java Map集合框架之LinkedHashMap

港控/mmm° 2022-08-20 06:14 290阅读 0赞

**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>
 {

LinkedHashMap采用的hash算法和HashMap相同，但是它重新定义了数组中保存的元素Entry，该Entry除了保存当前对象的引用外，还保存了其上一个元素before和下一个元素after的引用，从而在哈希表的基础上又构成了双向链接列表

**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;

Entry<K,V> header 双向链表的表头元素

**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);
 }

HashMap的Entry

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; 
 } 
 }

上面的两端代码可以看出LinkedHashMap的Entry元素继承了HashMap的Entry 并且保存了上一个元素的before和下一个元素的after

**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; 
 }

例子：

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.*;
 
 /**
 * 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
 * (insertion-order). Note that insertion order is not affected
 * if a key is re-inserted 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.)
 *
 * 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.)
 *
 * 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 (access-order). 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. No
 * other methods generate entry accesses. In particular, operations on
 * collection-views do not affect the order of iteration of the backing
 * map.
 *
 * 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.
 *
 * 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 size
 * of the map, regardless of its capacity. Iteration over a <tt>HashMap</tt>
 * is likely to be more expensive, requiring time proportional to its
 * capacity.
 *
 * A linked hash map has two parameters that affect its performance:
 * initial capacity and load factor. 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.
 *
 * 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 must 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. In access-ordered linked hash maps,
 * merely querying the map with <tt>get</tt> is a structural
 * modification.)
 *
 * The iterators returned by the <tt>iterator</tt> method of the collections
 * returned by all of this class's collection view methods are
 * fail-fast: 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.
 *
 * 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: the fail-fast behavior of iterators
 * should be used only to detect bugs.
 *
 * 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 iterator
 if (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.
 *
 * 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.)
 *
 * A return value of {@code null} does not necessarily
 * 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() methods
 Iterator<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 appropriate
 Entry<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.
 *
 * 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>
 *
 * 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 is permitted for this method to modify
 * the map directly, but if it does so, it must 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.
 *
 * 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;
 }
 }