File	Doc	Category	Size	Date	Package
IdentityHashMap.java	API Doc	Java SE 6 API	45298	Tue Jun 10 00:25:54 BST 2008	java.util

IdentityHashMap

• java.lang.Object
  • java.util.AbstractMap

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

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 int	capacity(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 void	clear() Removes all of the mappings from this map. The map will be empty after this call returns. modCount++; Object[] tab = table; for (int i = 0; i < tab.length; i++) tab[i] = null; size = 0;
public java.lang.Object	clone() 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> m = (IdentityHashMap<K,V>) super.clone(); m.entrySet = null; m.table = (Object[])table.clone(); return m; } catch (CloneNotSupportedException e) { throw new InternalError(); }
private void	closeDeletion(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 boolean	containsKey(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 boolean	containsMapping(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 the 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 boolean	containsValue(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.Set	entrySet() 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 Set<Map.Entry<K,V>> es = entrySet; if (es != null) return es; else return entrySet = new EntrySet();
public boolean	equals(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 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 V	get(java.lang.Object key) 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) 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 int	hash(java.lang.Object x, int length) Returns 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 int	hashCode() 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) 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 void	init(int initCapacity) 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. // 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 boolean	isEmpty() 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.Set	keySet() 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.Object	maskNull(java.lang.Object key) Use NULL_KEY for key if it is null. return (key == null ? NULL_KEY : key);
private static int	nextKeyIndex(int i, int len) Circularly traverses table of size len. return (i + 2 < len ? i + 2 : 0);
public V	put(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 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) 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 void	putAll(java.util.Map m) 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 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());
private void	putForCreate(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 void	readObject(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 V	remove(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 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.) 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 boolean	removeMapping(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 the 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 void	resize(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 int	size() 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.Object	unmaskNull(java.lang.Object key) Returns internal representation of null key back to caller as null. return (key == NULL_KEY ? null : key);
public java.util.Collection	values() 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. Collection<V> vs = values; if (vs != null) return vs; else return values = new Values();
private void	writeObject(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]); } }