java源码解析---HashMap

/*
 *  %W% %E%
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package explain;
import java.io.*;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * 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 %I%, %G%
 * @see     Object#hashCode()
 * @see     Collection
 * @see        Map
 * @see        TreeMap
 * @see        Hashtable
 * @since   1.2
 */

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.
     * 默认初始化容量为16---必须是2的次幂
     */
    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.
     * 最大的容量，当一个比较大的值通过构造器明确指出容器的容量是会被使用到，2的次幂小于等于1<<30
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     * 当构造器没有明确指出时，默认的负载因子，默认的负载因子为 0.75f
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     * 哈希表，大小根据需要调整，长度必须是2的次幂
     */
    transient Entry[] table;

    /**
     * The number of key-value mappings contained in this map.
     * HashMap实际存储key-value键值对的数量
     */
    transient int size;

    /**
     * The next size value at which to resize (capacity * load factor).
     * HashMap再次调整大小的阈值，该阈值的计算公式为（threshold = capacity * loadFactor）
     * （该阶段的阈值 = 该阶段的容器的容量 * 负载因子）
     * @serial
     */
    int threshold;

    /**
     * The load factor for the hash table.
     * 哈希表的负载因子
     * @serial
     */
    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).
     * 用于记录HashMap结构性修改的次数，结构性的修改是改变HashMap中key-value键值对的数量，或者是修改他内部的结构，
     * 该属性是为了在HashMap集合视图变量的时候，当出现并发修改的时候，产生快速失败，抛出 ConcurrentModificationException
     */
    transient volatile int modCount;

    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and load factor.
     * 构造一个包含指定初始化容量和负载因子的空的HashMap
     * @param  initialCapacity the initial capacity
     * 参数：初始化容量
     * @param  loadFactor      the load factor
     * 参数：负载因子
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     * 如果初始化容量是负数的或者负载因子是非正的，抛出IllegalArgumentException
     */
    public HashMap(int initialCapacity, float loadFactor) {
        //判断初始化容量是否小于零，如果小于零抛出非法的参数异常（IllegalArgumentException）
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        //判断初始化容量是否是最大容量2的30次方（1<<30）,如果大于最大值，初始化容量只能为最大值
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        //判断负载因子是否小于等于零或者不是一个数，抛出非法的参数异常（IllegalArgumentException）
        //Java.lang.Float.isNaN()方法 ： 
        //此方法如果此对象所表示的值是NaN(Not a Number不是一个数)，返回true，否则返回false
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);

        // Find a power of 2 >= initialCapacity
        // 找到一个大于等于传入的initialCapacity初始化容量的2的次幂，作为容量HashMap的初始化容量
        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;
        //负载因子赋值
        this.loadFactor = loadFactor;
        //根据上述阈值公式计算阈值
        threshold = (int)(capacity * loadFactor);
        //根据找到的2的次幂的容量，初始化Entry数组，大小为找到的容量
        table = new Entry[capacity];
        init();
    }

    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and the default load factor (0.75).
     * 构造一个包含指定初始化容量和默认负载因子的空的HashMap
     *
     * @param  initialCapacity the initial capacity.
     * 参数：初始化容量
     * @throws IllegalArgumentException if the initial capacity is negative.
     * 异常：在初始化容量为负数是抛出IllegalArgumentException
     */
    public HashMap(int initialCapacity) {
        //调用重载的构造器
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     * 构造一个包含默认初始化容量（16）和默认负载因子（0.75）的空的HashMap
     */
    public HashMap() {
        //负载因子赋值0.75
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        //根据上述阈值公式计算阈值
        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);//12 = 16 * 0.75
        //初始化Entry数组
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
        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>.
     * 根据指定的Map创建HashMap，该HashMap的负载因子是默认负载因子0.75和一个足够能够容纳这个Map的初始化容量
     * @param   m the map whose mappings are to be placed in this map
     * 参数：将要被放置在这个HashMap中的Map
     * @throws  NullPointerException if the specified map is null
     * 异常：当这个指定的Map是null是抛出NullPointException
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
        //threshold = capacity * loadFactor => capacity = threshold/loadFactor
        //调用第一个重载的构造器，选择初始化容量为（Map的大小(将其看作新建HashMap的阈值)/默认负载因子 + 1)和默认初始化容量相比较较大的那个
        // 将Map放入新构造的HashMap
        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.)
     * 子类初始化的钩子方法，在所有的构造器和伪构造器（例如 clone和readObject方法）中调用
     * 而且是在HashMap被初始化之后，任何键值对被添加之前（如果这个方法不存在，readObject必须清楚的知道子类的情况）
     */
    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.
     * 对于给予的hashcode，进一步处理，避免了糟糕的hash表的性能，这个至关重要的，因为HashMap用的是2的次幂长度的hash表
     * 否则在没有差别的hashcode就会遭遇碰撞 
     * 注意：key值为null的map的hash值是0，因此索引也就是一直为0
     */
    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).
        // 这个函数确保只有不同常量倍数的hashcode
        // 在每个位置上有限的碰撞次数（在默认的负载因子的情况大约是8）
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

