引用一张网友的图：
首先，理解这个数据结构，那么HashMap就理解了一半。重点看Entry的定义：
static class Entry<K, V> implements Map.Entry<K, V> {
final K key;
V value;
Entry<K, V> next; //这个很重要，也就是链表结构的关键。 在Entry内部持有一个Entry对象，行程一个链表结构。
final int hash;
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}再看put方法：
public V put(K key, V value) {
if (key == null)//这里有putForNullKey方法，HashMap支持null为键值，key=null的时候hash为0
return putForNullKey(value);//这里hash这个函数很有意思，主要是用于计算hashcode的hash值，再根据indexFor方法定义table中存放的位置。
int hash = hash(key.hashCode());
//定义捅的位置
int i = indexFor(hash, table.length);
//找到待添加元素的位置，如果没有，就不用执行for函数了
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;
}
}
//for循环中没有找到有hash相同的元素，则添加一个新的Entry
modCount++;
//hash值，key，value，桶的位置
addEntry(hash, key, value, i);
return null;
}
再看一个关键的addEntry方法：
void addEntry(int hash, K key, V value, int bucketIndex) {
//没啥好说的，直接添加一个table的桶位置（把table堪称一个bucket桶。。。）
Entry<K, V> e = table[bucketIndex];
//定位table的位置，然后，创建一个entry放在这个问题。
table[bucketIndex] = new Entry<K, V>(hash, key, value, e);
//在table这个大桶中添加了一个元素，则size个数增加1个，如果这个个数大于门限值，则捅扩大一倍！
if (size++ >= threshold)
resize(2 * table.length);
}
addEntry方法会引起扩容，每次添加后size加1！如果个数超出门限值，扩容都是以2的指数，因为每次都是扩大2倍。在jdk8中这源码有所改变。
再看扩容代码：扩容会创建一个新的数组，这是开销的主要来源
void resize(int newCapacity) {
//扩容
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
//这里创建一个新的容量的Entry。
Entry[] newTable = new Entry[newCapacity];
//把之前的元素全部存入到新的newTable中
transfer(newTable);
//table变量指向新的Table
table = newTable;
//更新门限值。
threshold = (int) (newCapacity * loadFactor);
}再看一个get方法：get方法中主要是因为HashMap的数据结构所致，在查询的时候，先找到table所在位置，也就是桶的位置，然后再遍历桶中链表中元素，如果元素的hash值相同，那么还需要进一步判断key的值，也就是equals方法。
public V get(Object key) {
if (key == null)
return getForNullKey();
//调用hashcode
int hash = hash(key.hashCode());
//通过hash值找到在table存储的位置，同理getForNullKey，位置相同，再遍历里面的链表结构，当hash相同，还要进一步判断key的值是否相同。
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.
* 默认key为null的哈希索引为0，所以这里取table[0],e.next也是一个Entry<K, V>对象，
* 这里，遍历e的链表头不为null，再调用下key的==来判断值是否相等。
* 其实这里就刚好说明2个问题，一，递增都是以table结构来存储数据，同时table里面的元素又是一个链表结构。
* 二，key的哈希相同的会放在同一个table的元素中，table元素是个链表结构，当key的哈希相同（存储地址在table的同一个位置），还要判断key的值是否相同。
*这也就是重写了hashcode还要重新equals方法
*
*/
private V getForNullKey() {
for (Entry<K, V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}至此，源码就基本差不多了。到了jdk8之后，增加了红黑色来代替链表结构。本质上，这是一个安全问题。链表的查询是线性的，会影响存储的效率。如果构造hash值相当的数据存储到HashMap中，将会导致cpu性能急剧被占用，而导致服务异常。
最后贴上jdk1.6.0_45中源码：
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.
*/
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.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient Entry[] table;
/**
* The number of key-value mappings contained in this map.
*/
transient int size;
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial 门限值
*/
int threshold;
/**
* The load factor for the hash table.
* 负载因子表示一个散列表的空间的使用程度，有这样一个公式：initailCapacity*loadFactor=HashMap的容量。
*
* 所以负载因子越大则散列表的装填程度越高，也就是能容纳更多的元素，元素多了，链表大了，所以此时索引效率就会降低。
*
* 反之，负载因子越小则链表中的数据量就越稀疏，此时会对空间造成烂费，但是此时索引效率高。
*
* @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 volatile int modCount;
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial capacity and
* load factor.
