java.util.IdentityHashMap Maven / Gradle / Ivy
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package java.util;
import java.io.*;
/**
* This class implements the Map interface with a hash table, using
* reference-equality in place of object-equality when comparing keys (and
* values). In other words, in an IdentityHashMap, two keys
* k1 and k2 are considered equal if and only if
* (k1==k2). (In normal Map implementations (like
* HashMap) two keys k1 and k2 are considered equal
* if and only if (k1==null ? k2==null : k1.equals(k2)).)
*
* This class is not a general-purpose Map
* implementation! While this class implements the Map interface, it
* intentionally violates Map's general contract, which mandates the
* use of the equals method when comparing objects. This class is
* designed for use only in the rare cases wherein reference-equality
* semantics are required.
*
*
A typical use of this class is topology-preserving object graph
* transformations, such as serialization or deep-copying. To perform such
* a transformation, a program must maintain a "node table" that keeps track
* of all the object references that have already been processed. The node
* table must not equate distinct objects even if they happen to be equal.
* Another typical use of this class is to maintain proxy objects. For
* example, a debugging facility might wish to maintain a proxy object for
* each object in the program being debugged.
*
*
This class provides all of the optional map operations, and permits
* null values and the null key. This class makes no
* guarantees as to the order of the map; in particular, it does not guarantee
* that the order will remain constant over time.
*
*
This class provides constant-time performance for the basic
* operations (get and put), assuming the system
* identity hash function ({@link System#identityHashCode(Object)})
* disperses elements properly among the buckets.
*
*
This class has one tuning parameter (which affects performance but not
* semantics): expected maximum size. This parameter is the maximum
* number of key-value mappings that the map is expected to hold. Internally,
* this parameter is used to determine the number of buckets initially
* comprising the hash table. The precise relationship between the expected
* maximum size and the number of buckets is unspecified.
*
*
If the size of the map (the number of key-value mappings) sufficiently
* exceeds the expected maximum size, the number of buckets is increased
* Increasing the number of buckets ("rehashing") may be fairly expensive, so
* it pays to create identity hash maps with a sufficiently large expected
* maximum size. On the other hand, iteration over collection views requires
* time proportional to the number of buckets in the hash table, so it
* pays not to set the expected maximum size too high if you are especially
* concerned with iteration performance or memory usage.
*
*
Note that this implementation is not synchronized.
* If multiple threads access an identity hash map concurrently, and at
* least one of the threads modifies the map structurally, it must
* be synchronized externally. (A structural modification is any operation
* that adds or deletes one or more mappings; merely changing the value
* associated with a key that an instance already contains is not a
* structural modification.) This is typically accomplished by
* synchronizing on some object that naturally encapsulates the map.
*
* If no such object exists, the map should be "wrapped" using the
* {@link Collections#synchronizedMap Collections.synchronizedMap}
* method. This is best done at creation time, to prevent accidental
* unsynchronized access to the map:
* Map m = Collections.synchronizedMap(new IdentityHashMap(...));
*
* The iterators returned by the iterator method of the
* collections returned by all of this class's "collection view
* methods" are fail-fast: if the map is structurally modified
* at any time after the iterator is created, in any way except
* through the iterator's own remove method, the iterator
* will throw a {@link ConcurrentModificationException}. Thus, in the
* face of concurrent modification, the iterator fails quickly and
* cleanly, rather than risking arbitrary, non-deterministic behavior
* at an undetermined time in the future.
*
*
Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification. Fail-fast iterators
* throw ConcurrentModificationException on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: fail-fast iterators should be used only
* to detect bugs.
*
*
Implementation note: This is a simple linear-probe hash table,
* as described for example in texts by Sedgewick and Knuth. The array
* alternates holding keys and values. (This has better locality for large
* tables than does using separate arrays.) For many JRE implementations
* and operation mixes, this class will yield better performance than
* {@link HashMap} (which uses chaining rather than linear-probing).
*
*
This class is a member of the
*
* Java Collections Framework.
*
* @see System#identityHashCode(Object)
* @see Object#hashCode()
* @see Collection
* @see Map
* @see HashMap
* @see TreeMap
* @author Doug Lea and Josh Bloch
* @since 1.4
*/
public class IdentityHashMap
extends AbstractMap
implements Map, java.io.Serializable, Cloneable
{
/**
* The initial capacity used by the no-args constructor.
