HashMap基于哈希表的Map接口的实现。此实现提供所有可选的映射操作，并允许使用null值和null键。此类不保证映射的顺序，特别是它不保证该顺序恒久不变。并且该类是非线程安全的。
HashMap的底层是一个数组加链表的集合。它的数据结构如下：
HashMap底层是一个table数组，数组的长度(table.length)始终要求是2的N次幂。每次在该HashMap中在新增一个元素时(内部会将该元素的信息封装成Entry<K,V>)，会先对该元素的键通过内部的一个函数(散列函数)求一个散列码(int型)，然后将该散列码与(数组的长度-1)进行与运算，得出table数组的下标，存在该处。如果该处已经有元素(不同的键的散列码可能相等)，就在这个新Entry<K,V>中的next属性保存上一次的Entry<K,V>的引用，依次，就在该数组的某项组成了链表。
其中table中的每一项都称为一个桶位。数组的长度要求是2的N次幂，可以保证在减1后所对应的二进制位均为1，并且(length-1)&(散列码)之后，会落在0-length之间，这样就能保证生成的数组下标不越界。
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable{//JDK1.7 java.util
//默认初始容量，必须为2的指数倍，因为可能使用该值初始化数组长度时。
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
//最大容量，必须为2的指数倍，因为可能使用该值初始化数组长度时。
static final int MAXIMUM_CAPACITY = 1 << 30;
//加载因子默认值。0.75是对空间和时间效率的一个平衡选择。当容量超出此最大容量时， resize 后的 HashMap 容量是容量的两倍
static final float DEFAULT_LOAD_FACTOR = 0.75f;
static final Entry<?,?>[] EMPTY_TABLE = {};//无数据时的空数组
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;//存储数据的Entry数组
transient int size;//HashMap中实际键值对的数量
//此 loadFactor和 capacity下允许的最大元素数目，超过这个数目就重新 resize，以降低实际的负载因子。threshold = capacity*loadFactor
int threshold;
final float loadFactor;//加载因子。loadFactor = size/capacity
transient int modCount;//修改次数
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;//0x7fffffff
public HashMap(int initialCapacity, float loadFactor) {//构造一个带指定初始容量和加载因子的空 HashMap
if (initialCapacity < 0)
//初始容量校验
throw new IllegalArgumentException("Illegal initial capacity: " +initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;//指定容量不能超过MAXIMUM_CAPACITY：1 << 30
//加载因子校验
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
//赋值
this.loadFactor = loadFactor;
threshold = initialCapacity;//初始时候直接将容量赋值给threshold。
init();
}
void init() {//空方法
}
//构造一个带指定初始容量和默认加载因子 (0.75) 的空 HashMap
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
//构造一个具有默认初始容量 (16)和默认加载因子 (0.75)的空 HashMap
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
//构造一个映射关系与指定 Map相同的新 HashMap
public HashMap(Map<? extends K, ? extends V> m) {
//容量为m的元素个数/0.75 和 默认初始容量中的较大值
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
inflateTable(threshold);
putAllForCreate(m);
}
private void putAllForCreate(Map<? extends K, ? extends V> m) {
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
putForCreate(e.getKey(), e.getValue());//遍历存放键值对
}
private void putForCreate(K key, V value) {
int hash = null == key ? 0 : hash(key);//散列key
int i = indexFor(hash, table.length);//获取底层数组的索引
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
//判断键相等的两个条件。散列码和key
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) {
e.value = value;
return;
}
}
createEntry(hash, key, value, i);
}
void createEntry(int hash, K key, V value, int bucketIndex) {
//获取原来此处已经存放的所有Entry<K,V>
Entry<K,V> e = table[bucketIndex];
//添加新的Entry<>，并且将新的Entry<>里的next属性指向上一个Entry<K,V>。依次递推，这样每次添加新的，都与原来的形成了一个链表。
table[bucketIndex] = new Entry<>(hash, key, value, e);//table[i]数组里的元素永远是最后添加进去的Entry<>
size++;//数量加1
}
private void inflateTable(int toSize) {
//2的N次方。见下面函数
int capacity = roundUpToPowerOf2(toSize);
//初始化threshold的值
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
table = new Entry[capacity];//初始化数组
initHashSeedAsNeeded(capacity);//初始化HashSeed值
}
private static int roundUpToPowerOf2(int number) {
//Integer.highestOneBit(int i)返回的则是跟它最靠近的比它小的2的N次方
return number >= MAXIMUM_CAPACITY
? MAXIMUM_CAPACITY
: (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;
}
transient int hashSeed = 0;//哈希因子。用于计算key的hash值
final boolean initHashSeedAsNeeded(int capacity) {//初始化哈希因子hashSeed，32位，高16位始终为0
boolean currentAltHashing = hashSeed != 0;
