本文章环境为java8
使用
public class Main {
public static void main(String[] args) {
LinkedHashMap<String,Integer> map = new LinkedHashMap<String,Integer>(15,1,true);
for(int i=0;i<10;i++){
map.put("key"+i, i);
}
map.get("key3");
for(String value:map.keySet()){
System.out.println(value);
}
}
}
原理
accessOrder用来设置是否访问有限，遍历顺序是访问顺序。默认是false，遍历顺序是插入顺序。
public class LinkedHashMap<K,V>
extends HashMap<K,V>
implements Map<K,V>
{
/**
* The iteration ordering method for this linked hash map: <tt>true</tt>
* for access-order, <tt>false</tt> for insertion-order.
*
* @serial
*/
final boolean accessOrder;
}
public V get(Object key) {
Node<K,V> e;
if ((e = getNode(hash(key), key)) == null)
return null;
if (accessOrder)
afterNodeAccess(e);
return e.value;
}
put操作
继承于HashMap，具体如何实现不做介绍
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
afterNodeRemoval、afterNodeInsertion、afterNodeAccess
LinkedHashMap对HashMap的afterNodeRemoval、afterNodeInsertion、afterNodeAccess方法进行重写
void afterNodeRemoval(Node<K,V> e) { // unlink
LinkedHashMap.Entry<K,V> p =
(LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
p.before = p.after = null;
if (b == null)
head = a;
else
b.after = a;
if (a == null)
tail = b;
else
a.before = b;
}
void afterNodeInsertion(boolean evict) { // possibly remove eldest
LinkedHashMap.Entry<K,V> first;
if (evict && (first = head) != null && removeEldestEntry(first)) {
K key = first.key;
removeNode(hash(key), key, null, false, true);
}
}
void afterNodeAccess(Node<K,V> e) { // move node to last
LinkedHashMap.Entry<K,V> last;
if (accessOrder && (last = tail) != e) {
LinkedHashMap.Entry<K,V> p =
(LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
p.after = null;
if (b == null)
head = a;
else
b.after = a;
if (a != null)
a.before = b;
else
last = b;
if (last == null)
head = p;
else {
p.before = last;
last.after = p;
}
tail = p;
++modCount;
}
}
removeEldestEntry
通过覆盖这个方法，加入一定的条件，满足条件返回true。当put进新的值方法返回true时，便移除该map中最老的键和值。
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return false;
}
测试
public class Main {
public static void main(String[] args) {
LinkedHashMap<Integer,Integer> map = new LinkedHashMap<Integer,Integer>(16, 0.75f, true){
public boolean removeEldestEntry(java.util.Map.Entry<Integer, Integer> eldest){
if(size() > 3){
return true;
}
return false;
}
};
map.put(1, 1);
map.put(2, 2);
map.put(3, 3);
System.out.println(map.size());
map.put(4, 4);
System.out.println(map.size());
}
}
缓存的应用
lru指的是least recently use，通过LinkedHashMap可以将访问多的元素放在列表头部，尾部是一些不经常访问的元素，可以回收
基于LinkedHashMap的lru设计
作者：How_2_Play_Life
链接：https://www.jianshu.com/p/987a25f9d470
來源：简书
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。
public class LRU<K,V> extends LinkedHashMap<K, V> implements Map<K, V>{
private static final long serialVersionUID = 1L;
public LRU(int initialCapacity,
float loadFactor,
boolean accessOrder) {
super(initialCapacity, loadFactor, accessOrder);
}
/**
* @description 重写LinkedHashMap中的removeEldestEntry方法，当LRU中元素多余6个时，
* 删除最不经常使用的元素
* @author rico
* @created 2017年5月12日 上午11:32:51
* @param eldest
* @return
* @see java.util.LinkedHashMap#removeEldestEntry(java.util.Map.Entry)
*/
@Override
protected boolean removeEldestEntry(java.util.Map.Entry<K, V> eldest) {
// TODO Auto-generated method stub
if(size() > 6){
return true;
}
return false;
}
public static void main(String[] args) {
LRU<Character, Integer> lru = new LRU<Character, Integer>(
16, 0.75f, true);
String s = "abcdefghijkl";
for (int i = 0; i < s.length(); i++) {
lru.put(s.charAt(i), i);
}
System.out.println("LRU中key为h的Entry的值为： " + lru.get('h'));
System.out.println("LRU的大小 ：" + lru.size());
System.out.println("LRU ：" + lru);
}
}
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