建议阅读本文前先了解HashMap，鄙人文章 HashMap解析
public class LinkedHashMap<K,V> extends HashMap<K,V>
static class Entry<K,V> extends HashMap.Node<K,V> {
Entry<K,V> before, after;
Entry(int hash, K key, V value, Node<K,V> next) {
super(hash, key, value, next);
}
}
很简单，LinkedHashMap的Entry只是比HashMap的Entry多了2个变量，before和after
先看put()
put()-->putVal()-->newNode()-->linkNodeLast()
transient LinkedHashMap.Entry<K,V> head;
transient LinkedHashMap.Entry<K,V> tail;
private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
LinkedHashMap.Entry<K,V> last = tail;
tail = p;
if (last == null)
head = p;
else {
p.before = last;
last.after = p;
}
}
put比较简单，修改last和新增node的before和atfer。
再看remove()，remove的时候肯定也要修改node前一个node的after，后一个node的before
remove()-->removeNode()-->afterNodeRemoval() //HashMap里是个空方法，LinkedHashMap实现了
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;
}
get()直接走HashMap
LinkedHashMap除了增加before和after之外，还有一个功能——accessOrder
public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder) {
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
accessOrder默认是false，如果是true，则每次put()更新旧值，或者get()的时候，都会执行方法
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;
}
}
这个方法就是把操作的node移到末尾，如此可以确定，末尾的node一定是最近操作的，而head当然是最长时间未操作的。这就是LRU算法，Android里的LruCache里面其实也只是维护了1个accessOrder=true的LinkedHashMap而已，当达到/超过限定的内存maxSize时，只要从head开始移除就可以了。
public LruCache(int maxSize) {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
this.maxSize = maxSize;
this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
}
//Android LinkedHashMap
public Entry<K, V> eldest() {
LinkedEntry<K, V> eldest = header.nxt;
return eldest != header ? eldest : null;
}