HashMap源码 详细解析

/* *  @(#)HashMap.java1.73 07/03/13 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */package java.util;import java.io.*;/** * Hash table based implementation of the <tt>Map</tt> interface.  This * implementation provides all of the optional map operations, and permits * <tt>null</tt> values and the <tt>null</tt> key.  (The <tt>HashMap</tt> * class is roughly equivalent to <tt>Hashtable</tt>, except that it is * unsynchronized and permits nulls.)  This class makes no guarantees as to * the order of the map; in particular, it does not guarantee that the order * will remain constant over time. * * <p>This implementation provides constant-time performance for the basic * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function * disperses the elements properly among the buckets.  Iteration over * collection views requires time proportional to the "capacity" of the * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number * of key-value mappings).  Thus, it's very important not to set the initial * capacity too high (or the load factor too low) if iteration performance is * important. * * <p>An instance of <tt>HashMap</tt> has two parameters that affect its * performance: <i>initial capacity</i> and <i>load factor</i>.  The * <i>capacity</i> is the number of buckets in the hash table, and the initial * capacity is simply the capacity at the time the hash table is created.  The * <i>load factor</i> is a measure of how full the hash table is allowed to * get before its capacity is automatically increased.  When the number of * entries in the hash table exceeds the product of the load factor and the * current capacity, the hash table is <i>rehashed</i> (that is, internal data * structures are rebuilt) so that the hash table has approximately twice the * number of buckets. * * <p>As a general rule, the default load factor (.75) offers a good tradeoff * between time and space costs.  Higher values decrease the space overhead * but increase the lookup cost (reflected in most of the operations of the * <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>).  The * expected number of entries in the map and its load factor should be taken * into account when setting its initial capacity, so as to minimize the * number of rehash operations.  If the initial capacity is greater * than the maximum number of entries divided by the load factor, no * rehash operations will ever occur. * * <p>If many mappings are to be stored in a <tt>HashMap</tt> instance, * creating it with a sufficiently large capacity will allow the mappings to * be stored more efficiently than letting it perform automatic rehashing as * needed to grow the table. * * <p><strong>Note that this implementation is not synchronized.</strong> * If multiple threads access a hash map concurrently, and at least one of * the threads modifies the map structurally, it <i>must</i> be * synchronized externally.  (A structural modification is any operation * that adds or deletes one or more mappings; merely changing the value * associated with a key that an instance already contains is not a * structural modification.)  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 HashMap(...));</pre> * * <p>The iterators returned by all of this class's "collection view methods" * are <i>fail-fast</i>: 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  Doug Lea * @author  Josh Bloch * @author  Arthur van Hoff * @author  Neal Gafter * @version 1.73, 03/13/07 * @see     Object#hashCode() * @see     Collection * @see    Map * @see    TreeMap * @see    Hashtable * @since   1.2 *//*** HashMap这个集合花了几天，看完了其实也没有什么难的啊，认真阅读，多读源代码，一步一个脚印，修炼成大牛  第一变过完了HashMap，还要多看几遍，我相信每一遍的效果都会不一样的，这个代码是java官方写的代码，有很多优点，仔细阅读，从中吸取更多的营养**/public class HashMap<K,V>    extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable{    /**     * The default initial capacity - MUST be a power of two.     */   //hash表默认的初始化容量,大小为16    static final int DEFAULT_INITIAL_CAPACITY = 16;    /**     * The maximum capacity, used if a higher value is implicitly specified     * by either of the constructors with arguments.     * MUST be a power of two <= 1<<30.     */  //hash表的最大容量为，2^30    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * The load factor used when none specified in constructor.     */  //装载因子默认为，0.75即:hash表中如果映射到表中的元素个数超过0.75*16=12个 //(这个里面的个数，指的是数组中拉链的个数，就是链表中有一个元素也算一个)，则会扩容,这样做法是避免发生大量的元素堆积    static final float DEFAULT_LOAD_FACTOR = 0.75f;    /**     * The table, resized as necessary. Length MUST Always be a power of two.     */  //hash表存的是Entry对象,Entry里面封装的有key 和value ,table 就是定义的Hash表    transient Entry[] table;    /**     * The number of key-value mappings contained in this map.     */  //就是hash表中存储总共的元素个数，就是Entry对象的个数    transient int size;    /**     * The next size value at which to resize (capacity * load factor).     * @serial     */  //就是Hash表中能存储的最大链表个数，表的大小*装载因子, 如果超过这个值，hash表就必须扩容    int threshold;    /**     * The load factor for the hash table.     *     * @serial     */  //装载因子的大小     final float loadFactor;    /**     * The number of times this HashMap has been structurally modified     * Structural modifications are those that change the number of mappings in     * the HashMap or otherwise modify its internal structure (e.g.,     * rehash).  This field is used to make iterators on Collection-views of     * the HashMap fail-fast.  (See ConcurrentModificationException).     */ //transient修饰后，就不需要序列化了,序列化的时候就不包含改属性了    transient volatile int modCount;    /**     * Constructs an empty <tt>HashMap</tt> 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     */  //Hash表的第一个构造方法，第一个参数初始化的容量，第二个参数是装载因子的大小    public HashMap(int initialCapacity, float loadFactor) {//如果Hash表初始容量<0 则抛出非法的初始化容量        if (initialCapacity < 0)            throw new IllegalArgumentException("Illegal initial capacity: " +                                               initialCapacity);//如果初始化容量大于最大容量(2^30)则 初始化容量默认为最大容量(2^30),就是即使你设置的初始容量超过2^30，但是你的初始容量依然为2^30        if (initialCapacity > MAXIMUM_CAPACITY)            initialCapacity = MAXIMUM_CAPACITY;//如果装载因子小于等于0，Float.isNan()(?)  将会抛出 非法的初始化装载因子        if (loadFactor <= 0 || Float.isNaN(loadFactor))            throw new IllegalArgumentException("Illegal load factor: " +                                               loadFactor);        // Find a power of 2 >= initialCapacity//我们设置的初始化容量，如果是2的次方，我们通过这循环，自动调整为大于设置这个初始化容量的第一个2的次方//(注：加入我的初始容量是9，那么我的初始容量为16，我的初始容量设置为10,11,12,13,14,15 ，则我的初始容量都转换为16)        int capacity = 1;        while (capacity < initialCapacity)            capacity <<= 1;        //将装载因子赋值为0.75f        this.loadFactor = loadFactor;//赋值hash表能够存储的拉链个数        threshold = (int)(capacity * loadFactor);//hash表的大小        table = new Entry[capacity];//然后调用init方法，其实这个init方法什么都没有实现，我们自己可以实现的        init();    }    /**     * Constructs an empty <tt>HashMap</tt> with the specified initial     * capacity and the default load factor (0.75).     *     * @param  initialCapacity the initial capacity.     * @throws IllegalArgumentException if the initial capacity is negative.     */  //第二个构造方法，只有一个参数是初始化容量，装载因子默认为0.75 //然后调用第二个构造方法，默认的装载因子为0.75    public HashMap(int initialCapacity) {        this(initialCapacity, DEFAULT_LOAD_FACTOR);    }    /**     * Constructs an empty <tt>HashMap</tt> with the default initial capacity     * (16) and the default load factor (0.75).     */ //第三个构造方法，空构造,hash默认大小为16，装载因子为0.75    public HashMap() {        this.loadFactor = DEFAULT_LOAD_FACTOR;        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);        table = new Entry[DEFAULT_INITIAL_CAPACITY];//调用初始化方法，这个方法是空方法        init();    }    /**     * Constructs a new <tt>HashMap</tt> with the same mappings as the     * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with     * default load factor (0.75) and an initial capacity sufficient to     * hold the mappings in the specified <tt>Map</tt>.     *     * @param   m the map whose mappings are to be placed in this map     * @throws  NullPointerException if the specified map is null     */ //第四个构造方法,还是调用带两个参数的构造方法    public HashMap(Map<? extends K, ? extends V> m) {        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);//这个方法 不懂啊        putAllForCreate(m);    }    // internal utilities    /**     * Initialization hook for subclasses. This method is called     * in all constructors and pseudo-constructors (clone, readObject)     * after HashMap has been initialized but before any entries have     * been inserted.  (In the absence of this method, readObject would     * require explicit knowledge of subclasses.)     */    void init() {    }    /**     * Applies a supplemental hash function to a given hashCode, which     * defends against poor quality hash functions.  This is critical     * because HashMap uses power-of-two length hash tables, that     * otherwise encounter collisions for hashCodes that do not differ     * in lower bits. Note: Null keys always map to hash 0, thus index 0.     */  //这个函数，是用来计算hash值的  传过来的是key.hashCode(); 计算key的hash值，然后映射到hash表中 key.hashCode() & length(table)-1    static int hash(int h) {        // This function ensures that hashCodes that differ only by        // constant multiples at each bit position have a bounded        // number of collisions (approximately 8 at default load factor).        