标签：
IdentityHashMap
1.内部通过数组存储键值对，相邻元素存在键值对
比如：i 位置是key，i+1位置是value
2.当hashcode相等，出现冲突的时候，通过线性探索发解决冲突问题
3.比较的是引用相等
IdentityHashMap与常用的HashMap的区别是：
前者比较key时是“引用相等”而后者是“对象相等”，即对于k1和k2，当k1==k2时，IdentityHashMap认为两个key相等，而HashMap只有在k1.equals(k2) == true 时才会认为两个key相等。
默认的加载因子为2/3,在重新哈希后，加载因子变为1/3.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，通过调用 reszie 方法将容量翻倍,重新进行哈希。增加桶数，重新哈希，可能相当昂贵。
继承AbstractMap
实现Map、java.io.Serializable、Cloneable
package java.util;
import java.io.*;
public class IdentityHashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, java.io.Serializable, Cloneable
{
/**
* 默认容量
*/
private static final int DEFAULT_CAPACITY = 32;
/**
* 最小容量
*/
private static final int MINIMUM_CAPACITY = 4;
/**
* 最大容量
*/
private static final int MAXIMUM_CAPACITY = 1 << 29;
/**
* 输出存储结构
*/
private transient Object[] table;
/**
* 键值对个数
* @serial
*/
private int size;
/**
* 修改次数
*/
private transient int modCount;
/**
* 更新容器时候的阈值= (capacity * load factor).
*/
private transient int threshold;
/**
* Value representing null keys inside tables.
*/
private static final Object NULL_KEY = new Object();
/**
* Use NULL_KEY for key if it is null.
*/
private static Object maskNull(Object key) {
return (key == null ? NULL_KEY : key);
}
/**
* Returns internal representation of null key back to caller as null.
*/
private static Object unmaskNull(Object key) {
return (key == NULL_KEY ? null : key);
}
/**
* 构造函数
*/
public IdentityHashMap() {
init(DEFAULT_CAPACITY);
}
/**
* 构造函数
*
* @param expectedMaxSize the expected maximum size of the map
* @throws IllegalArgumentException if <tt>expectedMaxSize</tt> is negative
*/
public IdentityHashMap(int expectedMaxSize) {
if (expectedMaxSize < 0)
throw new IllegalArgumentException("expectedMaxSize is negative: "
+ expectedMaxSize);
init(capacity(expectedMaxSize));
}
/**
* Returns the appropriate capacity for the specified expected maximum
* size. Returns the smallest power of two between MINIMUM_CAPACITY
* and MAXIMUM_CAPACITY, inclusive, that is greater than
* (3 * expectedMaxSize)/2, if such a number exists. Otherwise
* returns MAXIMUM_CAPACITY. If (3 * expectedMaxSize)/2 is negative, it
* is assumed that overflow has occurred, and MAXIMUM_CAPACITY is returned.
*/
private int capacity(int expectedMaxSize) {
// Compute min capacity for expectedMaxSize given a load factor of 2/3
int minCapacity = (3 * expectedMaxSize)/2;
// Compute the appropriate capacity
int result;
if (minCapacity > MAXIMUM_CAPACITY || minCapacity < 0) {
result = MAXIMUM_CAPACITY;
} else {
result = MINIMUM_CAPACITY;
while (result < minCapacity)
result <<= 1;
}
return result;
}
/**
* init
*/
private void init(int initCapacity) {
// assert (initCapacity & -initCapacity) == initCapacity; // power of 2
// assert initCapacity >= MINIMUM_CAPACITY;
// assert initCapacity <= MAXIMUM_CAPACITY;
threshold = (initCapacity * 2)/3; // 进行扩容时候的阈值
table = new Object[2 * initCapacity]; // 2倍，表示key value相邻存储
}
/**
* 构造函数，m集合元素加入到当前集合中
*/
public IdentityHashMap(Map<? extends K, ? extends V> m) {
// Allow for a bit of growth
this((int) ((1 + m.size()) * 1.1));
putAll(m);
}
/**
* size
*/
public int size() {
return size;
}
/**
* isEmpty
*/
public boolean isEmpty() {
return size == 0;
}
/**
* Returns index for Object x.