* MUST be a power of two. The value 32 corresponds to the
* (specified) expected maximum size of 21, given a load factor
* of 2/3.
*/
private static final int DEFAULT_CAPACITY = 32;
/**
* The minimum capacity, used if a lower value is implicitly specified
* by either of the constructors with arguments. The value 4 corresponds
* to an expected maximum size of 2, given a load factor of 2/3.
* MUST be a power of two.
*/
private static final int MINIMUM_CAPACITY = 4;
/**
* 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<<29.
*/
private static final int MAXIMUM_CAPACITY = 1 << 29;
/**
* The table, resized as necessary. Length MUST always be a power of two.
*/
private transient Object[] table;
/**
* The number of key-value mappings contained in this identity hash map.
*
* @serial
*/
private int size;
/**
* The number of modifications, to support fast-fail iterators
*/
private transient int modCount;
/**
* The next size value at which to resize (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);
}
/**
* Constructs a new, empty identity hash map with a default expected
* maximum size (21).
*/
public IdentityHashMap() {
init(DEFAULT_CAPACITY);
}
/**
* Constructs a new, empty map with the specified expected maximum size.
* Putting more than the expected number of key-value mappings into
* the map may cause the internal data structure to grow, which may be
* somewhat time-consuming.
*
* @param expectedMaxSize the expected maximum size of the map
* @throws IllegalArgumentException if expectedMaxSize 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;
}
/**
* Initializes object to be an empty map with the specified initial
* capacity, which is assumed to be a power of two between
* MINIMUM_CAPACITY and MAXIMUM_CAPACITY inclusive.
*/
private void init(int initCapacity) {
// assert (initCapacity & -initCapacity) == initCapacity; // power of 2
// assert initCapacity >= MINIMUM_CAPACITY;
// assert initCapacity <= MAXIMUM_CAPACITY;
threshold = (initCapacity * 2)/3;
table = new Object[2 * initCapacity];
}
/**
* Constructs a new identity hash map containing the keys-value mappings
* in the specified map.
*
* @param m the map whose mappings are to be placed into this map
* @throws NullPointerException if the specified map is null
*/
public IdentityHashMap(Map m) {
// Allow for a bit of growth
this((int) ((1 + m.size()) * 1.1));
putAll(m);
}
/**
* Returns the number of key-value mappings in this identity hash map.
*
* @return the number of key-value mappings in this map
*/
public int size() {
return size;
}
/**
* Returns true if this identity hash map contains no key-value
* mappings.
*
* @return true if this identity hash map contains no key-value
* mappings
*/
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);
}
/**
* Circularly traverses table of size len.
*/
private static int nextKeyIndex(int i, int len) {
return (i + 2 < len ? i + 2 : 0);
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key == k)},
* then this method returns {@code v}; otherwise it returns
* {@code null}. (There can be at most one such mapping.)
*
*
A return value of {@code null} does not necessarily
* 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) {
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 (V) tab[i + 1];
if (item == null)
return null;
i = nextKeyIndex(i, len);
}
}
/**
* Tests whether the specified object reference is a key in this identity
* hash map.
*
* @param key possible key
* @return true if the specified object reference is a key
* in this map
* @see #containsValue(Object)
*/
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);
}
}
/**
* Tests whether the specified object reference is a value in this identity
* hash map.
*
* @param value value whose presence in this map is to be tested
* @return true if this map maps one or more keys to the
* specified object reference
* @see #containsKey(Object)
*/
public boolean containsValue(Object value) {
Object[] tab = table;
for (int i = 1; i < tab.length; i += 2)
if (tab[i] == value && tab[i - 1] != null)
return true;
return false;
}
/**
* Tests if the specified key-value mapping is in the map.
*
* @param key possible key
* @param value possible value
* @return true if and only if the specified key-value
* mapping is in the map
*/
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;
if (item == null)
return false;
i = nextKeyIndex(i, len);
}
}
/**
* Associates the specified value with the specified key in this identity
* hash map. If the map previously contained a mapping for the key, the
* old value is replaced.