h ^= (h >>> 20) ^ (h >>> 12);        return h ^ (h >>> 7) ^ (h >>> 4);    }    /**     * Returns index for hash code h.     */  //通过key计算key映射到hash表的那个位置，然后把Entry<key,value>这个对象存入到hash表中，如果hash表的该位置有元素 //则存储这个位置链表的最前面，即使链头    static int indexFor(int h, int length) {        return h & (length-1);  //注意这个length-1 也是很有讲究的， 0 到length-1 表的索性是从0开始到length-1结束的  一定要注意了    }    /**     * Returns the number of key-value mappings in this map.     *     * @return the number of key-value mappings in this map     */  //hash表中的存入的Entry<Key,Value> 对象的总个数.    public int size() {        return size;    }    /**     * Returns <tt>true</tt> if this map contains no key-value mappings.     *     * @return <tt>true</tt> if this map contains no key-value mappings     */  //判断hash表是否为空    public boolean isEmpty() {        return size == 0;    }    /**     * 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.     *     * @see #put(Object, Object)     */    public V get(Object key) {        if (key == null)            return getForNullKey();        int hash = hash(key.hashCode());        for (Entry<K,V> e = table[indexFor(hash, table.length)];             e != null;             e = e.next) {            Object k;            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))                return e.value;        }        return null;    }    /**     * Offloaded version of get() to look up null keys.  Null keys map     * to index 0.  This null case is split out into separate methods     * for the sake of performance in the two most commonly used     * operations (get and put), but incorporated with conditionals in     * others.     */    private V getForNullKey() {        for (Entry<K,V> e = table[0]; e != null; e = e.next) {            if (e.key == null)                return e.value;        }        return null;    }    /**     * Returns <tt>true</tt> if this map contains a mapping for the     * specified key.     *     * @param   key   The key whose presence in this map is to be tested     * @return <tt>true</tt> if this map contains a mapping for the specified     * key.     */    public boolean containsKey(Object key) {        return getEntry(key) != null;    }    /**     * Returns the entry associated with the specified key in the     * HashMap.  Returns null if the HashMap contains no mapping     * for the key.     */    final Entry<K,V> getEntry(Object key) {        int hash = (key == null) ? 0 : hash(key.hashCode());        for (Entry<K,V> e = table[indexFor(hash, table.length)];             e != null;             e = e.next) {            Object k;            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k))))                return e;        }        return null;    }    /**     * Associates the specified value with the specified key in this map.     * If the map previously contained a mapping for the key, the old     * value is replaced.     *     * @param key key with which the specified value is to be associated     * @param value value to be associated with the specified key     * @return the previous value associated with <tt>key</tt>, or     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.     *         (A <tt>null</tt> return can also indicate that the map     *         previously associated <tt>null</tt> with <tt>key</tt>.)     */  //注意这个方法是hash表核心 元素是怎样放入hash表中去的 //是通过key 来找到Entry对象存入在hash表中的具体位置 //首先通过key 得到key.hashCode(), 然后将key.hashCode()传入hash(int key )得到hash值（从这一步我们可以看出把key的hash值进行了二次处理）  //然后，通过hash(key.hashCode())得到hash值：h  , h&length(table)-1 得到在表中的索引,从而就找到了存入在表中的位置 //最后如果这个位置为空，则将数据存入，如果不为空，则判断这个链表中的元素（Entry对象）是否与这个要存入的key的hash值相同，且key的内容相同，则认为存入的统一Entry<Key,Value>  //则将前一个覆盖，并把覆盖 的Entry<Key,Value>中value值返回     public V put(K key, V value) {//如果传入的key为null,则将key封装为一个Entry<Key,Value>对象存入hash表中的第一个位置        if (key == null)            return putForNullKey(value);//如果key值不为空,则得到key所映射的hash值        int hash = hash(key.hashCode());//通过hash值得到key映射在hash表中的位置        int i = indexFor(hash, table.length);//这个for很重要  找到key映射到hash表的位置后，将key和value封装成一个Entry<Key,Value> 对象。 //这个Entry对象中的属性有，key,value,通过key得到的hash值，和指向自己本身的一个引用next        for (Entry<K,V> e = table[i]; e != null; e = e.next) {            Object k;//这个if里面的条件判断，是这个hash表的核心很重要的。必须的深入的理解:1,首先我new出来的对象，把同一个对象往hashmap里面存，那么hash值一定相等// 那么key值必相等，如以同一对象往hash表里放，则一定符合条件，进入if。 将原来key对应的value给覆盖了，返回原value值，放入新value值。//第二问题：如果我们new出了两个对象 ，这两个对象，这两个对象完全一样，怎么办啊， 我们有需求就是两个内容相同的对象我们默认为一个key的 ，这个需求怎么办？怎么解决呢？