JDK之HashMap源码解读

一方面不断在看书，一方面HashMap的确比List源码难一点，所以发的有点迟。

JDK版本依旧是1.7。

在看HashMap源码之前希望大家看过ArrayList源码，这样对理解HashMap源码也有很大帮助。

希望大家看完后既能理解HashMap的源码也能收获HashMap代码编写风格(例如：被调用的方法都在调用者方法的下面，这是一个很好的编程风格，不记得这风格名字了，谁记得请留言)

2017/06/03更新：

重新复习一遍源码后，为了大家方便，总结了其中一些重点知识（源码还是要读，这里主要方便大家快速复习）

重点知识集合：

HashMap 的实例有两个参数影响其性能：初始容量 和 加载因子。 容量是哈希表中桶的数量，初始容量只是哈希表在创建时的容量。

加载因子是哈希表在其容量自动增加之前可以达到多满的一种尺度。 当哈希表中的条目数超出了加载因子与当前容量的乘积时，则要对该哈希表 进行

rehash 操作（即重建内部数据结构）

默认的加载因子（0.75f）是对时间和空间的一种很好的权衡

加载因子的值越高，空间的利用率越高，但是包括 get 和 set 在内的绝大多操作时间将会变慢。

提升HashMap效率的一个关键点就是尽量减少rehash操作，即 初始容量满足： “初始容量 * 加载因子 >= 存储数量 ”

容量必须为2^n原因：将indexFor()中的“取余运算”（hashCode%length）变为“位与运算”（hashCode&(length-1)）

（存储实际大小>容量*加载因子）时，HashMap将进行扩容(rehash)

HashMap底层是数组中存储链表。

HashMap中有一些钩子方法（模板方法模式），比如init()，自身调用但不实现，

由子类实现，而该方法中调用时会调用子类实现的init()

计算hash的时候，HashMap将对象分为String和其他Object来分别计算 9、key为Null都被放在table[0]上

package java.util;
import java.io.*;

/**
*允许使用 null 值和 null 键
*迭代所需的时间与 HashMap *实例的容量及其实际大小（键-值映射关系数）成比例。
*所以，如果迭代性能很重要，则不要将初始容量设置得太高（或将加载因子设置得太低）。
*HashMap 的实例有两个参数影响其性能：初始容量 和 加载因子。
*容量是哈希表中桶的数量，初始容量只是哈希表在创建时的容量。
*加载因子是哈希表在其容量自动增加之前可以达到多满的一种尺度。
*当哈希表中的条目数超出了加载因子与当前容量的乘积时，则要对该哈希表
*进行 rehash 操作（即重建内部数据结构），从而哈希表将具有大约两倍的桶数。
*
*在通常情况下，默认的加载因子（0.75）是对时间和空间的一种很好的权衡。
*加载因子的值越高，空间的利用率越高，但是包括 get 和 set
*在内的绝大多操作时间将会变慢。在设置初始容量时应该考虑到映射中所需的
*条目数及其加载因子，以便最大限度地减少 rehash 操作次数。
*注：这里是提升HashMap效率的一个关键点。因为每次进行rehash操作的时候，
*在扩充容量的同时，还会将存储的元素进行重新放置（包括计算、复制移动），会花费大量的时间。
*所以尽量减少rehash操作将会提升效率。如果在预先知道存储元素的数量的时候，
*初始容量满足： “初始容量 * 加载因子 >= 存储数量 ” ，将会极大提升效率。
*/

