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；

 

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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；

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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；

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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

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// 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

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// 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;         }     }