*/
private static int hash(Object x, int length) {
int h = System.identityHashCode(x);
// Multiply by -127, and left-shift to use least bit as part of hash
return ((h << 1) - (h << 8)) & (length - 1);
}
/**
*循环的方式，找到下一个key的id
*/
private static int nextKeyIndex(int i, int len) {
return (i + 2 < len ? i + 2 : 0);
}
/**
* V get(Object key)
*/
public V get(Object key) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len); // 根据hash计算应该在数组中的id
while (true) {
Object item = tab[i]; // 当前key
if (item == k) // 比较是否相等，相等下一个位置就是value
return (V) tab[i + 1];
if (item == null)
return null;
i = nextKeyIndex(i, len); // 上面不满足，根据上一个i 更新 i
}
}
/**
* containsKey 和 get 很类似
*/
public boolean containsKey(Object key) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
while (true) {
Object item = tab[i];
if (item == k)
return true;
if (item == null)
return false;
i = nextKeyIndex(i, len);
}
}
/**
* containsValue
*/
public boolean containsValue(Object value) {
Object[] tab = table;
for (int i = 1; i < tab.length; i += 2) // 顺序遍历，1 3 5 7 9 位置是value
if (tab[i] == value && tab[i - 1] != null)
return true;
return false;
}
/**
* containsMapping 和containsKey 很类似
*/
private boolean containsMapping(Object key, Object value) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
while (true) {
Object item = tab[i];
if (item == k)
return tab[i + 1] == value; // i 位置相等时 key相等，i+1 相等时value相等，比较的是引用相等
if (item == null)
return false;
i = nextKeyIndex(i, len);
}
}
/**
*put(K key, V value)
*/
public V put(K key, V value) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
Object item;
while ( (item = tab[i]) != null) {
if (item == k) {
V oldValue = (V) tab[i + 1];
tab[i + 1] = value; // key 存在更新value
return oldValue;
}
i = nextKeyIndex(i, len);
}
modCount++;
tab[i] = k; // key 不存在加入
tab[i + 1] = value;
if (++size >= threshold)
resize(len); // len == 2 * current capacity.
return null;
}
/**
* resize 需要移动大量元素，效率很低
*
* @param newCapacity the new capacity, must be a power of two.
*/
private void resize(int newCapacity) {
// assert (newCapacity & -newCapacity) == newCapacity; // power of 2
int newLength = newCapacity * 2;
Object[] oldTable = table;
int oldLength = oldTable.length;
if (oldLength == 2*MAXIMUM_CAPACITY) { // can‘t expand any further
if (threshold == MAXIMUM_CAPACITY-1)
throw new IllegalStateException("Capacity exhausted.");
threshold = MAXIMUM_CAPACITY-1; // Gigantic map!
return;
}
if (oldLength >= newLength)
return;
Object[] newTable = new Object[newLength];
threshold = newLength / 3;
for (int j = 0; j < oldLength; j += 2) {
Object key = oldTable[j];
if (key != null) {
Object value = oldTable[j+1];
oldTable[j] = null;
oldTable[j+1] = null;
int i = hash(key, newLength);
while (newTable[i] != null) // 找到可以插入的位置
i = nextKeyIndex(i, newLength);
newTable[i] = key;
newTable[i + 1] = value;
}
}
table = newTable;
}
/**
* putAll
* @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 n = m.size();
if (n == 0)
return;
if (n > threshold) // conservatively pre-expand
resize(capacity(n));
for (Entry<? extends K, ? extends V> e : m.entrySet())
put(e.getKey(), e.getValue());
}
/**
* remove
*/
public V remove(Object key) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
while (true) {
Object item = tab[i];
if (item == k) {
modCount++;
size--;
V oldValue = (V) tab[i + 1];
tab[i + 1] = null;
tab[i] = null;
closeDeletion(i);
return oldValue;
}
if (item == null)
return null;
i = nextKeyIndex(i, len);
}
}
/**
* removeMapping
*/
private boolean removeMapping(Object key, Object value) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
while (true) {
Object item = tab[i];
if (item == k) {
if (tab[i + 1] != value)
return false;
modCount++;
size--;
tab[i] = null;
tab[i + 1] = null;
closeDeletion(i);
return true;
}
if (item == null)
return false;
i = nextKeyIndex(i, len);
}
}
/**
* closeDeletion 作用：i位置元素删除了，更新后面元素的 位置，这里一个原因就是减少地址冲突
*/
private void closeDeletion(int d) {
// Adapted from Knuth Section 6.4 Algorithm R
Object[] tab = table;
int len = tab.length;
// Look for items to swap into newly vacated slot
// starting at index immediately following deletion,
// and continuing until a null slot is seen, indicating
// the end of a run of possibly-colliding keys.