    /**
     * Returns index for hash code h.
     * 根据hash值，返回索引
     */
    static int indexFor(int h, int length) {
        //hash值的取模操作，因为length保证是2的次幂
        //则 h%length <==> h & (length-1)
        //但是&操作效率高于%的操作效率
        return h & (length-1);
    }

    /**
     * Returns the number of key-value mappings in this map.
     * 
     * @return the number of key-value mappings in this map
     * 返回该Map中的Key-Value键值对的实际数目
     */
    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
     * 如果没有键值对返回true
     */
    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.
     * 返回指定Key对应的Value值，如果没有返回null      
     * <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.)
     * 更正式点说，如果存在这样个map(key==null ? k==null : key.equals(k)
     * 返回value，否则返回null，最多只有一个这个的键值对
     * <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.
     * 并不一定会返回null，如果返回null值，表示不存在指定key的map，或者可能是该key的Value就是null
     * 可以通过containsKey操作来判别
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        //如果Key等于null
        if (key == null)
            return getForNullKey();
        //根据hash值计算索引，遍历该索引下的链表，满足条件返回value
        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.
     * get()方法的分支查找为null的键值，null的键值对应的索引是0，
     * 分别针对get和put的情况，null值的特例被分出来，同时结合了其他条件
     */
    private V getForNullKey() {
        //遍历该索引下的链表，找到Key为null的Value返回，找不到返回null
        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.
     * 如果存在，指定key对用的键值对返回true
     * @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.
     * 根据指定key值返回对应的键值对对象，如果不存在这样的键值对返回null
     */
    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.
     * 将具体的Value和Key结合在一个Map中，如果之前存在过这样Key的键值对，该键值对
     * 的Value将被新Value覆盖
     * @param key key with which the specified value is to be associated
     * 参数：指定的Key
     * @param value value to be associated with the specified key
     * 参数：指定的Value
     * @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>.)
     * 返回：返回Key值对应的旧的value值，或者如果没有对用Key相同的键值对返回null
     * 返回null值，也表明Key值之前对用的Value值就是null
     */
    public V put(K key, V value) {
        //判断Key是否等于null
        if (key == null)
            return putForNullKey(value);
        //根据hash值计算索引
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        //遍历哈希表，如果存在Key相等的情况，用新的Value值覆盖旧的Value值，返回旧的Value值
        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;
            }
        }
        //如果不存在Key相等的情况，修改结构性改变计数
        modCount++;
        //新增键值对，添加到链表中
        addEntry(hash, key, value, i);
        return null;
    }

    /**
     * Offloaded version of put for null keys
     * put在Key为null情况下的分支方法
     */
    private V putForNullKey(V value) {
        //如果已经存在Key值为null，这用新的Value代替旧的Value
        //否则增加修改计数
        //创建新的键值对，hash值为0，键值为null,索引为0
        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;
            }
        }
        //如果不存在Key相等的情况，修改结构性改变计数
        modCount++;
        //新增键值对，添加到链表中
        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.
     * 这个方法代替通过构造器和伪构造器（clone, readObject），
     * 不会调整哈希表和检查并发修改，调用createEntry而不是addEntry
     */
    private void putForCreate(K key, V value) {
        // 计算hash值，
        // 如果key为null---> hash值为0
        // 如果key不为null---> 获取key对象的hashCode值调用静态的hash的方法进一步处理，获取hash值
        int hash = (key == null) ? 0 : hash(key.hashCode());
        //根据hash值计算key值在哈希表中的索引
        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.
         * 根据key查找已经存在的键值对，这个不会在clone和deserialize中出现，
         * 只会在这个Map是一个与放入顺序或者equals不一致的有序Map中出现
         */
        //变量HashMap的键值对，如果存在Key相等则直接用Value新值直接覆盖原来Value的旧值
        //注意：这里有两种情况，前提两者的hash值要相等 1. == 进行相比 2. equals进行相比 
        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;
            }
        }
        //如果没有key值相等的情况则创建新的结点
        createEntry(hash, key, value, i);
    }