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。
*左移计算：例如3<<4即3*(2^4)
*容量必须为2的n次方的原因：
*  ···
static int indexFor(int h, int length) {
return h & (length-1);
}
```
这是HashMap中的一个调用最频繁的方法，用于计算一个Key对应的Hash桶的索引， Hash桶放在一个数组中，这个方法返回的就是数组的索引，为了更加平均的分配容器内的元素，
采用的是取模运算来分配。参数里的h就是key的hashCode，length就是容量capacity。
这里假如h为70（二进制：**0100 0110**），length为64（二进制：**0100 0000**），length-1也就是63（二进制：**0011 1111**）。
```
h & (length-1) =01000110 & 00111111=110（十进制正好为6 ==h%length）
```
可以看到，如果length为2的N次方，取模运算可以变成位与运算，效率显著提高！但是要浪费一些容量的空间。
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

/**
*int最大值为2^31 - 1
*默认最大容量，但是如果在构造方法参数中给出了更大的值，则使用参数值
* 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.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;

/**
*默认加载因子,当(存储实际大小>容量*加载因子)的时候
*HashMap将进行扩容(rehash)
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;

/**
* An empty table instance to share when the table is not inflated.
*/
static final Entry<?,?>[] EMPTY_TABLE = {};

/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
/**
*从这里可以看到，HashMap其实是一个数组，
*每个数组中的元素又存储的是一个链表
*/
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

/**
*实际存储大小
* The number of key-value mappings contained in this map.
*/
transient int size;

/**
* The next size value at which to resize (capacity * load factor).
*负载值，下次扩容的临界值，为：(capacity * load factor)
* @serial
*/
// If table == EMPTY_TABLE then this is the initial capacity at which the
// table will be created when inflated.
int threshold;

/**
* The load factor for the hash table.
*
* @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).
*/
transient int modCount;

/**
*直译：当key为String并且hashMap的容量超过时，该字段提供了一个备用默认临界值。
*该字段供新hash算法使用，新hash算法将减少由于String key的weak hashCode计算而引起的冲突。
*如果想启用这个特性，你需要设置jdk.m
4000
ap.althashing.threshold这个系统属性的值为一个非负数（默认是-1）
*该值代表了一个集合大小的threshold，超过这个值，就会使用新的hash算法。
*需要注意的一点，只有当re-hash的时候，新的hash算法才会起作用
* The default threshold of map capacity above which alternative hashing is
* used for String keys. Alternative hashing reduces the incidence of
* collisions due to weak hash code calculation for String keys.
* <p/>
* This value may be overridden by defining the system property
* {@code jdk.map.althashing.threshold}. A property value of {@code 1}
* forces alternative hashing to be used at all times whereas
* {@code -1} value ensures that alternative hashing is never used.
*/
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;

/**
*Holder这个类用来在VM启动后初始化ALTERNATIVE_HASHING_THRESHOLD
* holds values which can't be initialized until after VM is booted.
*/
private static class Holder {

/**
* Table capacity above which to switch to use alternative hashing.
*/
static final int ALTERNATIVE_HASHING_THRESHOLD;

static {
String altThreshold = java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction(
"jdk.map.althashing.threshold"));

int threshold;
try {
threshold = (null != altThreshold)
? Integer.parseInt(altThreshold)
: ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;

// disable alternative hashing if -1
if (threshold == -1) {
//如果是-1则说明不是用，设置为Integer.MAX_VALUE
//也相当于ALTERNATIVE_HASHING_THRESHOLD_DEFAULT
threshold = Integer.MAX_VALUE;
}

if (threshold < 0) {
throw new IllegalArgumentException("value must be positive integer.");
}
} catch(IllegalArgumentException failed) {
throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
}

ALTERNATIVE_HASHING_THRESHOLD = threshold;
}
}

/**
*一个随机值，用来减少hashCode冲突。
* A randomizing value associated with this instance that is applied to
* hash code of keys to make hash collisions harder to find. If 0 then
* alternative hashing is disabled.
*/
transient int hashSeed = 0;

/**
* 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
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);

this.loadFactor = loadFactor;
threshold = initialCapacity;
/**
*在hashMap中init()方法中无内容，且访问为default。
*这个方法的作用在于多态。主要让子类重写，
*即提供一个方法给子类初始化。比如在LinkedHashMap中。
*实例：HashMap<?,?> map = new LinkedHashMap<?,?>();
*map.init()会调用LinkedHashMap中的init方法来。
*/
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.
*/
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).
*/
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}

/**
* 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
*/
public HashMap(Map<? extends K, ? extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,DEFAULT_INITIAL_CAPACITY)
, DEFAULT_LOAD_FACTOR);
inflateTable(threshold);

putAllForCreate(m);
}

// Find a power of 2 >= toSize
private static int roundUpToPowerOf2(int number) {
// assert number >= 0 : "number must be non-negative";
/**
* Integer.highestOneBit()的作用：
*比如int num = 170,那二进制就是10101010，
*highestOneBit就是把10101010变成10000000，
*即除最高位的1以外，其余都变为0
*/
/**
*而方法中number-1的意义在于roundUp,
*假如number = 4（100）,那么number-1 就是3（11），就会返回4（10 << 1）。
*假如number = 5（101）,那么number-1就是4（100）,就会返回8（100 << 1）
*/
return number >= MAXIMUM_CAPACITY
? MAXIMUM_CAPACITY
: (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;
}

/**
