java util包学习（9）HashMap源码分析

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

public class HashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
{

static final int DEFAULT_INITIAL_CAPACITY = 16;//默认容量

static final int MAXIMUM_CAPACITY = 1 << 30;//最大容量

static final float DEFAULT_LOAD_FACTOR = 0.75f;//加载因子
transient Entry[] table;//一个数组用hash函数定位

/*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。*/
transient int size;//长度

//使用transient修饰符来标识一个成员变量在序列化子系统中应被忽略

int threshold;

final float loadFactor;
/*Volatile修饰的成员变量在每次被线程访问时，都强迫从主内存中重读该成员变量的值。而且，当成员变量发生变化时，强迫线程将变化值回写到主内存。这样在任何时刻，两个不同的线程总是看到某个成员变量的同一个值。   */

transient volatile int modCount;//这也是用于快速失败的

public HashMap(int initialCapacity, float loadFactor) {//自己制定容量和加载因子
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)
capacity <<= 1;

this.loadFactor = loadFactor;
threshold = (int)(capacity * loadFactor);
table = new Entry[capacity];
init();
}

public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;
threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
table = new Entry[DEFAULT_INITIAL_CAPACITY];
init();
}

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);
}

void init() {
}

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);//无符号右移一位
return h ^ (h >>> 7) ^ (h >>> 4);//^异或运算
}

static int indexFor(int h, int length) {
return h & (length-1);//&位与
}

public int size() {
return size;
}

public boolean isEmpty() {
return size == 0;
}

public V get(Object key) {
if (key == null)
return getForNullKey();
int hash = hash(key.hashCode());//key使用hash函数定位
for (Entry e = table[indexFor(hash, table.length)];//取出值链式存储的
e != null;
e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k)))//hash一样equals一样就一样了
return e.value;
}
return null;
}

private V getForNullKey() {
for (Entry e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;//得到空键的值。。
}
return null;
}

public boolean containsKey(Object key) {
return getEntry(key) != null;//没有键的值对应是null的
}

final Entry getEntry(Object key) {
int hash = (key == null) ? 0 : hash(key.hashCode());//null hash就是0
for (Entry 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;
}

public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());//调用自己写的hashcode
int i = indexFor(hash, table.length);//索引之后才完成了定位啊
for (Entry 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++;//呵呵
addEntry(hash, key, value, i);
return null;
}

private V putForNullKey(V value) {
for (Entry e = table[0]; e != null; e = e.next) {
if (e.key == null) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;//NULL的值只能有一个put会产生覆盖
}
}
modCount++;
addEntry(0, null, value, 0);
return null;
}

private void putForCreate(K key, V value) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);

for (Entry 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());
}
}

//数组重现变化吧
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];
transfer(newTable);
table = newTable;
threshold = (int)(newCapacity * loadFactor);
}

void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
for (int j = 0; j < src.length; j++) {
Entry e = src[j];
if (e != null) {
src[j] = null;
do {
Entry next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];//复制老东西
newTable[i] = e;
e = next;
} while (e != null);
}
}
}

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

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 (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());
}
}

public V remove(Object key) {
Entry e = removeEntryForKey(key);
return (e == null ? null : e.value);
}

final Entry removeEntryForKey(Object key) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
Entry prev = table[i];
Entry e = prev;

while (e != null) {
Entry 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 removeMapping(Object o) {
if (!(o instanceof Map.Entry))
return null;

Map.Entry entry = (Map.Entry) o;
Object key = entry.getKey();
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
Entry prev = table[i];
Entry e = prev;

while (e != null) {
Entry 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++;
Entry[] tab = table;
for (int i = 0; i < tab.length; i++)
tab[i] = null;
size = 0;
}

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;
}

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;
}

public Object clone() {
HashMap result = null;
try {
result = (HashMap)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 implements Map.Entry {
final K key;//静态的值
V value;
Entry next;//难道还有指针么？？
final int hash;//hash不可变

Entry(int h, K k, V v, Entry 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) {//两个的比较方法就是调用比较地址和equals
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();
}

void recordAccess(HashMap m) {
}

void recordRemoval(HashMap m) {
}
}

void addEntry(int hash, K key, V value, int bucketIndex) {
Entry e = table[bucketIndex];
table[bucketIndex] = new Entry(hash, key, value, e);
if (size++ >= threshold)
resize(2 * table.length);//触发增加数组空间
}

void createEntry(int hash, K key, V value, int bucketIndex) {
Entry e = table[bucketIndex];
table[bucketIndex] = new Entry(hash, key, value, e);
size++;
}

private abstract class HashIterator implements Iterator {
Entry next; // next entry to return
int expectedModCount; // For fast-fail
int index; // current slot
Entry 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 nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry 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 {
public V next() {
return nextEntry().value;
}
}

private final class KeyIterator extends HashIterator {
public K next() {
return nextEntry().getKey();
}
}

private final class EntryIterator extends HashIterator<Map.Entry> {
public Map.Entry next() {
return nextEntry();
}
}

// Subclass overrides these to alter behavior of views' iterator() method
Iterator newKeyIterator() {
return new KeyIterator();
}
Iterator newValueIterator() {
return new ValueIterator();
}
Iterator<Map.Entry> newEntryIterator() {
return new EntryIterator();
}

private transient Set<Map.Entry> entrySet = null;

public Set keySet() {
Set ks = keySet;
return (ks != null ? ks : (keySet = new KeySet()));
}

private final class KeySet extends AbstractSet {
public Iterator 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();
}
}

public Collection values() {
Collection vs = values;
return (vs != null ? vs : (values = new Values()));
}

private final class Values extends AbstractCollection {
public Iterator iterator() {
return newValueIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
HashMap.this.clear();
}
}

public Set<Map.Entry> entrySet() {
return entrySet0();
}

private Set<Map.Entry> entrySet0() {
Set<Map.Entry> es = entrySet;
return es != null ? es : (entrySet = new EntrySet());
}

private final class EntrySet extends AbstractSet<Map.Entry> {
public Iterator<Map.Entry> iterator() {
return newEntryIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry) o;
Entry 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();
}
}

private void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
Iterator<Map.Entry> 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 e = i.next();
s.writeObject(e.getKey());
s.writeObject(e.getValue());
}
}
}

private static final long serialVersionUID = 362498820763181265L;

/**
* Reconstitute the HashMap 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; }
}

使用的是链表加hashcode 的混合，，稍微复杂一些，其实很多东西和学的差不多，，index 用的是位于其实就是mod吧，需要注意的就是增量因子，自己的容量，2的方最好否则会增加冲突的概率，还有就是Put比较hash的equals啊，hash相同产生冲突带来了不必要的开销要遍历整个链表，equals相同hashcode不同会放在不同的位置，，更加麻烦，，现实中尽量使hashcode和equals一样吧后面的博文来自转载，它告诉了我们whyhttp://beyond99.blog.51cto.com/1469451/429789