介绍
分析
本文主要介绍如何依赖LinkedHashMap实现LRU缓存。
LRU，也即Least Recently Used. 最近最少使用策略的缓存，可以动态删除本地老的数据，保留最新最活跃的热点数据。
实现
// LinkedHashMap提供的protected方法，根据实际场景，重写这个方法即可
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return false;
}
示例
public class LRUCache<K, V> extends LinkedHashMap<K, V> {
private final int maxEntries;
private static final int DEFAULT_INITIAL_CAPACITY = 16;
private static final float DEFAULT_LOAD_FACTOR = 0.75f;
public LRUCache(int initialCapacity,
float loadFactor,
int maxEntries) {
super(initialCapacity, loadFactor, true);
this.maxEntries = maxEntries;
}
public LRUCache(int initialCapacity,
int maxEntries) {
this(initialCapacity, DEFAULT_LOAD_FACTOR, maxEntries);
}
public LRUCache(int maxEntries) {
this(DEFAULT_INITIAL_CAPACITY, maxEntries);
}
// not very useful constructor
public LRUCache(Map<? extends K, ? extends V> m,
int maxEntries) {
this(m.size(), maxEntries);
putAll(m);
}
protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
return size() > maxEntries;
}
public static void main(String... args) {
Map<Integer, String> cache = new LRUCache<>(5);
for (int i = 0; i < 10; i++) {
cache.put(i, "hi");
}
// entries 0-4 have already been removed
// entries 5-9 are ordered
System.out.println("cache = " + cache);
System.out.println(cache.get(7));
// entry 7 has moved to the end
System.out.println("cache = " + cache);
for (int i = 10; i < 14; i++) {
cache.put(i, "hi");
}
// entries 5,6,8,9 have been removed (eldest entries)
// entry 7 is at the beginning now
System.out.println("cache = " + cache);
cache.put(42, "meaning of life");
// entry 7 is gone too
System.out.println("cache = " + cache);
}
// 输出为
cache = {5=hi, 6=hi, 7=hi, 8=hi, 9=hi}
hi
cache = {5=hi, 6=hi, 8=hi, 9=hi, 7=hi}
cache = {7=hi, 10=hi, 11=hi, 12=hi, 13=hi}
cache = {10=hi, 11=hi, 12=hi, 13=hi, 42=meaning of life}
}