Object item;
for (int i = nextKeyIndex(d, len); (item = tab[i]) != null;
i = nextKeyIndex(i, len) ) {
// The following test triggers if the item at slot i (which
// hashes to be at slot r) should take the spot vacated by d.
// If so, we swap it in, and then continue with d now at the
// newly vacated i. This process will terminate when we hit
// the null slot at the end of this run.
// The test is messy because we are using a circular table.
int r = hash(item, len);
if ((i < r && (r <= d || d <= i)) || (r <= d && d <= i)) {
tab[d] = item;
tab[d + 1] = tab[i + 1];
tab[i] = null;
tab[i + 1] = null;
d = i;
}
}
}
/**
* clear
*/
public void clear() {
modCount++;
Object[] tab = table;
for (int i = 0; i < tab.length; i++)
tab[i] = null;
size = 0;
}
/**
* equals
*/
public boolean equals(Object o) {
if (o == this) {
return true;
} else if (o instanceof IdentityHashMap) {
IdentityHashMap m = (IdentityHashMap) o;
if (m.size() != size)
return false;
Object[] tab = m.table;
for (int i = 0; i < tab.length; i+=2) {
Object k = tab[i];
if (k != null && !containsMapping(k, tab[i + 1]))
return false;
}
return true;
} else if (o instanceof Map) {
Map m = (Map)o;
return entrySet().equals(m.entrySet());
} else {
return false; // o is not a Map
}
}
/**
* hashCode
*/
public int hashCode() {
int result = 0;
Object[] tab = table;
for (int i = 0; i < tab.length; i +=2) {
Object key = tab[i];
if (key != null) {
Object k = unmaskNull(key);
result += System.identityHashCode(k) ^
System.identityHashCode(tab[i + 1]);
}
}
return result;
}
/**
* clone
*/
public Object clone() {
try {
IdentityHashMap<K,V> m = (IdentityHashMap<K,V>) super.clone();
m.entrySet = null;
m.table = table.clone();
return m;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}
// 迭代器
private abstract class IdentityHashMapIterator<T> implements Iterator<T> {
int index = (size != 0 ? 0 : table.length); // current slot.
int expectedModCount = modCount; // to support fast-fail
int lastReturnedIndex = -1; // to allow remove()
boolean indexValid; // To avoid unnecessary next computation
Object[] traversalTable = table; // reference to main table or copy
public boolean hasNext() {
Object[] tab = traversalTable;
for (int i = index; i < tab.length; i+=2) {
Object key = tab[i];
if (key != null) {
index = i;
return indexValid = true;
}
}
index = tab.length;
return false;
}
protected int nextIndex() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (!indexValid && !hasNext())
throw new NoSuchElementException();
indexValid = false;
lastReturnedIndex = index;
index += 2;
return lastReturnedIndex;
}
public void remove() {
if (lastReturnedIndex == -1)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
expectedModCount = ++modCount;
int deletedSlot = lastReturnedIndex;
lastReturnedIndex = -1;
// back up index to revisit new contents after deletion
index = deletedSlot;
indexValid = false;
// Removal code proceeds as in closeDeletion except that
// it must catch the rare case where an element already
// seen is swapped into a vacant slot that will be later
// traversed by this iterator. We cannot allow future
// next() calls to return it again. The likelihood of
// this occurring under 2/3 load factor is very slim, but
// when it does happen, we must make a copy of the rest of
// the table to use for the rest of the traversal. Since
// this can only happen when we are near the end of the table,
// even in these rare cases, this is not very expensive in
// time or space.
Object[] tab = traversalTable;
int len = tab.length;
int d = deletedSlot;
K key = (K) tab[d];
tab[d] = null; // vacate the slot
tab[d + 1] = null;
// If traversing a copy, remove in real table.
