Java-Collections Framework学习与总结-LinkedHashMap

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前面总结了java.util.HashMap，了解了其内部由散列表实现，每个桶内是一个单向链表。那有没有双向链表的实现呢？双向链表的实现会具备什么特性呢？来看一下HashMap的一个子类——java.util.LinkedHashMap。
        先读了一下源码的注释，首先LinkedHashMap中所有的Entry组成了一个双向链表，该链表定义了内部数据的迭代顺序，通常是按key插入的顺序(最近插入的放到链表的末尾，覆盖操作不会影响链表顺序)。LinkedHashMap还提供了构造方法LinkedHashMap(int,float,boolean)，如果第三个参数为true，那么内部数据的迭代顺序是按访问的某种顺序(访问时间由远到近)，最近访问的数据会放到链表的末尾。这样的结构很适合建立一个LRU Cache，所以基于LinkedHashMap来构建一个LRU Cache是很方便的(可参见removeEldestEntry方法)。
        对于大部分的操作来说，LinkedHashMap的性能比HashMap稍慢那么一点点(由于维护内部双向链表需要附加一些操作，但总体还是常数时间的)。LinkedHashMap的几种视图的迭代(XXIterator)要比HashMap快一些，由于它可以根据内部的双向链表来迭代，而HashMap需要遍历内部的散列表。
        其他特性继承自HashMap，来看下源码。

Java代码

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;
}

像LinkedList一样，内部存在一个表头header来作为双向链表的起点和终点(实际是一个环状)。accessOrder表示两种顺序——true为访问顺序；false为插入顺序。

Java代码

/**
* Called by superclass constructors and pseudoconstructors (clone,
* readObject) before any entries are inserted into the map. Initializes
* the chain.
*/
void init() {
header = new Entry<K,V>(-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;
}

还记得HashMap中的钩子方法init()，这里覆盖了init方法，在里面进行了双向链表的初始化。另外覆盖了transfer和containsValue方法，里面采用对链表的遍历，提高了一点儿性能。
接下来先看一下LinkedHashMap中Entry的代码。

Java代码

/**
* 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();
}
}

        重点看下recordAccess和recordRemoval方法。在分析总结HashMap的时候见过这两个钩子方法，在HashMap里，添加或者修改一个数据时(put)，会调用recordAccess；删除一个数据时会调用recordRemoval。
        LinkedHashMap的Entry里覆盖了这两个方法。在recordAccess里，如果accessOrder为true，说明是按访问顺序，那么改变双向链表的结构，把当前访问的Entry删掉，添加到链表的末尾。而在recordRemoval里则是从链表中删除当前的Entry。
        那么看下访问方法有什么变化。

Java代码

/**
* 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;
}

访问方法中调用了Entry的recordAccess方法。

Java代码

/**
* 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<K,V>(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;
}

这里覆盖了父类的addEntry方法，当添加一个数据时，首先调用createEntry方法，该方法也做了重写，加入了维护链表的逻辑，把新加的数据放到了表尾。然后在addEntry方法中有一个判断——通过调用removeEldestEntry方法来决定是否删除最老的(最长时间未访问的)数据。如果是，删除表头的数据；否则，判断是否需要扩容。所以子类可以覆盖removeEldestEntry方法来达到删除最老数据的目的，这在实现一个Cache的时候是非常有用的。
其余的代码也很容易看懂了，LinkedHashMap就总结到这儿。

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