    private void putAllForCreate(Map<? extends K, ? extends V> m) {
        //遍历Map，获取Key和Value，各自调用putForCreate方法
        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.
     * 再一次将Map中的内容放到一个新的更大容量的数组中去，当Map中的键值数量达到阈值的时候会调用这个方法
     * 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.
     * 如果当前的容量是HashMap的最大容量时，这个方法不会重新调整Map的大小，但是会把HashMap的阈值设置为Integer的最大值
     * 这就阻止了再次调用的发生
     * @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).
     * 参数：newCapacity新的容量，必须是2的次幂，必须大于当前的容量，除非当前容量是最大容量
     * （如果在这种情况下，该容量值也是不起作用的）
     */
    void resize(int newCapacity) {
        //1.将存放Map的Entry数组，临时存放起来到oldTable
        //2.获取旧的容量，并判断是否等于最大容量，如果等于最大容量，将阈值该为Integer的最大值，不进行调整大小直接返回
        //3.用newCapacity创建一个同样大小的Entry数组，这就是扩容后的新的数组容器
        //4.调用transfer方法进行复制，将所有的价值对搬移到新的容器中
        //5.table重新赋值，阈值重新计算
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        transfer(newTable);
        table = newTable;
        threshold = (int)(newCapacity * loadFactor);
    }

    /**
     * Transfers all entries from current table to newTable.
     * 将所有的键值对从当前的表搬到新的表，实质数组的复制
     */
    void transfer(Entry[] newTable) {
        //获取旧的table
        Entry[] src = table;
        int newCapacity = newTable.length;
        //遍历旧的键值对数组
        for (int j = 0; j < src.length; j++) {
            //获取一个桶的链表，对链表进行搬移
            Entry<K,V> e = src[j];
            //如果当前桶不为空，说明存在链表
            if (e != null) {
                //有助于垃圾回收
                src[j] = null;
                //遍历当前链表，知道链表为有结点为null为止
                do {
                    //获取链表的下一个节点的价值对对象
                    Entry<K,V> next = e.next;
                    //根据新的容量重新计算索引
                    int i = indexFor(e.hash, newCapacity);
                    //维护链表的结构是需要通过每个节点的next属性来做的
                    //可以这么认为table[i]/newTable[i]就是该链表的一个元素，因为新添加的元素总是要放在链表的第一个位置
                    //那怎么连起来呢？就要将newTable[i]作为新元素节点的下个节点(e.next = newTable[i])，
                    // 而将新元素赋值给newTable[i](newTable[i] = e)
                    e.next = newTable[i];
                    newTable[i] = e;
                    //为下一次循环做准备
                    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.
     * 将指定的Map复制到当前的Map中，对于指定Map有Key和当前Map重复的，这些键值对都会被替换掉
     * @param m mappings to be stored in this map
     * 参数：存储键值对的Map
     * @throws NullPointerException if the specified map is null
     * 异常：如果指定的Map为null，将会抛出NullPointerException
     */
    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.
         * 如果加入的键值对的数量大于或者等于阈值，就会对Map进行扩容
         * 这是一种保守的做法，很明显的条件是(m.size() + size) >= threshold
         * 但是，如果被添加的Keys与当前的Map中的Keys重复，这种条件可能导致Map的容量是合适容量的两倍
         * 通过使用保守的计算方式，将会导致至多一次的扩容操作。
         */
        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);
        }
        //遍历Map的Key和Value放入
        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.
     * 根据指定的Key删除键值对，如果存在
     * @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>.)
     *  返回值为null代表两种情况，第一，不存在指定的Key对应的键值对，第二，key值对应的Value值就是null
     */
    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.
     * 根据指定的Key，删除并返回键值对对象，返回为null，表示该HashMap中不含该Key对应的键值对
     */
    final Entry<K,V> removeEntryForKey(Object key) {
        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;
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k)))) {
                modCount++;
                size--;
                //如果就是链表的第一个元素，直接将链表的下一阶段next指向，链表头部
                if (prev == e)
                    table[i] = next;
                else
                    //前节点的next节点直接指向，当前节点的next节点，也就删除了当前节点在链表中的位置
                    prev.next = next;
                e.recordRemoval(this);
                return e;
            }
            //将做过循环的判断的e，作为下次循环的前一个节点
            prev = e;
            //将next作为下个循环节点
            e = next;
        }

        return e;
    }

    /**
     * Special version of remove for EntrySet.
     * 针对entryset的删除
     */
    //总体来说给removeEntryForKey没什么大的差别，添加了一下两点
    final Entry<K,V> removeMapping(Object o) {
        //1.添加了对于对象类型的判断
        if (!(o instanceof Map.Entry))
            return null;

        Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
        //2.获取Key值，而这是在removeEntryForKey方法中直接传入的
        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.
     * 清空HashMap
     */
    public void clear() {
        modCount++;
        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.
     * 如果有一个或者多个键值对包含指定的Value值则返回true
     * @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) {
     //如果为null做特殊处理
    if (value == null)
            return containsNullValue();
    //遍历Map，找到是否有Value值与参数相等的，注意比较是equals的比较
    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
     * 针对containsValue方法参数为null的特殊性做处理
     * 遍历Map，寻找是否有Value为null的键值对
     */
    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.
     * 返回当前HashMap实例的浅克隆，keys和values他们自身并没克隆
     * @return a shallow copy of this map
     */
    public Object clone() {
        HashMap<K,V> result = null;
    try {
        //调用父类开始clone
        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();
        //遍历放置当前的Map所有键值对到，克隆的result中，就像HashMap的下面的构造方法一样
        // public HashMap(Map<? extends K, ? extends V> m) {}
        result.putAllForCreate(this);

        return result;
    }
    //键值对静态内部类
    static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;
        final int hash;