// We can skip gap-closure on copy.
if (tab != IdentityHashMap.this.table) {
IdentityHashMap.this.remove(key);
expectedModCount = modCount;
return;
}
size--;
Object item;
for (int i = nextKeyIndex(d, len); (item = tab[i]) != null;
i = nextKeyIndex(i, len)) {
int r = hash(item, len);
// See closeDeletion for explanation of this conditional
if ((i < r && (r <= d || d <= i)) ||
(r <= d && d <= i)) {
// If we are about to swap an already-seen element
// into a slot that may later be returned by next(),
// then clone the rest of table for use in future
// next() calls. It is OK that our copy will have
// a gap in the "wrong" place, since it will never
// be used for searching anyway.
if (i < deletedSlot && d >= deletedSlot &&
traversalTable == IdentityHashMap.this.table) {
int remaining = len - deletedSlot;
Object[] newTable = new Object[remaining];
System.arraycopy(tab, deletedSlot,
newTable, 0, remaining);
traversalTable = newTable;
index = 0;
}
tab[d] = item;
tab[d + 1] = tab[i + 1];
tab[i] = null;
tab[i + 1] = null;
d = i;
}
}
}
}
private class KeyIterator extends IdentityHashMapIterator<K> {
public K next() {
return (K) unmaskNull(traversalTable[nextIndex()]);
}
}
private class ValueIterator extends IdentityHashMapIterator<V> {
public V next() {
return (V) traversalTable[nextIndex() + 1];
}
}
private class EntryIterator
extends IdentityHashMapIterator<Map.Entry<K,V>>
{
private Entry lastReturnedEntry = null;
public Map.Entry<K,V> next() {
lastReturnedEntry = new Entry(nextIndex());
return lastReturnedEntry;
}
public void remove() {
lastReturnedIndex =
((null == lastReturnedEntry) ? -1 : lastReturnedEntry.index);
super.remove();
lastReturnedEntry.index = lastReturnedIndex;
lastReturnedEntry = null;
}
private class Entry implements Map.Entry<K,V> {
private int index;
private Entry(int index) {
this.index = index;
}
public K getKey() {
checkIndexForEntryUse();
return (K) unmaskNull(traversalTable[index]);
}
public V getValue() {
checkIndexForEntryUse();
return (V) traversalTable[index+1];
}
public V setValue(V value) {
checkIndexForEntryUse();
V oldValue = (V) traversalTable[index+1];
traversalTable[index+1] = value;
// if shadowing, force into main table
if (traversalTable != IdentityHashMap.this.table)
put((K) traversalTable[index], value);
return oldValue;
}
public boolean equals(Object o) {
if (index < 0)
return super.equals(o);
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
return (e.getKey() == unmaskNull(traversalTable[index]) &&
e.getValue() == traversalTable[index+1]);
}
public int hashCode() {
if (lastReturnedIndex < 0)
return super.hashCode();
return (System.identityHashCode(unmaskNull(traversalTable[index])) ^
System.identityHashCode(traversalTable[index+1]));
}
public String toString() {
if (index < 0)
return super.toString();
return (unmaskNull(traversalTable[index]) + "="
+ traversalTable[index+1]);
}
private void checkIndexForEntryUse() {
if (index < 0)
throw new IllegalStateException("Entry was removed");
}
}
}
// Views
/**
* This field is initialized to contain an instance of the entry set
* view the first time this view is requested. The view is stateless,
* so there‘s no reason to create more than one.
*/
private transient Set<Map.Entry<K,V>> entrySet = null;
/**
* keySet
*/
public Set<K> keySet() {
Set<K> ks = keySet;
if (ks != null)
return ks;
else
return keySet = new KeySet();
}
private class KeySet extends AbstractSet<K> {
public Iterator<K> iterator() {
return new KeyIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
int oldSize = size;
IdentityHashMap.this.remove(o);
return size != oldSize;
}
/*
* Must revert from AbstractSet‘s impl to AbstractCollection‘s, as
* the former contains an optimization that results in incorrect
* behavior when c is a smaller "normal" (non-identity-based) Set.
