Class: java.util.Hashtable<K, V>
- public class Hashtable<K, V>
- extends Dictionary<K, V>
- implements Map<K, V>, Cloneable, Serializable
This class implements a hashtable, which maps keys to values. Any
non-
null object can be used as a key or as a value.
To successfully store and retrieve objects from a hashtable, the
objects used as keys must implement the hashCode
method and the equals method.
An instance of Hashtable has two parameters that affect its
performance: initial capacity and load factor. The
capacity is the number of buckets in the hash table, and the
initial capacity is simply the capacity at the time the hash table
is created. Note that the hash table is open: in the case of a "hash
collision", a single bucket stores multiple entries, which must be searched
sequentially. The load factor is a measure of how full the hash
table is allowed to get before its capacity is automatically increased.
The initial capacity and load factor parameters are merely hints to
the implementation. The exact details as to when and whether the rehash
method is invoked are implementation-dependent.
Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most Hashtable operations, including get and put).
The initial capacity controls a tradeoff between wasted space and the
need for rehash operations, which are time-consuming.
No rehash operations will ever occur if the initial
capacity is greater than the maximum number of entries the
Hashtable will contain divided by its load factor. However,
setting the initial capacity too high can waste space.
If many entries are to be made into a Hashtable,
creating it with a sufficiently large capacity may allow the
entries to be inserted more efficiently than letting it perform
automatic rehashing as needed to grow the table.
This example creates a hashtable of numbers. It uses the names of the numbers as keys:
Hashtable<String, Integer> numbers
= new Hashtable<String, Integer>();
numbers.put("one", 1);
numbers.put("two", 2);
numbers.put("three", 3);
To retrieve a number, use the following code:
Integer n = numbers.get("two");
if (n != null) {
System.out.println("two = " + n);
}
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 Hashtable 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 java.util.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. The Enumerations returned by Hashtable's keys and elements methods are not fail-fast.
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: the fail-fast behavior of iterators should be used only to detect bugs.
As of the Java 2 platform v1.2, this class was retrofitted to implement the java.util.Map interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, Hashtable is synchronized.
Inheritance
Superclass tree:- java.lang.Object
- java.util.Dictionary<K, V>
- java.util.Hashtable
- Map<K, V>
- Cloneable
- Serializable
Methods
-
Hashtabletop
public Hashtable()Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75). -
Hashtabletop
public Hashtable(int initialCapacity)Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75). -
Hashtabletop
public Hashtable(int initialCapacity, float loadFactor)Constructs a new, empty hashtable with the specified initial capacity and the specified load factor. -
Hashtabletop
public Hashtable(Map<? extends K, ? extends V> t)Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with an initial capacity sufficient to hold the mappings in the given Map and a default load factor (0.75). -
cleartop
public synchronized void clear()Clears this hashtable so that it contains no keys. -
clonetop
public synchronized Object clone()Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation. -
containstop
public synchronized boolean contains(Object value)Tests if some key maps into the specified value in this hashtable. This operation is more expensive than the containsKey method.Note that this method is identical in functionality to containsValue, (which is part of the java.util.Map interface in the collections framework).
-
containsKeytop
public synchronized boolean containsKey(Object key)Tests if the specified object is a key in this hashtable.- Specified by:
- containsKey from Map<K, V>
-
containsValuetop
public boolean containsValue(Object value)Returns true if this hashtable maps one or more keys to this value.Note that this method is identical in functionality to contains (which predates the java.util.Map interface).
