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IdentityHashMap.javaAPI DocJava SE 5 API44799Fri Aug 26 14:57:22 BST 2005java.util

IdentityHashMap

public class IdentityHashMap extends AbstractMap implements Cloneable, Map, Serializable
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 this 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 Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

Map m = Collections.synchronizedMap(new HashMap(...));

The iterators 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 or add methods, the iterator will throw a 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

Fields Summary
private static final int
DEFAULT_CAPACITY
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
MINIMUM_CAPACITY
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
MAXIMUM_CAPACITY
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 transient Object[]
table
The table, resized as necessary. Length MUST always be a power of two.
private int
size
The number of key-value mappings contained in this identity hash map.
private volatile transient int
modCount
The number of modifications, to support fast-fail iterators
private transient int
threshold
The next size value at which to resize (capacity * load factor).
private static final Object
NULL_KEY
Value representing null keys inside tables.
private transient Set
entrySet
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 static final long
serialVersionUID
Constructors Summary
public IdentityHashMap()
Constructs a new, empty identity hash map with a default expected maximum size (21).

        init(DEFAULT_CAPACITY);
    
public IdentityHashMap(int expectedMaxSize)
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

        if (expectedMaxSize < 0)
            throw new IllegalArgumentException("expectedMaxSize is negative: "
                                               + expectedMaxSize);
        init(capacity(expectedMaxSize));
    
public IdentityHashMap(Map m)
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.

        // Allow for a bit of growth
        this((int) ((1 + m.size()) * 1.1));
        putAll(m);
    
Methods Summary
private intcapacity(int 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.

        // 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;
    
public voidclear()
Removes all mappings from this map.

        modCount++;
        Object[] tab = table;
        for (int i = 0; i < tab.length; i++)
            tab[i] = null;
        size = 0;
    
public java.lang.Objectclone()
Returns a shallow copy of this identity hash map: the keys and values themselves are not cloned.

return
a shallow copy of this map.

        try {
            IdentityHashMap<K,V> t = (IdentityHashMap<K,V>) super.clone();
            t.entrySet = null;
            t.table = (Object[])table.clone();
            return t;
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }
    
private voidcloseDeletion(int d)
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

        // 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;
            }
        }
    
public booleancontainsKey(java.lang.Object key)
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)

        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);
        }
    
private booleancontainsMapping(java.lang.Object key, java.lang.Object value)
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 map.

        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);
        }
    
public booleancontainsValue(java.lang.Object value)
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)

        Object[] tab = table;
        for (int i = 1; i < tab.length; i+= 2)
            if (tab[i] == value)
                return true;

        return false;
    
public java.util.SetentrySet()
Returns a 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.

        Set<Map.Entry<K,V>> es = entrySet;
        if (es != null)
            return es;
        else
            return entrySet = new EntrySet();
    
public booleanequals(java.lang.Object o)
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 map 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)

        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
        }
    
public Vget(java.lang.Object key)
Returns the value to which the specified key is mapped in this identity hash map, or null if the map contains no mapping for this key. A return value of null does not necessarily indicate that the map contains no mapping for the key; it is also possible that the map explicitly maps the key to null. The containsKey method may be used to distinguish these two cases.

param
key the key whose associated value is to be returned.
return
the value to which this map maps the specified key, or null if the map contains no mapping for this key.
see
#put(Object, Object)

        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);
        }
    
private static inthash(java.lang.Object x, int length)
Return index for Object x.

        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);
    
public inthashCode()
Returns the hash code value for this map. The hash code of a map is defined to be the sum of the hashcode of each entry in the map's entrySet view. This ensures that t1.equals(t2) implies that t1.hashCode()==t2.hashCode() for any two IdentityHashMap instances t1 and t2, 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#hashCode()
see
Object#equals(Object)
see
#equals(Object)

        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;
    
private voidinit(int initCapacity)
Initialize 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.

        // assert (initCapacity & -initCapacity) == initCapacity; // power of 2
        // assert initCapacity >= MINIMUM_CAPACITY;
        // assert initCapacity <= MAXIMUM_CAPACITY;

        threshold = (initCapacity * 2)/3;
        table = new Object[2 * initCapacity];
    
public booleanisEmpty()
Returns true if this identity hash map contains no key-value mappings.

return
true if this identity hash map contains no key-value mappings.

        return size == 0;
    
public java.util.SetkeySet()
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)


                                                                                                                                                                                                                                                                                                
       
        Set<K> ks = keySet;
        if (ks != null)
            return ks;
        else
            return keySet = new KeySet();
    
private static java.lang.ObjectmaskNull(java.lang.Object key)
Use NULL_KEY for key if it is null.


                 

         
        return (key == null ? NULL_KEY : key);
    
private static intnextKeyIndex(int i, int len)
Circularly traverse table of size len.

        return (i + 2 < len ? i + 2 : 0);
    
public Vput(K key, V value)
Associates the specified value with the specified key in this identity hash map. If the map previously contained a mapping for this 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 the specified key.)
see
Object#equals(Object)
see
#get(Object)
see
#containsKey(Object)

        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;
    
public voidputAll(java.util.Map t)
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
t mappings to be stored in this map.
throws
NullPointerException if the specified map is null.

        int n = t.size();
        if (n == 0)
            return;
        if (n > threshold) // conservatively pre-expand
            resize(capacity(n));

	for (Entry<? extends K, ? extends V> e : t.entrySet())
            put(e.getKey(), e.getValue());
    
private voidputForCreate(K key, V value)
The put method for readObject. It does not resize the table, update modcount, etc.

        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;
    
private voidreadObject(java.io.ObjectInputStream s)
Reconstitute the IdentityHashMap instance from a stream (i.e., deserialize it).

        // 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);
        }
    
public Vremove(java.lang.Object key)
Removes the mapping for this key from this map if present.

param
key key whose mapping is to be removed from the map.
return
previous value associated with specified key, or null if there was no entry for key. (A null return can also indicate that the map previously associated null with the specified 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);
        }

    
private booleanremoveMapping(java.lang.Object key, java.lang.Object value)
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 map.

        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);
        }
    
private voidresize(int newCapacity)
Resize the table to hold given capacity.

param
newCapacity the new capacity, must be a power of two.

        // 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;
    
public intsize()
Returns the number of key-value mappings in this identity hash map.

return
the number of key-value mappings in this map.

        return size;
    
private static java.lang.ObjectunmaskNull(java.lang.Object key)
Return internal representation of null key back to caller as null

        return (key == NULL_KEY ? null : key);
    
public java.util.Collectionvalues()

Returns a 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.

return
a collection view of the values contained in this map.

        Collection<V> vs = values;
        if (vs != null)
            return vs;
        else
            return values = new Values();
    
private voidwriteObject(java.io.ObjectOutputStream s)
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.


                           	                                                                   
       
           
        // 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]);
            }
        }