*/
public boolean removeAll(Collection<?> c) {
boolean modified = false;
for (Iterator<K> i = iterator(); i.hasNext(); ) {
if (c.contains(i.next())) {
i.remove();
modified = true;
}
}
return modified;
}
public void clear() {
IdentityHashMap.this.clear();
}
public int hashCode() {
int result = 0;
for (K key : this)
result += System.identityHashCode(key);
return result;
}
}
/**
* values
*/
public Collection<V> values() {
Collection<V> vs = values;
if (vs != null)
return vs;
else
return values = new Values();
}
private class Values extends AbstractCollection<V> {
public Iterator<V> iterator() {
return new ValueIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsValue(o);
}
public boolean remove(Object o) {
for (Iterator<V> i = iterator(); i.hasNext(); ) {
if (i.next() == o) {
i.remove();
return true;
}
}
return false;
}
public void clear() {
IdentityHashMap.this.clear();
}
}
/**
*entrySet
*/
public Set<Map.Entry<K,V>> entrySet() {
Set<Map.Entry<K,V>> es = entrySet;
if (es != null)
return es;
else
return entrySet = new EntrySet();
}
private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public Iterator<Map.Entry<K,V>> iterator() {
return new EntryIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
return containsMapping(entry.getKey(), entry.getValue());
}
public boolean remove(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
return removeMapping(entry.getKey(), entry.getValue());
}
public int size() {
return size;
}
public void clear() {
IdentityHashMap.this.clear();
}
/*
* Must revert from AbstractSet‘s impl to AbstractCollection‘s, as
* the former contains an optimization that results in incorrect
* behavior when c is a smaller "normal" (non-identity-based) Set.
*/
public boolean removeAll(Collection<?> c) {
boolean modified = false;
for (Iterator<Map.Entry<K,V>> i = iterator(); i.hasNext(); ) {
if (c.contains(i.next())) {
i.remove();
modified = true;
}
}
return modified;
}
public Object[] toArray() {
int size = size();
Object[] result = new Object[size];
Iterator<Map.Entry<K,V>> it = iterator();
for (int i = 0; i < size; i++)
result[i] = new AbstractMap.SimpleEntry<>(it.next());
return result;
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
int size = size();
if (a.length < size)
a = (T[])java.lang.reflect.Array
.newInstance(a.getClass().getComponentType(), size);
Iterator<Map.Entry<K,V>> it = iterator();
for (int i = 0; i < size; i++)
a[i] = (T) new AbstractMap.SimpleEntry<>(it.next());
if (a.length > size)
a[size] = null;
return a;
}
}
private static final long serialVersionUID = 8188218128353913216L;
/**
* Save the state of the <tt>IdentityHashMap</tt> instance to a stream
* (i.e., serialize it).
*
* @serialData The <i>size</i> of the HashMap (the number of key-value
* mappings) (<tt>int</tt>), followed by the key (Object) and
* value (Object) for each key-value mapping represented by the
* IdentityHashMap. The key-value mappings are emitted in no
* particular order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out and any hidden stuff
s.defaultWriteObject();
// Write out size (number of Mappings)
s.writeInt(size);
// Write out keys and values (alternating)
Object[] tab = table;
for (int i = 0; i < tab.length; i += 2) {
Object key = tab[i];
if (key != null) {
s.writeObject(unmaskNull(key));
s.writeObject(tab[i + 1]);
}
}
}
/**
* Reconstitute the <tt>IdentityHashMap</tt> instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in any hidden stuff
s.defaultReadObject();
// Read in size (number of Mappings)
int size = s.readInt();
// Allow for 33% growth (i.e., capacity is >= 2* size()).
init(capacity((size*4)/3));
// Read the keys and values, and put the mappings in the table
for (int i=0; i<size; i++) {
K key = (K) s.readObject();
V value = (V) s.readObject();
putForCreate(key, value);
}
}
/**
* The put method for readObject. It does not resize the table,
* update modCount, etc.
*/
private void putForCreate(K key, V value)
throws IOException
{
K k = (K)maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
Object item;
while ( (item = tab[i]) != null) {
if (item == k)
throw new java.io.StreamCorruptedException();
i = nextKeyIndex(i, len);
}
tab[i] = k;
tab[i + 1] = value;
}
}
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原文地址：http://blog.csdn.net/qunxingvip/article/details/51932451