        /**
         * Creates new entry.
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }

        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;
        }

        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;
        }

        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.
         */
        //空方法，子类LinkedHashMap重写，实现功能
        void recordAccess(HashMap<K,V> m) {
        }

        /**
         * This method is invoked whenever the entry is
         * removed from the table.
         */
        //空方法，子类LinkedHashMap重写，实现功能
        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.
     * 添加一个新的指定key，Value和hashCode到具体的索引的桶中，该方法会在合适的时候重新调整hash表的大小
     * Subclass overrides this to alter the behavior of put method.
     * 子类可以重写该方法修改put的行为
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {
        //记录当前索引位置，将新建的键值对放在链表的第一位，该索引放在新建键值的next上
    Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
        //如果当前的键值对数量大于阈值，就要重新调整整个HashMap的大小，调整为之前容量的2倍
        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.
     * 这个方法很像addEntry方法，除了该版本可以应用于构造器或者伪构造器（clone，deserialization），将Map作为HashMap的一部分
     * 该版本并不需要担心重新调整Hash表，以为该方法用于初始化，在初始化的时候，已经保证了能够容纳Map的阈值了
     * Subclass overrides this to alter the behavior of HashMap(Map),
     * clone, and readObject.
     * 子类重写该方法，修改HashMap的clone，readObject的行为
     */
    void createEntry(int hash, K key, V value, int bucketIndex) {
    //将表索引暂时保存起来，代表链表的第一个节点，作为新建节点的下一个节点的指向
    //新添加的元素是要被放在链表的第一个位置的
    Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
        //key-Value键值对的实际值要增加
        size++;
    }
    //HashMap的迭代器的实现
    private abstract class HashIterator<E> implements Iterator<E> {
        Entry<K,V> next;    // next entry to return 下一个键值对
        int expectedModCount;    // For fast-fail 快速失败
        int index;        // current slot 当前位置
        Entry<K,V> current;    // current entry 当前键值对

        HashIterator() {
            expectedModCount = modCount;
            if (size > 0) { // advance to first entry
                Entry[] t = table; //找到第一个不为null的桶
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
        }

        public final boolean hasNext() {
            return next != null;
        }

        final Entry<K,V> nextEntry() {
            //并发修改判断，如果在遍历时，HashMap发生了结构性的变化，就会抛出异常
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Entry<K,V> e = next;
            //如果为null，表示遍历了所有都没找到，所以不存在
            if (e == null)
                throw new NoSuchElementException();
            //一直遍历下去，直到找到所以元素
            if ((next = e.next) == null) {
                Entry[] t = table;
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
        current = e;
            return e;
        }

        public void remove() {
            if (current == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Object k = current.key;
            current = null;
            //调用removeEntryForKey修改了modCount，
            HashMap.this.removeEntryForKey(k);
            //为了保证一致,避免并发异常
            expectedModCount = modCount;
        }

    }
    //Value迭代器
    private final class ValueIterator extends HashIterator<V> {
        public V next() {
            return nextEntry().value;
        }
    }
    //Key迭代器
    private final class KeyIterator extends HashIterator<K> {
        public K next() {
            return nextEntry().getKey();
        }
    }
    //Entry迭代器
    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
    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.
     * 返回该map中包含的key的一个set集合，由于该set是由map支持的，所以改变map同样会映射到set集合中，反之亦然
     * 如果在set遍历的过程中，Map被修改（除非通过迭代器自己的remove方法），那迭代器的结果是不明确的
     * set集合支持元素的移除，从map中删除相应的键值对，可以通过Iterator.remove，Set.remove，removeAll，retainAll，clear
     * 不支持add和addAll
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null ? ks : (keySet = new KeySet()));
    }

    private final class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return newKeyIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsKey(o);
        }
        public boolean remove(Object o) {
            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.
     */
    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
     */
    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());
    }

    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).
     * HashMap的序列化，将HashMap实例保存到一个流中
     * @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 stuff
    s.defaultWriteObject();

    // Write out number of buckets
    s.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).
     * 反序列化，重新构建HashMap
     */
    private void readObject(java.io.ObjectInputStream s)
         throws IOException, ClassNotFoundException
    {
    // Read in the threshold, loadfactor, and any hidden stuff
    s.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 HashMap
    for (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;   }
}