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1. LinkedHashMap概述：
LinkedHashMap是HashMap的一个子类，它保留插入的顺序，如果需要输出的顺序和输入时的相同，那么就选用LinkedHashMap。
LinkedHashMap是Map接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现提供所有可选的映射操作，并允许使用null值和null键。此类不保证映射的顺序，特别是它不保证该顺序恒久不变。
LinkedHashMap实现与HashMap的不同之处在于，后者维护着一个运行于所有条目的双重链接列表。此链接列表定义了迭代顺序，该迭代顺序可以是插入顺序或者是访问顺序。
注意，此实现不是同步的。如果多个线程同时访问链接的哈希映射，而其中至少一个线程从结构上修改了该映射，则它必须保持外部同步。
根据链表中元素的顺序可以分为：按插入顺序的链表，和按访问顺序(调用get方法)的链表。
默认是按插入顺序排序，如果指定按访问顺序排序，那么调用get方法后，会将这次访问的元素移至链表尾部，不断访问可以形成按访问顺序排序的链表。 可以重写removeEldestEntry方法返回true值指定插入元素时移除最老的元素。
2.LinkedHashMap接口
public class LinkedHashMap<K,V>extends HashMap<K,V>implements Map<K,V>
{LinedHashMap接口的成员变量
/*** The head of the doubly linked list.*/private transient Entry<K,V> header;/*** The iteration ordering method for this linked hash map: <tt>true</tt>* for access-order, <tt>false</tt> for insertion-order.** @serial*/private final boolean accessOrder;accessOrder ：true表示按照访问顺序迭代，false时表示按照插入顺序
LinkedHashMap的Entry
/*** LinkedHashMap entry.*/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);}
static class Entry<K,V> implements Map.Entry<K,V> { final K key; V value; Entry<K,V> next; final int hash; Entry(int h, K k, V v, Entry<K,V> n) { value = v; next = n; key = k; hash = h; }
}
LinkedHashMap并未重写父类HashMap的put方法，而是重写了父类HashMap的put方法调用的子方法void recordAccess(HashMap m) ，void addEntry(int hash, K key, V value, int bucketIndex) 和void createEntry(int hash, K key, V value, int bucketIndex)，提供了自己特有的双向链接列表的实现
LinkedHashMap的存储元素的方法
/*** 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);}}
/*** This override differs from addEntry in that it doesn't resize the* table or remove the eldest entry.*/void createEntry(int hash, K key, V value, int bucketIndex) {HashMap.Entry<K,V> old = table[bucketIndex];Entry<K,V> e = new Entry<>(hash, key, value, old);table[bucketIndex] = e;e.addBefore(header);size++;}对比HashMap的存储方法
public V put(K key, V value) { if (key == null) return putForNullKey(value); int hash = hash(key.hashCode()); int i = indexFor(hash, table.length); for (Entry<K,V> e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(hash, key, value, i); return null; } package com.my.collection;import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;public class MyLinkedHashMap {private static Map<String, Object> map = null;public static void initLinkedHashMap() {System.out.println("初始化Hashmap.......");// LinkedHashMap保存了记录的插入顺序，在用Iterator遍历LinkedHashMap时，//先得到的记录肯定是先插入的.在遍历// 的时候会比HashMap慢。map = new LinkedHashMap<>();map.put("1", "a");map.put("2", "b");// 不允许键重复,允许值重复,如果键重复,最后的key的值将会覆盖第一个key的值map.put("3", "c");map.put("4", "d");}/*** 遍历key和values*/public static void outEelem() {System.out.println("#####遍历key和values######");// 遍历方式一System.out.println("#####map.keySet()遍历key和values######");for (String key : map.keySet()) {System.out.println("key" + key + "and value" + map.get(key));}// 遍历方式二：Iterator<Map.Entry<String, Object>> it = map.entrySet().iterator();System.out.println("#####通过Map.Entry<String, Object>使用Iterator遍历key和values######");while (it.hasNext()) {Map.Entry<String, Object> entry = it.next();System.out.println("key= " + entry.getKey() + " and value= "+ entry.getValue());}// 遍历方式三：System.out.println("##### 通过Map.Entry<String, Object> entry 遍历key和values######");for (Map.Entry<String, Object> entry : map.entrySet()) {System.out.println("key= " + entry.getKey() + " and value= "+ entry.getValue());}// 遍历方式四：System.out.println("#####通过Map.values()遍历所有的value，但不能遍历key######");for (Object value : map.values()) {System.out.println("Values " + value);}}//true表示按照访问顺序迭代，false时表示按照插入顺序 public static void main(String[] args) {initLinkedHashMap();outEelem();}
}
最后附上LinkedHashMap的源码：
/** Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.*********************/package java.util;