//第二个问题，解决办法 是我们必须从写key的hashCode()和equals的方法,这样才可以的，如果不重写 key的hashCode和equals方法，他会调用父类的//即Object的hashCode()和equals() Object的hashCode()返回的这个对象在堆内存中的首地址，而equals是比较 两个对象的地址//那么如果不重写hashCode(),则两个不同对象的内存地址肯定不会相同，则hash值肯定不会相同，如果重写了hashCode()方法，重写的hashCode()的功能是，内容相同的key即使不是一个对象，//则hashCode()获得值是相等的，这个hashCode()是根据内容获得的，如果key的内容相同，则内容相同的hashCode()相等.//第三个问题？必须重写eqauls方法 ，用来比较两个对象的key的内容相同, 这样如果两个对象的内容相同 ，则hashCode()值和equals()比较为真，则两个内容相同的对象会被新的覆盖            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {                V oldValue = e.value;                e.value = value;                e.recordAccess(this);                return oldValue;            }        }        modCount++;//如果要存入hash变的key和拉链中的存入的key都不相等，则将key,value封装为Entry<Key,Value>对象存入拉链的头部.//注意在这个方法里面进行扩容        addEntry(hash, key, value, i);//这个返回null也很有讲究，判断一个key在拉链中是否有重复，可以用这null来判断，如果返回的null则key在拉链中是第一次出现，如果不空，则不是第一次进入.        return null;    }    /**     * Offloaded version of put for null keys     */  //该方法的作用是 如果key=null ,则将进行单独处理，将这个空加入hash表的第一个位置，即table[0]这个位置    private V putForNullKey(V value) {        for (Entry<K,V> e = table[0]; e != null; e = e.next) {            if (e.key == null) {                V oldValue = e.value;                e.value = value;                e.recordAccess(this);                return oldValue;            }        }        modCount++;//将为null的key封装为 Entry<Null,Value>对象 存入hash表 其中Entry对象中的hash值为0        addEntry(0, null, value, 0);        return null;    }    /**     * This method is used instead of put by constructors and     * pseudoconstructors (clone, readObject).  It does not resize the table,     * check for comodification, etc.  It calls createEntry rather than     * addEntry.     */ //这个方法是私有方法，外部无法调用，和put(Key key ,Value value)的主要区别是： //遇到重复的key时，put方法将其覆盖，返回被覆盖的value值，但是putForCreate方法直接覆盖原来的value，不返回原来的value值    private void putForCreate(K key, V value) {        int hash = (key == null) ? 0 : hash(key.hashCode());        int i = indexFor(hash, table.length);        /**         * Look for preexisting entry for key.  This will never happen for         * clone or deserialize.  It will only happen for construction if the         * input Map is a sorted map whose ordering is inconsistent w/ equals.         */        for (Entry<K,V> e = table[i]; e != null; e = e.next) {            Object k;            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k)))) {                e.value = value;                return;            }        }        createEntry(hash, key, value, i);    } //这个是私有的方法，泛型必须要深入的理解,这个传递的是一个泛型类型    private void putAllForCreate(Map<? extends K, ? extends V> m) {//这个是泛型里面又嵌套了泛型,认真的琢磨        for (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) {            Map.Entry<? extends K, ? extends V> e = i.next();            putForCreate(e.getKey(), e.getValue());        }    }    /**     * Rehashes the contents of this map into a new array with a     * larger capacity.  This method is called automatically when the     * number of keys in this map reaches its threshold.     *     * If current capacity is MAXIMUM_CAPACITY, this method does not     * resize the map, but sets threshold to Integer.MAX_VALUE.     * This has the effect of preventing future calls.     *     * @param newCapacity the new capacity, MUST be a power of two;     *        must be greater than current capacity unless current     *        capacity is MAXIMUM_CAPACITY (in which case value     *        is irrelevant).     */  //这个方法是进行扩容的，但是hash表的 table.length*装在因子， 表中无位置的时候，需要对hash表进行扩容。    void resize(int newCapacity) {        Entry[] oldTable = table;//首先得到老的hash表        int oldCapacity = oldTable.length; //然后得到老的hash表的长度，如果老的hash表以及达到最大值时：2^30 则无法进行扩容，不进行扩容。        if (oldCapacity == MAXIMUM_CAPACITY) {            threshold = Integer.MAX_VALUE;            return;        }        Entry[] newTable = new Entry[newCapacity]; //设置扩容后的hash表容量，在这里其实是重新建立一个hash表而已//这个函数的作用是将扩展前表的元素移动到新扩展的表里面来，很耗时的。        transfer(newTable);        table = newTable;         threshold = (int)(newCapacity * loadFactor);  //改变表中能够容纳拉链的条数。    }    /**     * Transfers all entries from current table to newTable.     */ //这个函数的主要作用是进行扩容，看看他是怎样扩容的    void transfer(Entry[] newTable) {//首先得到扩容前的表        Entry[] src = table;//得到新表的大小        int newCapacity = newTable.length;//下面这个循环的作用是来进行扩容的//用来循环扩容前表的内容        for (int j = 0; j < src.length; j++) {            Entry<K,V> e = src[j];            if (e != null) {                src[j] = null;                do {//下面几行代码的作用是很关键的，看看sun程序员的代码风格。//取出表中该位置的下一个元素                    Entry<K,V> next = e.next;//然后取出hash值，根据hash值计算在扩容后表中的位置。                    