*
* @param initialCapacity
* the initial capacity
* @param loadFactor
* the load factor
* @throws IllegalArgumentException
* if the initial capacity is negative or the load factor is
* nonpositive
*/
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);
// Find a power of 2 >= initialCapacity
int capacity = 1;
while (capacity < initialCapacity)
//如果指定的初始化容量大于默认的容量，则扩容2倍
capacity <<= 1;
this.loadFactor = loadFactor;
//门限值
threshold = (int) (capacity * loadFactor);
//底层的存在kv使用entry数组
table = new Entry[capacity];
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.loadFactor = DEFAULT_LOAD_FACTOR;
threshold = (int) (DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
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>.
*
* @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);
putAllForCreate(m);
}
// internal utilities
/**
* Initialization hook for subclasses. This method is called in all constructors
* and pseudo-constructors (clone, readObject) after HashMap has been
* initialized but before any entries have been inserted. (In the absence of
* this method, readObject would require explicit knowledge of subclasses.)
*/
void init() {
}
/**
* Applies a supplemental hash function to a given hashCode, which defends
* against poor quality hash functions. This is critical because HashMap uses
* power-of-two length hash tables, that otherwise encounter collisions for
* hashCodes that do not differ in lower bits. Note: Null keys always map to
* hash 0, thus index 0.
* hash计算方法，高位移动
* 这里需要将高位数据移位到低位进行异或运算呢？这是因为有些数据计算出的哈希值差异主要在高位，而 HashMap 里的哈希寻址是忽略容量以上的高位的，那么这种处理就可以有效避免类似情况下的哈希碰撞。
*/
static int hash(int h) {
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
/**
* Returns index for hash code h.
*/
static int indexFor(int h, int length) {
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)
return getForNullKey();
//调用hashcode
int hash = hash(key.hashCode());
//通过hash值找到在table存储的位置，同理getForNullKey，位置相同，再遍历里面的链表结构，当hash相同，还要进一步判断key的值是否相同。
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.
* 默认key为null的哈希索引为0，所以这里取table[0],e.next也是一个Entry<K, V>对象，
* 这里，遍历e的链表头不为null，再调用下key的==来判断值是否相等。
* 其实这里就刚好说明2个问题，一，递增都是以table结构来存储数据，同时table里面的元素又是一个链表结构。
* 二，key的哈希相同的会放在同一个table的元素中，table元素是个链表结构，当key的哈希相同（存储地址在table的同一个位置），还要判断key的值是否相同。
*这也就是重写了hashcode还要重新equals方法
*
*/
private V getForNullKey() {
for (Entry<K, V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}
/**
* Returns <tt>true</tt> if this map contains a mapping for the specified key.
*
* @param key
* The key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified key.
*/
public boolean containsKey(Object key) {
return getEntry(key) != null;
}
/**
* Returns the entry associated with the specified key in the HashMap. Returns
* null if the HashMap contains no mapping for the key.
*/
final Entry<K, V> getEntry(Object key) {
int hash = (key == null) ? 0 : hash(key.hashCode());
for (Entry<K, V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}
/**
* Associates the specified value with the specified key in this map. If the map
* previously contained a mapping for the key, the old value is replaced.
*
* @param key
* key with which the specified value is to be associated
* @param value
* value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or <tt>null</tt> if
* there was no mapping for <tt>key</tt>. (A <tt>null</tt> return can
* also indicate that the map previously associated <tt>null</tt> with
* <tt>key</tt>.)
*/
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
//定义捅的位置
int i = indexFor(hash, table.length);
//找到待添加元素的位置，如果没有，就不用执行for函数了
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;
}
}
//for循环中没有找到有hash相同的元素，则添加一个新的Entry
modCount++;
//hash值，key，value，桶的位置
addEntry(hash, key, value, i);
return null;
}
/**
* Offloaded version of put for null keys
* 存放null值
*/
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
* (clone, readObject). It does not resize the table, check for comodification,
* etc. It calls createEntry rather than addEntry.
*/
private void putForCreate(K key, V value) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
/**
* Look for preexisting entry for key. This will never happen for clone or
* deserialize. It will only happen for construction if the input Map is a
* sorted map whose ordering is inconsistent w/ equals.
*/
for (Entry<K, V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) {
e.value = value;
return;
}
}
createEntry(hash, key, value, i);
}
private void putAllForCreate(Map<? extends K, ? extends V> m) {
for (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext();) {
Map.Entry<? extends K, ? extends V> e = i.next();
putForCreate(e.getKey(), e.getValue());
}
}
/**
* Rehashes the contents of this map into a new array with a larger capacity.
* This method is called automatically when the number of keys in this map
* reaches its threshold.
*
* If current capacity is MAXIMUM_CAPACITY, this method does not resize the map,
* but sets threshold to Integer.MAX_VALUE. This has the effect of preventing
* future calls.
