前段时间看了看HashMap 源代码，趁着端午小长假记录一下自己的学习所得。 HashMap实际存储数据的对象是一个数组，数组的每第一个元素都是一个单链的头部。数组的每一个元素是一个内部类Entry 的对象。
// 默认的Entry数组的大小
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.
* Entry数组的最大值
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
*默认加载因子是构造容器阈值的一个参数。
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The table, resized as necessary. Length MUST Always be a power of two.
* Enrty 数组 存储实际数据的地方，长度必须是2的次幂
*/
transient Entry[] table;
/**
* The number of key-value mappings contained in this map.
* 容器的大小
*/
transient int size;
/**
* The next size value at which to resize (capacity * load factor).
* @serial
* 容器的最大容量，由Entry数组长度＊加载因子决定。当装载的对象大于这个最 大容量的时候，就会进行扩容操作。
*/
int threshold;
/**
* 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).
* 容器被改变的次数。新加入一个不存在的key的时候也算是改变容器
*/
transient int modCount;
先看看最为重要的 内部Entry 类。这个类实现Map的Entry<K,V>接口。有这么几个方法:getKey,getValue,setValue,equals,hashCode:
static class Entry<K,V> implements Map.Entry<K,V> {
//前面说过，HashMap是由数组链表构成的。这里的next就是指向链表的下一个节点
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.
* 这个方法 在LinkedHashMap里面被重写了，LinkedHashMap可以按照存入的顺序获取对象，也能按照访问顺序获取对象，当按照访问顺序获取时：1，当put方法被调用，并且key已经存在的时候，调用该方法。2,当get方法被调用也需要调整访问顺序。以后写LinkedHashMap在详细写。
*/
void recordAccess(HashMap<K,V> m) {
}
/**
* This method is invoked whenever the entry is
* removed from the table.
* 当对象被从容器中移除时调用
*/
void recordRemoval(HashMap<K,V> m) {
}
}
接下来看看构造函数：
/**
* 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
//table 数组的长度必须是2的次方
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
this.loadFactor = loadFactor;
threshold = (int)(capacity * loadFactor);
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.
* 使用默认加载因子构造一个HashMap对象
*/
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).
* 使用默认加载因子和默认table容量构造一个HashMap对象
*/
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
* 根据一个Map来构造另一个HashMap
*/
public HashMap(Map<? extends K, ? extends V> m) {
//table的大小取（m的大小／默认加载因子＋1）和默认容量中较大的值，使用默认加载因子。因此下面putAllForCreate就不用考虑容器扩容的问题。
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
putAllForCreate(m);
}
看看put方法：
/**
* 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>.)
* 计算Key对象的hash值，根据hash值和table的长度，计算出这个key应该处在的table数组的位置。之后遍历这个位置的链表如果发现key已经存在了，那么就替换这个key对应的value值，并返回旧的value值，否则就增加一个节点到这个链表结构中，并且容器修改修改次数＋＋。
*/
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
//调用key的hashCode方法，计算hash值。所以使用hashMap的对象要根据实际情况重写hashCode方法，默认的HashCode方法返回的是对象的内存地址。
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
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;
}
}
modCount++;
//如果key不存在的话，增加一个节点
addEntry(hash, key, value, i);
return null;
}
现在看看 hash方法,indexFor方法
/**
* 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.
*/
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).