boolean useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean switching = currentAltHashing ^ useAltHashing;
if (switching) {
//对hashSeed赋值
hashSeed = useAltHashing
? sun.misc.Hashing.randomHashSeed(this)
: 0;
}
return switching;
}
private static class Holder {
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) {
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;
}
}
final int hash(Object k) {//散列函数。根据 key求散列码
int h = hashSeed;
if (0 != h && k instanceof String) {//单独计算String的hash值
return sun.misc.Hashing.stringHash32((String) k);
}
//计算算法
h ^= k.hashCode();
h ^= (h >>> 20) ^ (h >>> 12);//异或
return h ^ (h >>> 7) ^ (h >>> 4);
}
public int size() {//返回键值对的数量
return size;
}
public boolean isEmpty() {//是否为空
return size == 0;
}
public V get(Object key) {//返回指定键所映射的值
if (key == null)
return getForNullKey();//允许键为null
Entry<K,V> entry = getEntry(key);
return null == entry ? null : entry.getValue();
}
private V getForNullKey() {//当键为null时
if (size == 0) {
return null;
}
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;//返回键为null时的value
}
return null;//没有则返回null
}
public boolean containsKey(Object key) {//是否包含对于指定键的映射关系
return getEntry(key) != null;
}
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;//map为空则返回null
}
int hash = (key == null) ? 0 : hash(key);//根据key求散列码
for (Entry<K,V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) {
Object k;
//判断键相等的两个条件。散列码和key
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}
static int indexFor(int h, int length) {
return h & (length-1);//初始化时始终保证length为2的N次幂，减1后，与h求与运算始终在区间[0,length)中，不会越界
}
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);//此时会初始化底层数组，数组长度将是2的N次幂
}
if (key == null)
return putForNullKey(value);//允许键值为null
int hash = hash(key);//对key求散列码
int i = indexFor(hash, table.length);//找到table的索引
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
//hash值判断和键需要同时判断
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);//空方法
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}
private V putForNullKey(V value) {//允许键值为null
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null) {
V oldValue = e.value;//若存在该键，则更新值
e.value = value;
e.recordAccess(this);//空方法
return oldValue;//返回旧值
}
}
//若不存在该键
modCount++;//修改次数+1
addEntry(0, null, value, 0);
return null;
}
void addEntry(int hash, K key, V value, int bucketIndex) {
//元素是否超过阈值，该桶位是否存放了元素
if ((size >= threshold) && (null != table[bucketIndex])) {
resize(2 * table.length);//扩容，重新散列，即重新分布数组
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);//重新计算散列码(桶位的索引)
}
//元素太多超过阈值并且该桶位还没有存放元素
createEntry(hash, key, value, bucketIndex);//增加键值对
}
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;//原来的长度为默认最大值时
return;
}
Entry[] newTable = new Entry[newCapacity];//新建数组
transfer(newTable, initHashSeedAsNeeded(newCapacity));//重新初始化哈希因子hashSeed
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry<K,V> e : table) {//遍历table
while(null != e) {
//遍历数组里的Entry
Entry<K,V> next = e.next;
if (rehash) {
e.hash = null == e.key ? 0 : hash(e.key);//计算散列码
}
int i = indexFor(e.hash, newCapacity);
//将原来数据存放在心的数组中
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
}
//将指定映射的所有映射关系复制到此映射中
public void putAll(Map<? extends K, ? extends V> m) {
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;
if (table == EMPTY_TABLE) {//是否为空，为空需要进行初始化
inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));
}
if (numKeysToBeAdded > threshold) {//判断是否超过了阈值
int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
if (targetCapacity > MAXIMUM_CAPACITY)
targetCapacity = MAXIMUM_CAPACITY;
int newCapacity = table.length;