* Inflates the table.
*扩容的后重置hashSeed，
*这个方法只有在table为空的情况下才会调用来创建table
*即table初始化
*/
private void inflateTable(int toSize) {
// Find a power of 2 >= toSize
int capacity = roundUpToPowerOf2(toSize);

threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
table = new Entry[capacity];
initHashSeedAsNeeded(capacity);
}

// 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.)
*初始化方法，这里为空是为了让子类重写(多态)
*如LinkedHashMap中就重写了这个方法，那么
*HashMap map = new LinkedHashMap<T>();
*map.init中就调用的LinkedHashMap的init()
*/
void init() {
}

/**
* Initialize the hashing mask value. We defer initialization until we
* really need it.
*初始化hashSeed值
*/
final boolean initHashSeedAsNeeded(int capacity) {
boolean currentAltHashing = hashSeed != 0;
boolean useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
//按异或运算
boolean switching = currentAltHashing ^ useAltHashing;
if (switching) {
hashSeed = useAltHashing
? sun.misc.Hashing.randomHashSeed(this)
: 0;
}
return switching;
}

/**
* Retrieve object hash code and applies a supplemental hash function to the
* result hash, 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.
*如果key为String，就直接调用stringHash32返回hash值，如果是其他对象，则
*进行计算返回。（key为Null那么hash值为0）
*/
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}

h ^= k.hashCode();

// 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.
*/
static int indexFor(int h, int length) {
// assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
return h & (length-1);
}

/**
* Returns the number of key-value mappings in this map.
*
* @return the number of key-value mappings in this map
*/
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
*/
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)
//查找key为Null时的value,
//key为Null都放在table[0]上
return getForNullKey();
//没找到就返回null
Entry<K,V> entry = getEntry(key);

return null == entry ? null : entry.getValue();
}

/**
* 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() {
if (size == 0) {
return null;
}
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}

157f8
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) {
if (size == 0) {
return null;
}

//计算key的hash值
int hash = (key == null) ? 0 : hash(key);
//indexFor通过hash值与table.length取余来计算key所在table的位置
for (Entry<K,V> e = table[indexFor(hash, table.length)];
//遍历entry链表，直到最后元素
e != null;
e = e.next) {
Object k;
//判断是同一元素：首先hash要相同，其次key值要相同或相等
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>.)
*/
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
if (key == null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
//判断链表上是否存在同一元素，如果存在就替换
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
//recordAccess()方法与init()一样，都是为了给
//子类重写。比如LinkedHashMap中，它有插入顺序和
//访问顺序，recoredAccess就是记录访问顺序的。
//访问顺序即最新访问的放到entry链表表头.
//LinkedHashMap中有个accessOrder，如果accessOrder为
//false，则为插入顺序，插入顺序即插入到链表表尾，
//true，则为访问顺序，被访问后该entry被移到表头。
//但put()Map中没有的值时，不会触发该方法。
//recordAccess在HashMap中主要在put重复元素的时候被调用,
//相当于该重复元素被访问了。
e.recordAccess(this);
return oldValue;
}
}

modCount++;
addEntry(hash, key, value, i);
return null;
}

/**
* Offloaded version of put for null keys
*/
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++;
addEntry(0, null, value, 0);
return null;
}

/**
* This method is used instead of put by constructors and
* pseudoconstructors（pseudo伪） (clone, readObject).  It does not resize the table,
* check for comodification, etc.  It calls createEntry rather than
* addEntry.
*它和put()不同。putForCreate()是内部方法，它被构造函数等调用，用来创建HashMap
*而put()是对外提供的往HashMap中添加元素的方法。
*/
private void putForCreate(K key, V value) {
int hash = null == key ? 0 : hash(key);
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);
}

private void putAllForCreate(Map<? extends K, ? extends V> m) {
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
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).
*当size达到threshold时为table扩容,但如果数组大小已经为
*MAXIMUM_CAPACITY了，那就调整threshold为MAXIMUM_CAPACITY
*而不再扩容。
*/
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}

Entry[] newTable = new Entry[newCapacity];
//将table中元素转到新数组中
transfer(newTable, initHashSeedAsNeeded(newCapacity));
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}

/**
* Transfers all entries from current table to newTable.
*将table中元素全部转移到新table中，
*reshsh即是否重新计算hash值
*/
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry<K,V> e : table) {
while(null != e) {
Entry<K,V> next = e.next;
if (rehash) {
e.hash = null == e.key ? 0 : hash(e.key);
}
int i = indexFor(e.hash, newCapacity);
//链表节点操作
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
}

/**
* 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
*
*/
public void putAll(Map<? extends K, ? extends V> m) {
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;

if (table == EMPTY_TABLE) {
inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));
}

/*
* 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 (Map.Entry<? extends K, ? extends V> e : m.entrySet())
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>.)
