java.util: public class: IdentityHashMap

java.util
public class: IdentityHashMap [javadoc | source]

java.lang.Object
   java.util.AbstractMap<K, V>
      java.util.IdentityHashMap

All Implemented Interfaces:
Cloneable, Map, java$io$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 (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 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 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 HashMap (which uses chaining rather than linear-probing).

This class is a member of the Java Collections Framework.

Also see:

System#identityHashCode(Object)

Object#hashCode()

author: Doug - Lea and Josh Bloch

since: 1.4 -

Fields inherited from java.util.AbstractMap:
keySet, values

Constructor:

Constructor:
public IdentityHashMap() { init(DEFAULT_CAPACITY); } Constructs a new, empty identity hash map with a default expected maximum size (21).
public IdentityHashMap(int expectedMaxSize) { if (expectedMaxSize < 0) throw new IllegalArgumentException("expectedMaxSize is negative: " + expectedMaxSize); init(capacity(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. Parameters: `expectedMaxSize` - the expected maximum size of the map Throws: `IllegalArgumentException` - if `expectedMaxSize` is negative
public IdentityHashMap(Map<? extends K, ? extends V> m) { // Allow for a bit of growth this((int) ((1 + m.size()) * 1.1)); putAll(m); } Constructs a new identity hash map containing the keys-value mappings in the specified map. Parameters: `m` - the map whose mappings are to be placed into this map Throws: `NullPointerException` - if the specified map is null

 public IdentityHashMap() {
        init(DEFAULT_CAPACITY);
}

Constructs a new, empty identity hash map with a default expected maximum size (21).

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

Parameters:

expectedMaxSize - the expected maximum size of the map

Throws:

IllegalArgumentException - if expectedMaxSize is negative

 public IdentityHashMap(Map<? extends K, ? extends V> m) {
        // Allow for a bit of growth
        this((int) ((1 + m.size()) * 1.1));
        putAll(m);
}

Constructs a new identity hash map containing the keys-value mappings in the specified map.

Parameters:

m - the map whose mappings are to be placed into this map

Throws:

NullPointerException - if the specified map is null

Method from java.util.IdentityHashMap Summary:
clear, clone, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, values

Methods from java.util.AbstractMap:
clear, clone, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, toString, values

Methods from java.lang.Object:
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method from java.util.IdentityHashMap Detail:
public void clear() { modCount++; Object[] tab = table; for (int i = 0; i < tab.length; i++) tab[i] = null; size = 0; } Removes all of the mappings from this map. The map will be empty after this call returns.
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(); } } Returns a shallow copy of this identity hash map: the keys and values themselves are not cloned.
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 key in this identity hash map.
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 whether the specified object reference is a value in this identity hash map.
public Set<K, V> entrySet() { Set< Map.Entry< K,V > > es = entrySet; if (es != null) return es; else return entrySet = new EntrySet(); } 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 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.
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 } } 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.
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); } } 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 containsKey operation may be used to distinguish these two cases.
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 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 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.
public boolean isEmpty() { return size == 0; } Returns `true` if this identity hash map contains no key-value mappings.
public Set<K> keySet() { Set< K > ks = keySet; if (ks != null) return ks; else return keySet = new 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.
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; } 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.
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()); } 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.
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 mapping for this key from this map if present.
public int size() { return size; } Returns the number of key-value mappings in this identity hash map.
public Collection<V> values() { Collection< V > vs = values; if (vs != null) return vs; else return values = new Values(); } 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.

Method from java.util.IdentityHashMap Detail:

 public  void clear() {
        modCount++;
        Object[] tab = table;
        for (int i = 0; i <  tab.length; i++)
            tab[i] = null;
        size = 0;
}

Removes all of the mappings from this map. The map will be empty after this call returns.

 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();
        }
}

Returns a shallow copy of this identity hash map: the keys and values themselves are not cloned.

 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 key in this identity hash map.

 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 whether the specified object reference is a value in this identity hash map.

 public Set<K, V> entrySet() {
        Set< Map.Entry< K,V > > es = entrySet;
        if (es != null)
            return es;
        else
            return entrySet = new EntrySet();
}

Set

Map.Entry

Iterator.remove

Set.remove

removeAll

retainAll

clear

add

addAll

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 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.

 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
        }
}

true

m

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.

 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);
        }
}

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 containsKey operation may be used to distinguish these two cases.

 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;
}

entrySet()

m1.equals(m2)

m1.hashCode()==m2.hashCode()

IdentityHashMap

m1

m2

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.

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

true

 public Set<K> keySet() {
        Set< K > ks = keySet;
        if (ks != null)
            return ks;
        else
            return keySet = new KeySet();
}

Iterator.remove

Set.remove

removeAll

retainAll

clear

add

addAll

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.

 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;
}

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.

 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());
}

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.

 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 mapping for this key from this map if present.

 public int size() {
        return size;
}

Returns the number of key-value mappings in this identity hash map.

 public Collection<V> values() {
        Collection< V > vs = values;
        if (vs != null)
            return vs;
        else
            return values = new Values();
}

Collection

Iterator.remove

Collection.remove

removeAll

retainAll

clear

add

addAll

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.