while (newCapacity < targetCapacity)
newCapacity <<= 1;//一直乘以2，保证是2的N次幂
if (newCapacity > table.length)
resize(newCapacity);//重新散列
}
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
put(e.getKey(), e.getValue());//存放元素
}
public V remove(Object key) {//从此映射中移除指定键的映射关系
Entry<K,V> e = removeEntryForKey(key);
return (e == null ? null : e.value);
}
final Entry<K,V> removeEntryForKey(Object key) {
if (size == 0) {
return null;//当map为空时，直接null
}
int hash = (key == null) ? 0 : hash(key);//获取散列码
int i = indexFor(hash, table.length);//获取索引
Entry<K,V> prev = table[i];//拿到桶位中的Entry<K,V>
Entry<K,V> e = prev;
while (e != null) {
Entry<K,V> next = e.next;//获取上一个Entry<K,V>
Object k;
//hash和key必须都相等
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) {
modCount++;//修改次数+1
size--;//元素个数减1
if (prev == e)
//如果prev == e，即要删除的键值对是最晚添加进去的，此时需要把上一个Entry复制给table[i]数组里的值
table[i] = next;
else
//如果要删除的键值对不是最晚添加进去的，此时需要把上一个Entry复制给下一个Entry里的next
prev.next = next;
e.recordRemoval(this);//空方法
return e;
}
prev = e;
e = next;//继续遍历
}
return e;
}
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;
}
public void clear() {
modCount++;//修改次数+1
Arrays.fill(table, null);//全部赋值为null，方便回收
size = 0;//size为0
}
public boolean containsValue(Object value) {//如果此映射包含指定值，则返回 true
if (value == null)
return containsNullValue();//允许值为null
Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)//遍历数组
for (Entry e = tab[i] ; e != null ; e = e.next)//遍历数组里的Entry
if (value.equals(e.value))
return true;
return false;
}
private boolean containsNullValue() {//值为null时
Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)//遍历数组
for (Entry e = tab[i] ; e != null ; e = e.next)//遍历数组里的Entry
if (e.value == null)
return true;
return false;
}
public Object clone() {//浅复制
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);//存放所有键值对
return result;
}
//静态类。实现了Map.Entry<K,V>接口
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;//存放当前键值对的key
V value;//存放当前键值对的value
Entry<K,V> next;//指向上一个Entry<K,V>
int hash;//存放当前所处桶位的散列码
Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}
public final K getKey() {//获取key
return key;
}
public final V getValue() {//获取value
return value;
}
public final V setValue(V newValue) {//修改value
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {//equals
if (!(o instanceof Map.Entry))
return false;//必须是Map.Entry或其子类
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() {//哈希值，键与值异或
return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
}
public final String toString() {//toString
return getKey() + "=" + getValue();
}
void recordAccess(HashMap<K,V> m) {//空方法
}
void recordRemoval(HashMap<K,V> m) {//空方法
}
}
public Set<K> keySet() {//返回key的set集合
Set<K> ks = keySet;//继承自父类的keySet，键集合
return (ks != null ? ks : (keySet = new KeySet()));
}
private final class KeySet extends AbstractSet<K> {
public Iterator<K> iterator() {//迭代器
return newKeyIterator();
}
public int size() {//元素个数
return size;//父类的size
}
public boolean contains(Object o) {
return containsKey(o);//是否包含某个key
}
public boolean remove(Object o) {//移除
return HashMap.this.removeEntryForKey(o) != null;//直接调用外部类的移除
}
public void clear() {//清空
HashMap.this.clear();//直接调用外部类的清空
}
}
Iterator<K> newKeyIterator() {
return new KeyIterator();
}
private final class KeyIterator extends HashIterator<K> {
public K next() {
return nextEntry().getKey();
}
}
private abstract class HashIterator<E> implements Iterator<E> {
Entry<K,V> next; //下一次迭代时返回的Entry
int expectedModCount; // 期望修改次数 fast-fail机制
int index; // 当前桶位索引
Entry<K,V> current; // 当前的Entry
HashIterator() {