import java.io.*;/*** <p>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. This linked list defines the iteration ordering,* which is normally the order in which keys were inserted into the map* (<i>insertion-order</i>). 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.)** <p>This implementation spares its clients from the unspecified, generally* chaotic ordering provided by {@link HashMap} (and {@link Hashtable}),* without incurring the increased cost associated with {@link TreeMap}. It* can be used to produce a copy of a map that has the same order as the* original, regardless of the original map's implementation:* <pre>* void foo(Map m) {* Map copy = new LinkedHashMap(m);* ...* }* </pre>* This technique is particularly useful if a module takes a map on input,* copies it, and later returns results whose order is determined by that of* the copy. (Clients generally appreciate having things returned in the same* order they were presented.)** <p>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>). This kind of map is well-suited to* building LRU caches. Invoking the <tt>put</tt> or <tt>get</tt> method* results in an access to the corresponding entry (assuming it exists after* the invocation completes). The <tt>putAll</tt> method generates one entry* access for each mapping in the specified map, in the order that key-value* mappings are provided by the specified map's entry set iterator. <i>No* other methods generate entry accesses.</i> In particular, operations on* collection-views do <i>not</i> affect the order of iteration of the backing* map.** <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to* impose a policy for removing stale mappings automatically when new mappings* are added to the map.** <p>This class provides all of the optional <tt>Map</tt> operations, and* permits null elements. Like <tt>HashMap</tt>, it provides constant-time* performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and* <tt>remove</tt>), assuming the hash function disperses elements* properly among the buckets. Performance is likely to be just slightly* below that of <tt>HashMap</tt>, due to the added expense of maintaining the* linked list, with one exception: Iteration over the collection-views* of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i>* of the map, regardless of its capacity. Iteration over a <tt>HashMap</tt>* is likely to be more expensive, requiring time proportional to its* <i>capacity</i>.** <p>A linked hash map has two parameters that affect its performance:* <i>initial capacity</i> and <i>load factor</i>. They are defined precisely* as for <tt>HashMap</tt>. Note, however, that the penalty for choosing an* excessively high value for initial capacity is less severe for this class* than for <tt>HashMap</tt>, as iteration times for this class are unaffected* by capacity.** <p><strong>Note that this implementation is not synchronized.</strong>* 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. This is typically accomplished by* synchronizing on some object that naturally encapsulates the map.** If no such object exists, the map should be "wrapped" using the* {@link Collections#synchronizedMap Collections.synchronizedMap}* method. This is best done at creation time, to prevent accidental* unsynchronized access to the map:<pre>* Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre>** A structural modification is any operation that adds or deletes one or more* mappings or, in the case of access-ordered linked hash maps, affects* iteration order. In insertion-ordered linked hash maps, merely changing* the value associated with a key that is already contained in the map is not* a structural modification. <strong>In access-ordered linked hash maps,* merely querying the map with <tt>get</tt> is a structural* modification.</strong>)** <p>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}. Thus, in the face of concurrent* modification, the iterator fails quickly and cleanly, rather than risking* arbitrary, non-deterministic behavior at an undetermined time in the future.** <p>Note that the fail-fast behavior of an iterator cannot be guaranteed* as it is, generally speaking, impossible to make any hard guarantees in the* presence of unsynchronized concurrent modification. Fail-fast iterators* throw <tt>ConcurrentModificationException</tt> on a best-effort basis.* Therefore, it would be wrong to write a program that depended on this* exception for its correctness: <i>the fail-fast behavior of iterators* should be used only to detect bugs.</i>** <p>This class is a member of the* <a href="{@docRoot}/../technotes/guides/collections/index.html">* Java Collections Framework</a>.** @param <K> the type of keys maintained by this map* @param <V> the type of mapped values** @author Josh Bloch* @see Object#hashCode()* @see Collection* @see Map* @see HashMap* @see TreeMap* @see Hashtable* @since 1.4*/public class LinkedHashMap<K,V>extends HashMap<K,V>implements Map<K,V>
