一、类图
二、要点解读
a. 使用双向链表连接了所有 entry，使迭代有序（与插入顺序一致）
b. LinkedHashMap 支持 LRU 排序，适合用作 LRU cache（重写 removeEldestEntry）
构造函数，最后一个参数指明是否按访问顺序排序
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return false; //可以改成 size()> xxx ，size 超过多少就开始移除 LRU 最老的
}
c. 对 Set View 的操作，不影响后端的 Map iterator 迭代器
d. 遍历与 size 有关，与 Capacity 无关，这点与 HashMap 不同
e. LinkedHashMap capacity 设置较大也不影响迭代时间，因为不依赖容量
f. 最老的节点在 head，最新的在 tail
g. forEach 等这种方式迭代时，在迭代完才检查异常，ConcurrentModificationException 是迭代完才抛出的
public void forEach(BiConsumer<? super K, ? super V> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e.key, e.value);
if (modCount != mc)
throw new ConcurrentModificationException();
}
而通过 iterator 迭代时，则是在调用 next() 方法时检查和抛出 ConcurrentModificationException 异常的
iterator 的 remove 方法能够自动修正结构改变计数，因此不会抛 ConcurrentModificationException
public final void remove() {
Node<K,V> p = current;
if (p == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
current = null;
K key = p.key;
removeNode(hash(key), key, null, false, false);
expectedModCount = modCount;
}
h. 用双向链表是为了方便链表操作，比如按访问顺序排序时，很方便调整指针
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;
}
}
get 的效率和 HashMap 几乎是一样的，就是调用的 HashMap 父类的方法
i. LinkedHashMap 是如何与 HashMap 结合的，这要归功于 HashMap 保留了些钩子函数，例如
// Callbacks to allow LinkedHashMap post-actions
void afterNodeAccess(Node<K,V> p) { }
void afterNodeInsertion(boolean evict) { }
void afterNodeRemoval(Node<K,V> p) { }/*
* The following package-protected methods are designed to be
* overridden by LinkedHashMap, but not by any other subclass.
* Nearly all other internal methods are also package-protected
* but are declared final, so can be used by LinkedHashMap, view
* classes, and HashSet.
*/// Create a regular (non-tree) node
Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {
return new Node<>(hash, key, value, next);
}// For conversion from TreeNodes to plain nodes
Node<K,V> replacementNode(Node<K,V> p, Node<K,V> next) {
return new Node<>(p.hash, p.key, p.value, next);
}// Create a tree bin node
TreeNode<K,V> newTreeNode(int hash, K key, V value, Node<K,V> next) {
return new TreeNode<>(hash, key, value, next);
}// For treeifyBin
TreeNode<K,V> replacementTreeNode(Node<K,V> p, Node<K,V> next) {
return new TreeNode<>(p.hash, p.key, p.value, next);
}
LinkedHashMap 通过重写这些方法，在数据改变时，维护自己的链表结构，就 OK 了
就拿 put 操作来说
原来 HashMap 的 put 操作是这样的
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;
}Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {
return new Node<>(hash, key, value, next);
}// HashMap 默认 newNode 就只是返回了新建的节点，afterxxx 函数是空的
Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
LinkedHashMap.Entry<K,V> p =
new LinkedHashMap.Entry<K,V>(hash, key, value, e);
linkNodeLast(p);
return p;
}// LinkedHashMap 重写了该方法，嵌入了链表连接的操作 linkNodeLast(p)
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;
}
}// afterxxx 方法添加了对应的方法体内容，用于在 Hash 结构改编后 更新链表结构