*
* @param key the key with which the specified value is to be associated
* @param value the value to be associated with the specified key
* @return the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
* @see Object#equals(Object)
* @see #get(Object)
* @see #containsKey(Object)
*/
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;
return oldValue;
}
i = nextKeyIndex(i, len);
}
modCount++;
tab[i] = k;
tab[i + 1] = value;
if (++size >= threshold)
resize(len); // len == 2 * current capacity.
return null;
}
/**
* Resize the table to hold given capacity.
*
* @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)
i = nextKeyIndex(i, newLength);
newTable[i] = key;
newTable[i + 1] = value;
}
}
table = newTable;
}
/**
* 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 m) {
int n = m.size();
if (n == 0)
return;
if (n > threshold) // conservatively pre-expand
resize(capacity(n));
for (Entry e : m.entrySet())
put(e.getKey(), e.getValue());
}
/**
* Removes the mapping for this key from this map if present.
*
* @param key key whose mapping is to be removed from the map
* @return the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
*/
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);
}
}
/**
* Removes the specified key-value mapping from the map if it is present.
*
* @param key possible key
* @param value possible value
* @return true if and only if the specified key-value
* mapping was in the map
*/
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);
}
}
/**
* Rehash all possibly-colliding entries following a
* deletion. This preserves the linear-probe
* collision properties required by get, put, etc.
*
* @param d the index of a newly empty deleted slot
*/
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;
}
}
}
/**
* Removes all of the mappings from this map.
* The map will be empty after this call returns.
*/
public void clear() {
modCount++;
Object[] tab = table;
for (int i = 0; i < tab.length; i++)
tab[i] = null;
size = 0;
}
/**
* Compares the specified object with this map for equality. Returns
* true if the given object is also a map and the two maps
* represent identical object-reference mappings. More formally, this
* map is equal to another map m if and only if
* this.entrySet().equals(m.entrySet()).
*
*
Owing to the reference-equality-based semantics of this map it is
* possible that the symmetry and transitivity requirements of the
* Object.equals contract may be violated if this map is compared
* to a normal map. However, the Object.equals contract is
* guaranteed to hold among IdentityHashMap instances.
*
* @param o object to be compared for equality with this map
* @return true if the specified object is equal to this map
* @see Object#equals(Object)
*/
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
}
}
/**
* Returns the hash code value for this map. The hash code of a map is
* defined to be the sum of the hash codes of each entry in the map's
* entrySet() view. This ensures that m1.equals(m2)
* implies that m1.hashCode()==m2.hashCode() for any two
* IdentityHashMap instances m1 and m2, as
* required by the general contract of {@link Object#hashCode}.
*
*
Owing to the reference-equality-based semantics of the
* Map.Entry instances in the set returned by this map's
* entrySet method, it is possible that the contractual
* requirement of Object.hashCode mentioned in the previous
* paragraph will be violated if one of the two objects being compared is
* an IdentityHashMap instance and the other is a normal map.
*
* @return the hash code value for this map
* @see Object#equals(Object)
* @see #equals(Object)
*/
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;
}
/**
* Returns a shallow copy of this identity hash map: the keys and values
* themselves are not cloned.
*
* @return a shallow copy of this map
*/
public Object clone() {
try {
IdentityHashMap m = (IdentityHashMap) super.clone();
m.entrySet = null;
m.table = table.clone();
return m;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}
private abstract class IdentityHashMapIterator implements Iterator {
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 {
public K next() {
return (K) unmaskNull(traversalTable[nextIndex()]);
}
}
private class ValueIterator extends IdentityHashMapIterator {
public V next() {
return (V) traversalTable[nextIndex() + 1];
}
}
private class EntryIterator
extends IdentityHashMapIterator>
{
private Entry lastReturnedEntry = null;
public Map.Entry 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 {
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> entrySet = null;
/**
* Returns an identity-based 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, the results of the iteration are
* undefined. The set supports element removal, which removes the
* corresponding mapping from the map, via the Iterator.remove,
* Set.remove, removeAll, retainAll, and
* clear methods. It does not support the add or
* addAll methods.