*
* @param newCapacity
* the new capacity, MUST be a power of two; must be greater than
* current capacity unless current capacity is MAXIMUM_CAPACITY (in
* which case value is irrelevant).
*/
void resize(int newCapacity) {
//扩容
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
//这里创建一个新的容量的Entry。
Entry[] newTable = new Entry[newCapacity];
//把之前的元素全部存入到新的newTable中
transfer(newTable);
//table变量指向新的Table
table = newTable;
//更新门限值。
threshold = (int) (newCapacity * loadFactor);
}
/**
* Transfers all entries from current table to newTable.
*/
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
for (int j = 0; j < src.length; j++) {
Entry<K, V> e = src[j];
if (e != null) {
src[j] = null;
do {
Entry<K, V> next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
} while (e != null);
}
}
}
/**
* Copies all of the mappings from the specified map to this map. These mappings
* will replace any mappings that this map had for any of the keys currently in
* the specified map.
*
* @param m
* mappings to be stored in this map
* @throws NullPointerException
* if the specified map is null
*/
public void putAll(Map<? extends K, ? extends V> m) {
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;
/*
* Expand the map if the map if the number of mappings to be added is greater
* than or equal to threshold. This is conservative; the obvious condition is
* (m.size() + size) >= threshold, but this condition could result in a map with
* twice the appropriate capacity, if the keys to be added overlap with the keys
* already in this map. By using the conservative calculation, we subject
* ourself to at most one extra resize.
*/
if (numKeysToBeAdded > threshold) {
int targetCapacity = (int) (numKeysToBeAdded / loadFactor + 1);
if (targetCapacity > MAXIMUM_CAPACITY)
targetCapacity = MAXIMUM_CAPACITY;
int newCapacity = table.length;
while (newCapacity < targetCapacity)
//扩容一倍
newCapacity <<= 1;
if (newCapacity > table.length)
resize(newCapacity);
}
for (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext();) {
Map.Entry<? extends K, ? extends V> e = i.next();
put(e.getKey(), e.getValue());
}
}
/**
* Removes the mapping for the specified key from this map if present.
*
* @param key
* key whose mapping is to be removed from the map
* @return the previous value associated with <tt>key</tt>, or <tt>null</tt> if
* there was no mapping for <tt>key</tt>. (A <tt>null</tt> return can
* also indicate that the map previously associated <tt>null</tt> with
* <tt>key</tt>.)
*/
public V remove(Object key) {
//删除元素，如果不存在就返回null
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) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
//删除的时候先根据hash定位大table中位置，如果为空，就返回e，e也是等于table[i]等于null
Entry<K, V> prev = table[i];
Entry<K, V> e = prev;
//如果在table有元素。
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;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
}
/**
* Special version of remove for EntrySet.
*/
final Entry<K, V> removeMapping(Object o) {
if (!(o instanceof Map.Entry))
return null;
Map.Entry<K, V> entry = (Map.Entry<K, V>) o;
Object key = entry.getKey();
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
Entry<K, V> prev = table[i];
Entry<K, V> e = prev;
while (e != null) {
Entry<K, V> next = e.next;
if (e.hash == hash && e.equals(entry)) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
}
/**
* Removes all of the mappings from this map. The map will be empty after this
* call returns.
*/
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.
*
* @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;
}
result.table = new Entry[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;
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.
*/
void recordAccess(HashMap<K, V> m) {
}
/**
* This method is invoked whenever the entry is removed from the table.
*/
void recordRemoval(HashMap<K, V> m) {
}
}
/**
* Adds a new entry with the specified key, value and hash code to the specified
* bucket. It is the responsibility of this method to resize the table if
* appropriate.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
//没啥好说的，直接添加一个table的桶位置（把table堪称一个bucket桶。。。）
Entry<K, V> e = table[bucketIndex];
//定位table的位置，然后，创建一个entry放在这个问题。
table[bucketIndex] = new Entry<K, V>(hash, key, value, e);
//在table这个大桶中添加了一个元素，则size个数增加1个，如果这个个数大于门限值，则捅扩大一倍！
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.
*
* Subclass overrides this to alter the behavior of HashMap(Map), clone, and
* readObject.
*/
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry<K, V> e = table[bucketIndex];
table[bucketIndex] = new Entry<K, V>(hash, key, value, e);
//和addEntry的区别是，最后没有判断是否要扩容
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;
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 {
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).
*/
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;
}
}
参考文献：
http://hg.openjdk.java.net/jdk/jdk/file/5b75d7485f2a/src/java.base/share/classes/java/util/HashMap.java
https://blog.csdn.net/luanlouis/article/details/41576373