{private static final long serialVersionUID = 3801124242820219131L;/*** The head of the doubly linked list.*/private transient Entry<K,V> header;/*** The iteration ordering method for this linked hash map: <tt>true</tt>* for access-order, <tt>false</tt> for insertion-order.** @serial*/private final boolean accessOrder;/*** Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance* with the specified initial capacity and load factor.** @param initialCapacity the initial capacity* @param loadFactor the load factor* @throws IllegalArgumentException if the initial capacity is negative* or the load factor is nonpositive*/public LinkedHashMap(int initialCapacity, float loadFactor) {super(initialCapacity, loadFactor);accessOrder = false;}/*** Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance* with the specified initial capacity and a default load factor (0.75).** @param initialCapacity the initial capacity* @throws IllegalArgumentException if the initial capacity is negative*/public LinkedHashMap(int initialCapacity) {super(initialCapacity);accessOrder = false;}/*** 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;}/*** Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with* the same mappings as the specified map. The <tt>LinkedHashMap</tt>* instance is created with a default load factor (0.75) and an initial* capacity sufficient to hold the mappings in the specified map.** @param m the map whose mappings are to be placed in this map* @throws NullPointerException if the specified map is null*/public LinkedHashMap(Map<? extends K, ? extends V> m) {super(m);accessOrder = false;}/*** Constructs an empty <tt>LinkedHashMap</tt> instance with the* specified initial capacity, load factor and ordering mode.** @param initialCapacity the initial capacity* @param loadFactor the load factor* @param accessOrder the ordering mode - <tt>true</tt> for* access-order, <tt>false</tt> for insertion-order* @throws IllegalArgumentException if the initial capacity is negative* or the load factor is nonpositive*/public LinkedHashMap(int initialCapacity,float loadFactor,boolean accessOrder) {super(initialCapacity, loadFactor);this.accessOrder = accessOrder;}/*** Called by superclass constructors and pseudoconstructors (clone,* readObject) before any entries are inserted into the map. Initializes* the chain.*/void init() {header = new Entry<>(-1, null, null, null);header.before = header.after = header;}/*** Transfers all entries to new table array. This method is called* by superclass resize. It is overridden for performance, as it is* faster to iterate using our linked list.*/void transfer(HashMap.Entry[] newTable) {int newCapacity = newTable.length;for (Entry<K,V> e = header.after; e != header; e = e.after) {int index = indexFor(e.hash, newCapacity);e.next = newTable[index];newTable[index] = e;}}/*** Returns <tt>true</tt> if this map maps one or more keys to the* specified value.** @param value value whose presence in this map is to be tested* @return <tt>true</tt> if this map maps one or more keys to the* specified value*/public boolean containsValue(Object value) {// Overridden to take advantage of faster iteratorif (value==null) {for (Entry e = header.after; e != header; e = e.after)if (e.value==null)return true;} else {for (Entry e = header.after; e != header; e = e.after)if (value.equals(e.value))return true;}return false;}/*** Returns the value to which the specified key is mapped,* or {@code null} if this map contains no mapping for the key.** <p>More formally, if this map contains a mapping from a key* {@code k} to a value {@code v} such that {@code (key==null ? k==null :* key.equals(k))}, then this method returns {@code v}; otherwise* it returns {@code null}. (There can be at most one such mapping.)** <p>A return value of {@code null} does not <i>necessarily</i>* indicate that the map contains no mapping for the key; it's also* possible that the map explicitly maps the key to {@code null}.* The {@link #containsKey containsKey} operation may be used to* distinguish these two cases.*/public V get(Object key) {Entry<K,V> e = (Entry<K,V>)getEntry(key);if (e == null)return null;e.recordAccess(this);return e.value;}/*** Removes all of the mappings from this map.* The map will be empty after this call returns.*/public void clear() {super.clear();header.before = header.after = header;}/*** LinkedHashMap entry.