- Specified by:
- containsValue from Map<K, V>
-
elementstop
public synchronized Enumeration<V> elements()Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.- Override hierarchy:
- elements from Dictionary<K, V>
-
entrySettop
Returns a java.util.Hashtable$KeySet view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. -
equalstop
public synchronized boolean equals(Object o)Compares the specified Object with this Map for equality, as per the definition in the Map interface. -
gettop
public synchronized V get(Object key)Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.More formally, if this map contains a mapping from a key k to a value v such that (key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)
- Override hierarchy:
- get from Dictionary<K, V>
-
getEnumerationtop
private <T> Enumeration<T> getEnumeration(int type) -
getIteratortop
private <T> Iterator<T> getIterator(int type) -
hashCodetop
public synchronized int hashCode()Returns the hash code value for this Map as per the definition in the Map interface. -
isEmptytop
public synchronized boolean isEmpty()Tests if this hashtable maps no keys to values.- Override hierarchy:
- isEmpty from Dictionary<K, V>
-
keySettop
public Set<K> keySet()Returns a java.util.Hashtable$KeySet 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 (except through the iterator's own remove operation), 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 operations. It does not support the add or addAll operations. -
keystop
public synchronized Enumeration<K> keys()Returns an enumeration of the keys in this hashtable.- Override hierarchy:
- keys from Dictionary<K, V>
-
puttop
public synchronized V put(K key, V value)Maps the specifiedkeyto the specifiedvaluein this hashtable. Neither the key nor the value can benull.The value can be retrieved by calling the
getmethod with a key that is equal to the original key.- Override hierarchy:
- put from Dictionary<K, V>
-
putAlltop
public synchronized void putAll(Map<? extends K, ? extends V> t)Copies all of the mappings from the specified map to this hashtable. These mappings will replace any mappings that this hashtable had for any of the keys currently in the specified map. -
readObjecttop
private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundExceptionReconstitute the Hashtable from a stream (i.e., deserialize it). -
reconstitutionPuttop
private void reconstitutionPut(Hashtable.Entry[] tab, K key, V value) throws StreamCorruptedExceptionThe put method used by readObject. This is provided because put is overridable and should not be called in readObject since the subclass will not yet be initialized.This differs from the regular put method in several ways. No checking for rehashing is necessary since the number of elements initially in the table is known. The modCount is not incremented because we are creating a new instance. Also, no return value is needed.
-
rehashtop
protected void rehash()Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor. -
removetop
public synchronized V remove(Object key)Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.- Override hierarchy:
- remove from Dictionary<K, V>
-
sizetop
public synchronized int size()Returns the number of keys in this hashtable.- Override hierarchy:
- size from Dictionary<K, V>
-
toStringtop
public synchronized String toString()Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters ", " (comma and space). Each entry is rendered as the key, an equals sign =, and the associated element, where the toString method is used to convert the key and element to strings. -
valuestop
public Collection<V> values()Returns a java.util.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 (except through the iterator's own remove operation), 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 operations. It does not support the add or addAll operations. -
writeObjecttop
synchronized private void writeObject(ObjectOutputStream s) throws IOExceptionSave the state of the Hashtable to a stream (i.e., serialize it).
Fields
-
ENTRIES
static final private int ENTRIES = 2 -
KEYS
static final private int KEYS = 0 -
VALUES
static final private int VALUES = 1 -
count
transient private int countThe total number of entries in the hash table. -
entrySet
-
keySet
transient volatile private Set<K> keySetEach of these fields are initialized to contain an instance of the appropriate view the first time this view is requested. The views are stateless, so there's no reason to create more than one of each. -
loadFactor
private float loadFactorThe load factor for the hashtable. -
modCount
transient private int modCountThe number of times this Hashtable has been structurally modified Structural modifications are those that change the number of entries in the Hashtable or otherwise modify its internal structure (e.g., rehash). This field is used to make iterators on Collection-views of the Hashtable fail-fast. (See ConcurrentModificationException). -
serialVersionUID
static final private long serialVersionUID = 1421746759512286392use serialVersionUID from JDK 1.0.2 for interoperability -
table
The hash table data. -
threshold
private int thresholdThe table is rehashed when its size exceeds this threshold. (The value of this field is (int)(capacity * loadFactor).) -
values
transient volatile private Collection<V> values