//使用无符号右移操作符和异或操作符得到一个hash值来降低hash值冲突的概率
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
/**
* Returns index for hash code h.返回数组下标，length-1是为了防止下标越界
*/
static int indexFor(int h, int length) {
return h & (length-1);
}
现在看看 key=null的情况
private V putForNullKey(V value) {
//null不具备hashCode方法。所以null 的hash值被默认设置为0.因此只有可能被放在table的第一个元素上。
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++;
//如果不存在，增加一个节点，并且设置hash值为0,数组下表为0
addEntry(0, null, value, 0);
return null;
}
一个小问题，为什么不在put方法里进行判断key是否为null。如果是就将i和hash赋值为0，为什么单独写一个putForNullKey，其中有大部分的代码都是可以重用的。
下面看看addEntry方法
void addEntry(int hash, K key, V value, int bucketIndex) {
// 获取 table bucketIndex处的元素，该元素是此处链表的表头
Entry<K,V> e = table[bucketIndex];
//将这个元素作为 新生成的Entry对象的next 指向的下一个元素。新生成的对象替代该元素成为此处链表的表头
table[bucketIndex] = new Entry<>(hash, key, value, e);
//判断是否已经超过容器的极限值
if (size++ >= threshold)
//是的话，进行容器扩容操作
resize(2 * table.length);
}
接下来看看resize方法
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
//如果table的长度已经是最大值了，那么就将容器的容量极限值设置为integer的最大值，并且不在进行扩容操作。
threshold = Integer.MAX_VALUE;
return;
}
//否则进行扩容操作。
Entry[] newTable = new Entry[newCapacity];
transfer(newTable);
table = newTable;
//重新设置容器的极限值
threshold = (int)(newCapacity * loadFactor);
}
transfer方法，该方法在linkedHashMap 出于效率的考虑重写过，这个以后在说。
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
//遍历老的容器，将容器的元素放到新的newTable数组链表中
for (int j = 0; j < src.length; j++) {
Entry<K,V> e = src[j];
if (e != null) {
src[j] = null;
//遍历链表
do {
Entry<K,V> next = e.next;
//获取该节点在新容器里的数组下标
int i = indexFor(e.hash, newCapacity);
//将e插入到新链表的表头
e.next = newTable[i];
newTable[i] = e;
//e 指向老链表的下一个元素
e = next;
} while (e != null);
}
}
}
注意到经过扩容之后，原先到HashMap的元素顺序已经完全打乱了。
还记得根据一个map创造一个HashMap对象的构造方法吗？构造方法调用了如下方法：
private void putAllForCreate(Map<? extends K, ? extends V> m) {
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
putForCreate(e.getKey(), e.getValue());
}
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);
}
大部分代码比较直白，看看createEntry方法。相比与addEntry方法只是不需要扩容了，因为在构造函数里已经对容量做了计算
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++;
}
在看和get相关的方法：
public V get(Object key) {
if (key == null)
return getForNullKey();
int hash = hash(key.hashCode());
//是一个o（n）的遍历
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;
}
private V getForNullKey() {
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}
//根据key获取一个Entry对象
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;
}
都很简单，也不多说了。 再来看看HashMap的迭代器设计,map类本身并没有提供迭代器方法。你只能获取一个map的key的Set或者value的Collection或者entry的Set，然后调用这些set对象的迭代器方法获取迭代器对象。
//这是Values 私有类。可以通过values()方法获取到一个map的Values对象，这个对象的iterator方法可以获取到该对象的迭代器对象。
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();
}
}
Iterator<V> newValueIterator() {
return new ValueIterator();
}
//value迭代器对象，继承自HashIterator 重写了next方法，如果是EntrySet的话就是return nextEntry（），如果是KeySet就是return nextEntry().key。nextEntry()方法是HashIterator对象的方法。
private final class ValueIterator extends HashIterator<V> {
public V next() {
return nextEntry().value;
}
}
HashMap的私有迭代器抽象类，EntrySet的迭代器类，values的迭代器，keySet的迭代器类都是继承自这个类。注意到没有，这个类实现类Iterator接口，但是却没有实现next方法。还记得前一篇文章说的吗?抽象类实现接口的意义。因为这个next方法会根据几个子迭代器不同而改变，但是迭代器的其它方法却都是一样的。所以采用这种设计模式。
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 方法不能被重写
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;
}
}
接下来看看EntrySet和KeySet的迭代器设计，和Values的大同小异,就是迭代器的next方法的返回值不一样。
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();
}
}
Iterator<K> newKeyIterator() {
return new KeyIterator();
}
private final class KeyIterator extends HashIterator<K> {
public K next() {
return nextEntry().getKey();
}
}
在贴一下EntySet的代码：
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();
}
}
Iterator<Map.Entry<K,V>> newEntryIterator() {
return new EntryIterator();
}
private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
public Map.Entry<K,V> next() {
return nextEntry();
}
}