int i = indexFor(e.hash, newCapacity);//将新表中的元素连接到e后面                    e.next = newTable[i];//将元素放入扩容后的表中                    newTable[i] = e;//将下一个元素赋值给e                    e = next;                } while (e != null);            }        }    }    /**     * Copies all of the mappings from the specified map to this map.     * These mappings will replace any mappings that this map had for     * any of the keys currently in the specified map.     *     * @param m mappings to be stored in this map     * @throws NullPointerException if the specified map is null     */ //这个函数，关于泛型的，以后要仔细琢磨    public void putAll(Map<? extends K, ? extends V> m) {        int numKeysToBeAdded = m.size();        if (numKeysToBeAdded == 0)            return;        /*         * Expand the map if the map if the number of mappings to be added         * is greater than or equal to threshold.  This is conservative; the         * obvious condition is (m.size() + size) >= threshold, but this         * condition could result in a map with twice the appropriate capacity,         * if the keys to be added overlap with the keys already in this map.         * By using the conservative calculation, we subject ourself         * to at most one extra resize.         */        if (numKeysToBeAdded > threshold) {            int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);            if (targetCapacity > MAXIMUM_CAPACITY)                targetCapacity = MAXIMUM_CAPACITY;            int newCapacity = table.length;            while (newCapacity < targetCapacity)                newCapacity <<= 1;            if (newCapacity > table.length)                resize(newCapacity);        }        for (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) {            Map.Entry<? extends K, ? extends V> e = i.next();            put(e.getKey(), e.getValue());        }    }    /**     * Removes the mapping for the specified key from this map if present.     *     * @param  key key whose mapping is to be removed from the map     * @return the previous value associated with <tt>key</tt>, or     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.     *         (A <tt>null</tt> return can also indicate that the map     *         previously associated <tt>null</tt> with <tt>key</tt>.)     */  //这个函数的作用是通过key删除hash表中的Entry<K,V>对象，然后返回被删除元素的value    public V remove(Object key) {        Entry<K,V> e = removeEntryForKey(key);        return (e == null ? null : e.value);    }    /**     * Removes and returns the entry associated with the specified key     * in the HashMap.  Returns null if the HashMap contains no mapping     * for this key.     */ //final修饰的方法不能够被重写 //这个方法的作用是，通过key删除hash表中的元素，然后返回被删除元素的value    final Entry<K,V> removeEntryForKey(Object key) {//通过key.hashCode()得到hash值，然后通过hash值在表中找到元素要存储的位置。        int hash = (key == null) ? 0 : hash(key.hashCode());        int i = indexFor(hash, table.length);//得到位置后，取出hash表中的值        Entry<K,V> prev = table[i];//将hash表中的值赋值给e        Entry<K,V> e = prev;        //对这个位置进行循环，取出所有的Entry<K,V>对象值。        while (e != null) {//首先得到要删除元素的指向下一个元素的引用            Entry<K,V> next = e.next;            Object k;            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k)))) {                modCount++;                size--;//hash表中的第一个元素就是要移除的值，则直接将下一个相邻的元素赋值到表中的链头                if (prev == e)                    table[i] = next;                else                    prev.next = next;                e.recordRemoval(this);                return e;            }//prev保存的当前元素的上一个Entry<K,V>对象            prev = e;//e保存的是当前对象。            e = next;        }        return e;    }    /**     * Special version of remove for EntrySet.     */    final Entry<K,V> removeMapping(Object o) {        if (!(o instanceof Map.Entry))            return null;        Map.Entry<K,V> entry = (Map.Entry<K,V>) o;        Object key = entry.getKey();        int hash = (key == null) ? 0 : hash(key.hashCode());        int i = indexFor(hash, table.length);        Entry<K,V> prev = table[i];        Entry<K,V> e = prev;        while (e != null) {            Entry<K,V> next = e.next;            if (e.hash == hash && e.equals(entry)) {                modCount++;                size--;                if (prev == e)                    table[i] = next;                else                    prev.next = next;                e.recordRemoval(this);                return e;            }            prev = e;            e = next;        }        return e;    }    /**     * Removes all of the mappings from this map.     * The map will be empty after this call returns.     */ //这个函数的作用是清除hash表中所有元素的值，就是清空hash表    public void clear() {        modCount++;//这个hash表table 是个全局变量        Entry[] tab = table;        for (int i = 0; i < tab.length; i++)            tab[i] = null;        size = 0;    }    /**     * 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     */  //用于判断Hash表中是否包含指定的value，如果包含则返回true，否则返回false    public boolean containsValue(Object value) {//如果value为null则单独开个函数进行判断if (value == null)            return containsNullValue();Entry[] tab = table;        for (int i = 0; i < tab.length ; i++)            for (Entry e = tab[i] ; e != null ; e = e.next)                if (value.equals(e.value))                    return true;return false;    }    /**     * Special-case code for containsValue with null argument     */ //value值为Null的情况下，为什么要单独开函数判断啊，sun公司写的代码真是耐人寻味啊    private boolean containsNullValue() {Entry[] tab = table;        for (int i = 0; i < tab.length ; i++)            for (Entry e = tab[i] ; e != null ; e = e.next)                if (e.value == null)                    return true;return false;    }    /**     * Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and     * values themselves are not cloned.     *     * @return a shallow copy of this map     */    public Object clone() {        HashMap<K,V> result = null;try {    result = (HashMap<K,V>)super.clone();} catch (CloneNotSupportedException e) {    // assert false;}        result.table = new Entry[table.length];        result.entrySet = null;        result.modCount = 0;        result.size = 0;        result.init();        result.putAllForCreate(this);        return result;    }//静态内部类，内部类可以操作外部类的所有变量,sun公司为什么要用静态内部类    static class Entry<K,V> implements Map.Entry<K,V> {//key用final修饰是不可变的         final K key;        V value;        Entry<K,V> next;//hash值也是不可变的        final int hash;        /**         * Creates new entry.         */ //构造方法，创建这个Entry<K,V>需要传递hash值，key,value,下一个Entry<K,V>对象的引用        Entry(int h, K k, V v, Entry<K,V> n) {            value = v;            next = n;            key = k;            hash = h;        }        //final修饰的方法不能够被重写//得到key        public final K getKey() {            return key;        }        public final V getValue() {            return value;        }        public final V setValue(V newValue) {    V oldValue = value;            value = newValue;            return oldValue;        }        //重写了equals和hashCode方法//equals比较的是key和value的内容相同则就认为是同一个Entry对象        public final boolean equals(Object o) {            if (!(o instanceof Map.Entry))                return false;            Map.Entry e = (Map.Entry)o;            Object k1 = getKey();            Object k2 = e.getKey();            if (k1 == k2 || (k1 != null && k1.equals(k2))) {                Object v1 = getValue();                Object v2 = e.getValue();                if (v1 == v2 || (v1 != null && v1.equals(v2)))                    return true;            }            return false;        }        //重写hashCode()方法        public final int hashCode() {            return (key==null   ? 0 : key.hashCode()) ^                   (value==null ? 0 : value.hashCode());        }        public final String toString() {            return getKey() + "=" + getValue();        }        /**         * This method is invoked whenever the value in an entry is         * overwritten by an invocation of put(k,v) for a key k that's already         * in the HashMap.         */        void recordAccess(HashMap<K,V> m) {        }        /**         * This method is invoked whenever the entry is         * removed from the table.         */        void recordRemoval(HashMap<K,V> m) {        }    }    /**     * Adds a new entry with the specified key, value and hash code to     * the specified bucket.  It is the responsibility of this     * method to resize the table if appropriate.     *     * Subclass overrides this to alter the behavior of put method.     */ //addEntry的作用是将key和value封装为Entry对象加入到hash表中去。    void addEntry(int hash, K key, V value, int bucketIndex) {Entry<K,V> e = table[bucketIndex];        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);//如果超出了容量，则需要进行扩容        if (size++ >= threshold)            resize(2 * table.length);    }    /**     * Like addEntry except that this version is used when creating entries     * as part of Map construction or "pseudo-construction" (cloning,     * deserialization).  This version needn't worry about resizing the table.     *     * Subclass overrides this to alter the behavior of HashMap(Map),     * clone, and readObject.     */ //和addEntry这个方法一样，主要区别就是这个方法没有判断扩容，不管 hash表是否达到最大的threshold,都将Entry对象加入到hash表中去    void createEntry(int hash, K key, V value, int bucketIndex) {Entry<K,V> e = table[bucketIndex];        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);        size++;    }//这个抽象类写 在这里的目的是什么，完全用不到啊？？？？？？sun公司到低是什么目的啊？？？？？//我明白了，这个是给自己调用的，主要是关于hashMap迭代用的    private abstract class HashIterator<E> implements Iterator<E> {        Entry<K,V> next;// next entry to return        int expectedModCount;// For fast-fail        int index;// current slot        Entry<K,V> current;// current entry//        HashIterator() {            expectedModCount = modCount;            if (size > 0) { // advance to first entry                Entry[] t = table;//在hash表中找到第一个非空的元素                while (index < t.length && (next = t[index++]) == null)                    ;            }        }        public final boolean hasNext() {            return next != null;        }        final Entry<K,V> nextEntry() {            if (modCount != expectedModCount)                throw new ConcurrentModificationException();            Entry<K,V> e = next;            if (e == null)                throw new NoSuchElementException(); //如果hash表中只有一个元素，继续遍历找到hash表中第二个非空的元素            if ((next = e.next) == null) {                Entry[] t = table;                while (index < t.length && (next = t[index++]) == null)                    ;            }    current = e;            return e;        }        //在hash表移除当前遍历的元素        public void remove() {            if (current == null)                throw new IllegalStateException();            if (modCount != expectedModCount)                throw new ConcurrentModificationException();            Object k = current.key;            current = null;            HashMap.this.removeEntryForKey(k);            expectedModCount = modCount;        }    }    //这一步为什么用nextEntry()什么意思啊？？？真是搞不明白啊//难道一个类不类，可以调用另外一个类不类的方法吗？不是很明白呀//回去代码实战一把,如果说，一个内部类和调用另外一个内部类的方法，这个我可以理解了//下面这三个方法：KeyIterator 、ValueIterator、EntryIterator使用的很巧妙啊，下次自己造个集合需要写迭代器和模仿hashMap来写    private final class ValueIterator extends HashIterator<V> {        public V next() {            return nextEntry().value;        }    }    private final class KeyIterator extends HashIterator<K> {        public K next() {            return nextEntry().getKey();        }    }    private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {        public Map.Entry<K,V> next() {            return nextEntry();        }    }    // Subclass overrides these to alter behavior of views' iterator() method//这三个方法实现的也很好，堪称完美啊！  HashMap内部写的迭代器太完美了啊    Iterator<K> newKeyIterator()   {        return new KeyIterator();    }    Iterator<V> newValueIterator()   {        return new ValueIterator();    }    Iterator<Map.Entry<K,V>> newEntryIterator()   {        return new EntryIterator();    }    // Views    private transient Set<Map.Entry<K,V>> entrySet = null;    /**     * Returns a {@link Set} view of the keys contained in this map.     * The set is backed by the map, so changes to the map are     * reflected in the set, and vice-versa.  If the map is modified     * while an iteration over the set is in progress (except through     * the iterator's own <tt>remove</tt> operation), the results of     * the iteration are undefined.  The set supports element removal,     * which removes the corresponding mapping from the map, via the     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>     * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>     * operations.     */ //将所有key封装到Set集合中，这里面得到Set集合，也是在hashmap内部实现的，KeySet继承AbstractSet<K>    public Set<K> keySet() {        Set<K> ks = keySet;        return (ks != null ? ks : (keySet = new KeySet()));    } //其实得到KeySet也没什么大不了的哈，源代码实现很简单的    private final class KeySet extends AbstractSet<K> {//得到的迭代器，是hashMap的迭代器.        public Iterator<K> iterator() {            return newKeyIterator();        }        public int size() {            return size;        }        public boolean contains(Object o) {            return containsKey(o);        }        public boolean remove(Object o) {//这中写法，很奇特，以前没有用这种方式写过啊            return HashMap.this.removeEntryForKey(o) != null;        }        public void clear() {            HashMap.this.clear();        }    }    /**     * Returns a {@link Collection} view of the values contained in this map.     * The collection is backed by the map, so changes to the map are     * reflected in the collection, and vice-versa.  If the map is     * modified while an iteration over the collection is in progress     * (except through the iterator's own <tt>remove</tt> operation),     * the results of the iteration are undefined.  