概述
LinkedHashMap与HashMap的最大区别是：LinkedHashMap的迭代器遍历顺序与put元素的顺序是一致的。
Entry<K,V>结构
private static class Entry<K,V> extends HashMap.Entry<K,V> {
// These fields comprise the doubly linked list used for iteration.
Entry<K,V> before, after;
Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
super(hash, key, value, next);
}
/**
* Removes this entry from the linked list.
*/
private void remove() {
before.after = after;
after.before = before;
}
/**
* Inserts this entry before the specified existing entry in the list.
*/
private void addBefore(Entry<K,V> existingEntry) {
after = existingEntry;
before = existingEntry.before;
before.after = this;
after.before = this;
}
/**
* This method is invoked by the superclass whenever the value
* of a pre-existing entry is read by Map.get or modified by Map.set.
* If the enclosing Map is access-ordered, it moves the entry
* to the end of the list; otherwise, it does nothing.
*/
void recordAccess(HashMap<K,V> m) {
LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
if (lm.accessOrder) {
lm.modCount++;
remove();
addBefore(lm.header);
}
}
void recordRemoval(HashMap<K,V> m) {
remove();
}
}
从代码中可以看出，Entry<K,V>在HashMap.Entry的基础上增加了两个指针，从而构成双向链表。
存储结构如下:
linkedHashmap.png
