Map---HashMap-LinkedHashMap
概述
LinkedHashMap是HashMap的直接子类;
二者唯一的区别是LinkedHashMap在HashMap的基础上，采用双向链表(doubly-linked list)的形式将所有entry连接起来，这样是为保证元素的迭代顺序跟插入顺序相同。
上图给出了LinkedHashMap的结构图，主体部分跟HashMap完全一样，多了header指向双向链表的头部(是一个哑元)，该双向链表的迭代顺序就是entry的插入顺序。
除了可以保迭代历顺序，这种结构还有一个好处 : 迭代LinkedHashMap时不需要像HashMap那样遍历整个table，而只需要直接遍历header指向的双向链表即可，
也就是说LinkedHashMap的迭代时间就只跟entry的个数相关，而跟table的大小无关。
有两个参数可以影响LinkedHashMap的性能: 初始容量(inital capacity)和负载系数(load factor)。
初始容量指定了初始table的大小，负载系数用来指定自动扩容的临界值。
当entry的数量超过capacity*load_factor时，容器将自动扩容并重新哈希。对于插入元素较多的场景，将初始容量设大可以减少重新哈希的次数。
将对象放入到LinkedHashMap或LinkedHashSet中时，有两个方法需要特别关心: hashCode()和equals()。
hashCode()方法决定了对象会被放到哪个bucket里，当多个对象的哈希值冲突时，equals()方法决定了这些对象是否是“同一个对象”。
所以，如果要将自定义的对象放入到LinkedHashMap或LinkedHashSet中，需要@Override hashCode()和equals()方法。
Hash table and linked list implementation of the <tt>Map</tt> interface, with predictable iteration order.
This implementation differs from <tt>HashMap</tt> in that it maintains a doubly-linked list running through all of its entries.
Map接口的hash表+链表实现（有可预测的iterator顺序）；
LinkedHashMap是双向链表；
This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (<i>insertion-order</i>).
定义了iterator顺序，一般是 插入顺序；
Note that insertion order is not affected if a key is <i>re-inserted</i> into the map.
(A key <tt>k</tt> is reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to the invocation.)
A special {@link #LinkedHashMap(int,float,boolean) constructor} is provided to create a linked hash map whose order of iteration is the order in which its entries were last accessed, from least-recently accessed to most-recently (<i>access-order</i>).
提供了一个 LinkedHashMap(int,float,boolean)构造器 创建一个linked hash map
This class provides all of the optional <tt>Map</tt> operations, and permits null elements.
LinkedHashMap允许null值；
A linked hash map has two parameters that affect its performance:<i>initial capacity</i> and <i>load factor</i>.
LinkedHashMap有2个影响性能的参数： initial capacity、load factor
Note that this implementation is not synchronized.
If multiple threads access a linked hash map concurrently, and at least one of the threads modifies the map structurally, it <em>must</em> be synchronized externally.
If no such object exists, the map should be "wrapped" using the {@link Collections#synchronizedMap Collections.synchronizedMap} method.
LinkedHashMap是线程非同步的；
如果多个线程并发访问LinkedHashMap，需要在外部同步；
同步方式：Collections.synchronizedMap；
The iterators returned by the <tt>iterator</tt> method of the collections returned by all of this class's collection view methods are <em>fail-fast</em>: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own <tt>remove</tt> method, the iterator will throw a {@link ConcurrentModificationException}.
iterator方法是fail-fast：如果在iterator的同时对结构进行修改，将会抛出ConcurrentModificationException；
public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V> {
static class Entry<K,V> extends HashMap.Node<K,V> {
LinkedHashMap.Entry<K,V> before, after;
Entry(int hash, K key, V value, Node<K,V> next) {
super(hash, key, value, next);
}
}
/**
* The head (eldest) of the doubly linked list.
*/
transient LinkedHashMap.Entry<K,V> head;
/**
* The tail (youngest) of the doubly linked list.
*/
transient LinkedHashMap.Entry<K,V> tail;
// The iteration ordering method for this linked hash map: true for access-order, false for insertion-order.
// 迭代排序：true->access-order、false->insertion-order
final boolean accessOrder;
// Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance with the default initial capacity (16) and load factor (0.75).
public LinkedHashMap() {
super();
accessOrder = false;
}
public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) {
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
}
LinkedHashMap的扩展
accessOrder
构造参数accessOrder为true，get时会将该元素move到队尾；
LinkedHashMap<String, String> linkedHashMap = new LinkedHashMap<>(10, 0.75F,true);
linkedHashMap.put("a", "a");
linkedHashMap.put("b", "b");
linkedHashMap.put("c", "c");
linkedHashMap.put("d", "d");
System.out.println(linkedHashMap.toString()); // {a=a, b=b, c=c, d=d}
linkedHashMap.get("c");
System.out.println(linkedHashMap.toString()); // {a=a, b=b, d=d, c=c}
linkedHashMap.entrySet().stream().forEach(e -> System.out.print(" "+e)); // a=a b=b d=d c=c
removeEldestEntry
LinkedHashMap每次插入新的元素，都会调用removeEldestEntry方法，如果返回true，移除最老的元素；
static class FIFOCache<K, V> extends LinkedHashMap<K, V>{
private final int cacheSize;
public FIFOCache(int cacheSize){
this.cacheSize = cacheSize;
}
// 当Entry个数超过cacheSize时，删除最老的Entry
@Override
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return size() > cacheSize;
}
}
FIFOCache<String, String> fifoCache = new FIFOCache<>(4);
fifoCache.put("a", "a");
fifoCache.put("b", "b");
fifoCache.put("c", "c");
fifoCache.put("d", "d");
fifoCache.put("e", "e");
System.out.print(fifoCache); // {b=b, c=c, d=d, e=e}
链路
Put
// java.util.HashMap.put
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
// java.util.HashMap.putVal
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); // 创建新的node && link到列表尾部
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;
}
// java.util.LinkedHashMap.newNode
HashMap.Node<K,V> newNode(int hash, K key, V value, HashMap.Node<K,V> e) {
LinkedHashMap.Entry<K,V> p = new LinkedHashMap.Entry<K,V>(hash, key, value, e);
linkNodeLast(p);
return p;
}
// java.util.LinkedHashMap.linkNodeLast
private void linkNodeLast(LinkedHashMap.Entry<K,V> p) { // link到列表尾部
LinkedHashMap.Entry<K,V> last = tail;
tail = p;
if (last == null)
head = p;
else {
p.before = last;
last.after = p;
}
}
// java.util.LinkedHashMap.afterNodeInsertion
void afterNodeInsertion(boolean evict) { // possibly remove eldest
LinkedHashMap.Entry<K,V> first;
if (evict && (first = head) != null && removeEldestEntry(first)) { // removeEldestEntry每次Put都要看下是否需要移除最老的元素
K key = first.key;
removeNode(hash(key), key, null, false, true);
}
}
get
// java.util.LinkedHashMap.get
public V get(Object key) {
HashMap.Node<K,V> e;
if ((e = getNode(hash(key), key)) == null)
return null;
if (accessOrder)
afterNodeAccess(e); // 如果accessOrder=true -> 将访问的元素插入尾部
return e.value;
}
// java.util.LinkedHashMap.afterNodeAccess
void afterNodeAccess(HashMap.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;
}
}
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