*
* While the object returned by this method implements the
* Set interface, it does not obey Set's general
* contract. Like its backing map, the set returned by this method
* defines element equality as reference-equality rather than
* object-equality. This affects the behavior of its contains,
* remove, containsAll, equals, and
* hashCode methods.
*
*
The equals method of the returned set returns true
* only if the specified object is a set containing exactly the same
* object references as the returned set. The symmetry and transitivity
* requirements of the Object.equals contract may be violated if
* the set returned by this method is compared to a normal set. However,
* the Object.equals contract is guaranteed to hold among sets
* returned by this method.
*
*
The hashCode method of the returned set returns the sum of
* the identity hashcodes of the elements in the set, rather than
* the sum of their hashcodes. This is mandated by the change in the
* semantics of the equals method, in order to enforce the
* general contract of the Object.hashCode method among sets
* returned by this method.
*
* @return an identity-based set view of the keys contained in this map
* @see Object#equals(Object)
* @see System#identityHashCode(Object)
*/
public Set keySet() {
Set ks = keySet;
if (ks != null)
return ks;
else
return keySet = new KeySet();
}
private class KeySet extends AbstractSet {
public Iterator 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 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;
}
}
/**
* 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,
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the Iterator.remove,
* Collection.remove, removeAll,
* retainAll and clear methods. It does not
* support the add or addAll methods.
*
* While the object returned by this method implements the
* Collection interface, it does not obey
* Collection's general contract. Like its backing map,
* the collection returned by this method defines element equality as
* reference-equality rather than object-equality. This affects the
* behavior of its contains, remove and
* containsAll methods.
*/
public Collection values() {
Collection vs = values;
if (vs != null)
return vs;
else
return values = new Values();
}
private class Values extends AbstractCollection {
public Iterator iterator() {
return new ValueIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsValue(o);
}
public boolean remove(Object o) {
for (Iterator i = iterator(); i.hasNext(); ) {
if (i.next() == o) {
i.remove();
return true;
}
}
return false;
}
public void clear() {
IdentityHashMap.this.clear();
}
}
/**
* Returns a {@link Set} view of the mappings contained in this map.
* Each element in the returned set is a reference-equality-based
* Map.Entry. 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,
* the results of the iteration are undefined. The set supports
* element removal, which removes the corresponding mapping from
* the map, via the Iterator.remove, Set.remove,
* removeAll, retainAll and clear
* methods. It does not support the add or
* addAll methods.
*
* Like the backing map, the Map.Entry objects in the set
* returned by this method define key and value equality as
* reference-equality rather than object-equality. This affects the
* behavior of the equals and hashCode methods of these
* Map.Entry objects. A reference-equality based Map.Entry
* e is equal to an object o if and only if o is a
* Map.Entry and e.getKey()==o.getKey() &&
* e.getValue()==o.getValue(). To accommodate these equals
* semantics, the hashCode method returns
* System.identityHashCode(e.getKey()) ^
* System.identityHashCode(e.getValue()).
*
*
Owing to the reference-equality-based semantics of the
* Map.Entry instances in the set returned by this method,
* it is possible that the symmetry and transitivity requirements of
* the {@link Object#equals(Object)} contract may be violated if any of
* the entries in the set is compared to a normal map entry, or if
* the set returned by this method is compared to a set of normal map
* entries (such as would be returned by a call to this method on a normal
* map). However, the Object.equals contract is guaranteed to
* hold among identity-based map entries, and among sets of such entries.
*
*
* @return a set view of the identity-mappings contained in this map
*/
public Set> entrySet() {
Set> es = entrySet;
if (es != null)
return es;
else
return entrySet = new EntrySet();
}
private class EntrySet extends AbstractSet> {
public Iterator> 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> 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> it = iterator();
for (int i = 0; i < size; i++)
result[i] = new AbstractMap.SimpleEntry<>(it.next());
return result;
}
@SuppressWarnings("unchecked")
public T[] toArray(T[] a) {
int size = size();
if (a.length < size)
a = (T[])java.lang.reflect.Array
.newInstance(a.getClass().getComponentType(), size);
Iterator> 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 IdentityHashMap instance to a stream
* (i.e., serialize it).
*
* @serialData The size of the HashMap (the number of key-value
* mappings) (int), 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 IdentityHashMap 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