*/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();}}private abstract class LinkedHashIterator<T> implements Iterator<T> {Entry<K,V> nextEntry = header.after;Entry<K,V> lastReturned = null;/*** The modCount value that the iterator believes that the backing* List should have. If this expectation is violated, the iterator* has detected concurrent modification.*/int expectedModCount = modCount;public boolean hasNext() {return nextEntry != header;}public void remove() {if (lastReturned == null)throw new IllegalStateException();if (modCount != expectedModCount)throw new ConcurrentModificationException();LinkedHashMap.this.remove(lastReturned.key);lastReturned = null;expectedModCount = modCount;}Entry<K,V> nextEntry() {if (modCount != expectedModCount)throw new ConcurrentModificationException();if (nextEntry == header)throw new NoSuchElementException();Entry<K,V> e = lastReturned = nextEntry;nextEntry = e.after;return e;}}private class KeyIterator extends LinkedHashIterator<K> {public K next() { return nextEntry().getKey(); }}private class ValueIterator extends LinkedHashIterator<V> {public V next() { return nextEntry().value; }}private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {public Map.Entry<K,V> next() { return nextEntry(); }}// These Overrides alter the behavior of superclass view iterator() methodsIterator<K> newKeyIterator() { return new KeyIterator(); }Iterator<V> newValueIterator() { return new ValueIterator(); }Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); }/*** This override alters behavior of superclass put method. It causes newly* allocated entry to get inserted at the end of the linked list and* removes the eldest entry if appropriate.*/void addEntry(int hash, K key, V value, int bucketIndex) {createEntry(hash, key, value, bucketIndex);// Remove eldest entry if instructed, else grow capacity if appropriateEntry<K,V> eldest = header.after;if (removeEldestEntry(eldest)) {removeEntryForKey(eldest.key);} else {if (size >= threshold)resize(2 * table.length);}}/*** This override differs from addEntry in that it doesn't resize the* table or remove the eldest entry.*/void createEntry(int hash, K key, V value, int bucketIndex) {HashMap.Entry<K,V> old = table[bucketIndex];Entry<K,V> e = new Entry<>(hash, key, value, old);table[bucketIndex] = e;e.addBefore(header);size++;}/*** Returns <tt>true</tt> if this map should remove its eldest entry.* This method is invoked by <tt>put</tt> and <tt>putAll</tt> after* inserting a new entry into the map. It provides the implementor* with the opportunity to remove the eldest entry each time a new one* is added. This is useful if the map represents a cache: it allows* the map to reduce memory consumption by deleting stale entries.** <p>Sample use: this override will allow the map to grow up to 100* entries and then delete the eldest entry each time a new entry is* added, maintaining a steady state of 100 entries.* <pre>* private static final int MAX_ENTRIES = 100;** protected boolean removeEldestEntry(Map.Entry eldest) {* return size() > MAX_ENTRIES;* }* </pre>** <p>This method typically does not modify the map in any way,* instead allowing the map to modify itself as directed by its* return value. It <i>is</i> permitted for this method to modify* the map directly, but if it does so, it <i>must</i> return* <tt>false</tt> (indicating that the map should not attempt any* further modification). The effects of returning <tt>true</tt>* after modifying the map from within this method are unspecified.** <p>This implementation merely returns <tt>false</tt> (so that this* map acts like a normal map - the eldest element is never removed).** @param eldest The least recently inserted entry in the map, or if* this is an access-ordered map, the least recently accessed* entry. This is the entry that will be removed it this* method returns <tt>true</tt>. If the map was empty prior* to the <tt>put</tt> or <tt>putAll</tt> invocation resulting* in this invocation, this will be the entry that was just* inserted; in other words, if the map contains a single* entry, the eldest entry is also the newest.* @return <tt>true</tt> if the eldest entry should be removed* from the map; <tt>false</tt> if it should be retained.*/protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {return false;}
}
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