expectedModCount = modCount;//初始化期望修改次数，此时等于实际修改次数
if (size > 0) {
Entry[] t = table;
//获取第一个不为null的桶并赋值给next。此为迭代的起点。没有元素时，next为null
while (index < t.length && (next = t[index++]) == null)
;
}
}
//是否可以继续迭代
public final boolean hasNext() {
return next != null;
}
final Entry<K,V> nextEntry() {
if (modCount != expectedModCount)//实际修改次数与期望修改次数比较
throw new ConcurrentModificationException();
Entry<K,V> e = next;
if (e == null)
throw new NoSuchElementException();//此时next即为null，没有可迭代的元素
if ((next = e.next) == null) {
Entry[] t = table;
//为下一次迭代做准备，越过数组中的null项
while (index < t.length && (next = t[index++]) == null)
;
}
current = e;
return e;
}
public void remove() {//移除
if (current == null)
throw new IllegalStateException();//此时没有元素移除
if (modCount != expectedModCount)
throw new ConcurrentModificationException();//实际修改次数与期望修改次数比较
Object k = current.key;//拿到当前键值对的key
current = null;//清空
HashMap.this.removeEntryForKey(k);//调用外部类的删除
expectedModCount = modCount;//删除时，修改次数会+1，重新赋值
}
}
public Collection<V> values() {//值的集合
Collection<V> vs = values;//继承自父类
return (vs != null ? vs : (values = new Values()));
}
private final class Values extends AbstractCollection<V> {
public Iterator<V> iterator() {
return newValueIterator();//返回迭代器
}
public int size() {//元素个数
return size;//重复值算不同的值
}
public boolean contains(Object o) {//是否包含某个值
return containsValue(o);
}
public void clear() {//清空
HashMap.this.clear();
}
}
Iterator<V> newValueIterator() {
return new ValueIterator();//迭代器
}
private final class ValueIterator extends HashIterator<V> {//迭代器内部实现
public V next() {
return nextEntry().value;//迭代值
}
}
private transient Set<Map.Entry<K,V>> entrySet = null;//存放键值对
public Set<Map.Entry<K,V>> entrySet() {
return entrySet0();
}
private Set<Map.Entry<K,V>> entrySet0() {//内部实现
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() {//迭代器
return newEntryIterator();
}
//
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;//必须是Map.Entry或其子类
Map.Entry<K,V> e = (Map.Entry<K,V>) o;
Entry<K,V> candidate = getEntry(e.getKey());
return candidate != null && candidate.equals(e);//Entry的比较
}
public boolean remove(Object o) {
return removeMapping(o) != null;//移除
}
public int size() {
return size;//大小
}
public void clear() {
HashMap.this.clear();//清空
}
}
Iterator<Map.Entry<K,V>> newEntryIterator() {
return new EntryIterator();
}
//EntryIterator内部实现
private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
public Map.Entry<K,V> next() {
return nextEntry();
}
}
private static final long serialVersionUID = 362498820763181265L;//版本号
//序列化
private void writeObject(java.io.ObjectOutputStream s) throws IOException{
s.defaultWriteObject();
if (table==EMPTY_TABLE) {
s.writeInt(roundUpToPowerOf2(threshold));//写入threshold
} else {
s.writeInt(table.length);//写入length
}
s.writeInt(size);//写入size
if (size > 0) {
for(Map.Entry<K,V> e : entrySet0()) {
s.writeObject(e.getKey());//写入键
s.writeObject(e.getValue());//写入值
}
}
}
//反序列化
private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException{
s.defaultReadObject();
if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
}
table = (Entry<K,V>[]) EMPTY_TABLE;
s.readInt(); // ignored.
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), HashMap.MAXIMUM_CAPACITY);
if (mappings > 0) {
inflateTable(capacity);
} else {
threshold = capacity;
}
init();
for (int i = 0; i < mappings; i++) {
K key = (K) s.readObject();
V value = (V) s.readObject();
putForCreate(key, value);
}
}
int capacity() { return table.length; }//获取长度
float loadFactor() { return loadFactor; }//获取加载因子
}
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