*/
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 Entry<K,V> removeEntryForKey(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
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--;
if (prev == e)
table[i] = next;
else
prev.next = next;
/**
*和recordAccess()等方法一样，该函数在HashMap中也是空的，主要用来重写.
*LinkedHashMap没有重写remove(Object key)方法，重写了被remove调用的recordRemoval方法
*注：这种设计模式叫“模板方法模式”，其中像recordRemovel()这种空或默认被子类重写的
*的方法叫钩子(好形象的命名)
*在LinkedHashMap里面，recordRemoval()用来移除header链表里面Entry的after和before关系
*/
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}

return e;
}

/**
* Special version of remove for EntrySet using {@code Map.Entry.equals()}
* for matching.
*
*/
final Entry<K,V> removeMapping(Object o) {
if (size == 0 || !(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);
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.
*/
public void clear() {
modCount++;
//将数组内元素都变为Null
Arrays.fill(table, 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
*/
public boolean containsValue(Object value) {
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
*/
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;
}
if (result.table != EMPTY_TABLE) {
result.inflateTable(Math.min(
(int) Math.min(
size * Math.min(1 / loadFactor, 4.0f),
// we have limits...
HashMap.MAXIMUM_CAPACITY),
table.length));
}
result.entrySet = null;
result.modCount = 0;
result.size = 0;
result.init();
result.putAllForCreate(this);

return result;
}

static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
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 Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
}

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.
*/
/**
*recordAccess()方法与init()一样，都是为了给
* 子类重写。比如LinkedHashMap中，它有插入顺序和
* 访问顺序，recoredAccess就是记录访问顺序的。
* 访问顺序即最新访问的放到entry链表表头.
* LinkedHashMap中有个accessOrder，如果accessOrder为
* false，则为插入顺序，插入顺序即插入到链表表尾，
* true，则为访问顺序，被访问后该entry被移到表头。
* 但put()Map中没有的值时，不会触发该方法。
* recordAccess在HashMap中主要在put重复元素的时候被调用,
* 相当于该重复元素被访问了。
*/
void recordAccess(HashMap<K,V> m) {
}

/**
* This method is invoked whenever the entry is
* removed from the table.
*/
/**
*和recordAccess()等方法一样，该函数在HashMap中也是空的，主要用来重写.
*LinkedHashMap没有重写remove(Object key)方法，重写了被remove调用的recordRemoval方法
*注：这种设计模式叫“模板方法模式”，其中像recordRemovel()这种空或默认被子类重写的
*的方法叫钩子
*在LinkedHashMap里面，recordRemoval()用来移除header链表里面Entry的after和before关系
*/
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.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) {
resize(2 * table.length);
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}

createEntry(hash, key, value, bucketIndex);
}

/**
* 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.
*/
/**
*从这里可以看出，新增的entry放在链表表头
*/
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
}

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;
/**
*一直循环找到第一个不为null的entry为止
*注意：index++ 相当于t[index]，然后在循环中index++;
*/
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();

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;
HashMap.this.removeEntryForKey(k);
expectedModCount = modCount;
}
}

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
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.
*/
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).
*
* @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
{
// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();

// Write out number of buckets
if (table==EMPTY_TABLE) {
s.writeInt(roundUpToPowerOf2(threshold));
} else {
s.writeInt(table.length);
}

// Write out size (number of Mappings)
s.writeInt(size);

// Write out keys and values (alternating)
if (size > 0) {
for(Map.Entry<K,V> e : entrySet0()) {
s.writeObject(e.getKey());
s.writeObject(e.getValue());
}
}
}

private static final long serialVersionUID = 362498820763181265L;

/**
* Reconstitute the {@code HashMap} instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the threshold (ignored), loadfactor, and any hidden stuff
s.defaultReadObject();
if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
}

// set other fields that need values
table = (Entry<K,V>[]) EMPTY_TABLE;

// Read in number of buckets
/**
* writeObject中按顺序输出两个int，
*所以这里也按顺序读入两个。
*/
s.readInt(); // ignored.

// Read number of mappings
int mappings = s.readInt();
if (mappings < 0)
throw new InvalidObjectException("Illegal mappings count: " +
mappings);

// capacity chosen by number of mappings and desired load (if >= 0.25)
int capacity = (int) Math.min(
mappings * Math.min(1 / loadFactor, 4.0f),
// we have limits...
HashMap.MAXIMUM_CAPACITY);

// allocate the bucket array;
if (mappings > 0) {
inflateTable(capacity);
//当mappings<0时会抛出异常结束，所以这里只有mappings = 0 的时候进来，
//但当mappings为0的时候，capacity也为0
} else {
threshold = capacity;
}

init();  // Give subclass a chance to do its thing.

// Read the keys and values, and put the mappings in the HashMap
for (int i = 0; i < mappings; 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;   }
}