The collection     * supports element removal, which removes the corresponding     * mapping from the map, via the <tt>Iterator.remove</tt>,     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not     * support the <tt>add</tt> or <tt>addAll</tt> operations.     */ //得到Collection集合,这个集合中包含newValueIterator迭代器    public Collection<V> values() {        Collection<V> vs = values;        return (vs != null ? vs : (values = new Values()));    }    private final class Values extends AbstractCollection<V> {        public Iterator<V> iterator() {            return newValueIterator();        }        public int size() {            return size;        }        public boolean contains(Object o) {            return containsValue(o);        }        public void clear() {            HashMap.this.clear();        }    }    /**     * Returns a {@link Set} view of the mappings contained in this map.     * The set is backed by the map, so changes to the map are     * reflected in the set, and vice-versa.  If the map is modified     * while an iteration over the set is in progress (except through     * the iterator's own <tt>remove</tt> operation, or through the     * <tt>setValue</tt> operation on a map entry returned by the     * iterator) the results of the iteration are undefined.  The set     * supports element removal, which removes the corresponding     * mapping from the map, via the <tt>Iterator.remove</tt>,     * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and     * <tt>clear</tt> operations.  It does not support the     * <tt>add</tt> or <tt>addAll</tt> operations.     *     * @return a set view of the mappings contained in this map     */ //提供给外部调用，得到一个Set集合，这个Set集合里面存放的都是Entry对象    public Set<Map.Entry<K,V>> entrySet() {return entrySet0();    }    private Set<Map.Entry<K,V>> entrySet0() {        Set<Map.Entry<K,V>> es = entrySet;        return es != null ? es : (entrySet = new EntrySet());    }    //EntrySet实现也很简单，首先返回个遍历Entry的迭代器，以及判断集合中是否包含指定的对象    private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {        public Iterator<Map.Entry<K,V>> iterator() {            return newEntryIterator();        }        public boolean contains(Object o) {            if (!(o instanceof Map.Entry))                return false;            Map.Entry<K,V> e = (Map.Entry<K,V>) o;            Entry<K,V> candidate = getEntry(e.getKey());            return candidate != null && candidate.equals(e);        }        public boolean remove(Object o) {            return removeMapping(o) != null;        }        public int size() {            return size;        }        public void clear() {            HashMap.this.clear();        }    }    /**     * Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,     * serialize it).     *     * @serialData The <i>capacity</i> of the HashMap (the length of the     *   bucket array) is emitted (int), followed by the     *   <i>size</i> (an int, the number of key-value     *   mappings), followed by the key (Object) and value (Object)     *   for each key-value mapping.  The key-value mappings are     *   emitted in no particular order.     */    private void writeObject(java.io.ObjectOutputStream s)        throws IOException    {Iterator<Map.Entry<K,V>> i =    (size > 0) ? entrySet0().iterator() : null;// Write out the threshold, loadfactor, and any hidden stuffs.defaultWriteObject();// Write out number of bucketss.writeInt(table.length);// Write out size (number of Mappings)s.writeInt(size);        // Write out keys and values (alternating)if (i != null) {    while (i.hasNext()) {Map.Entry<K,V> e = i.next();s.writeObject(e.getKey());s.writeObject(e.getValue());    }        }    }    private static final long serialVersionUID = 362498820763181265L;    /**     * Reconstitute the <tt>HashMap</tt> instance from a stream (i.e.,     * deserialize it).     */    private void readObject(java.io.ObjectInputStream s)         throws IOException, ClassNotFoundException    {// Read in the threshold, loadfactor, and any hidden stuffs.defaultReadObject();// Read in number of buckets and allocate the bucket array;int numBuckets = s.readInt();table = new Entry[numBuckets];        init();  // Give subclass a chance to do its thing.// Read in size (number of Mappings)int size = s.readInt();// Read the keys and values, and put the mappings in the HashMapfor (int i=0; i<size; i++) {    K key = (K) s.readObject();    V value = (V) s.readObject();    putForCreate(key, value);}    }    // These methods are used when serializing HashSets    int   capacity()     { return table.length; }    float loadFactor()   { return loadFactor;   }}