java.util.IdentityHashMap Maven / Gradle / Ivy
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This work corresponds to the API signatures of JSR 219: Foundation
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*/
package java.util;
/**
* 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 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
*/
public class IdentityHashMap extends AbstractMap
implements Map, java.io.Serializable, Cloneable
{
/**
* The number of key-value mappings contained in this identity hash map.
*
* @serial
*/
private int size;
/**
* Constructs a new, empty identity hash map with a default expected
* maximum size (21).
*/
public IdentityHashMap() { }
/**
* 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) { }
/**
* 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) { }
/**
* 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 0;
}
/**
* 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 false;
}
/**
* 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)
*/
public Object get(Object key) {
return null;
}
/**
* 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) {
return false;
}
/**
* 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) {
return false;
}
/**
* 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)
*/
public Object put(Object key, Object value) {
return null;
}
/**
* 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.
*/
public void putAll(Map t) { }
/**
* 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.)
*/
public Object remove(Object key) {
return null;
}
/**
* Removes all mappings from this map.
*/
public void clear() { }
/**
* 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)
*/
public boolean equals(Object o) {
return false;
}
/**
* 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)
*/
public int hashCode() {
return 0;
}
/**
* 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() {
return 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() {
return null;
}
/**
*
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.
*/
public Collection values() {
return null;
}
/**
* 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.
*/
public Set entrySet() {
return null;
}
/**
* Reconstitute the IdentityHashMap instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException
{ }
/**
* 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
{ }
}