Hashtable

public class Hashtable extends Dictionary<K, V>
implements Map<K, V> Cloneable Serializable
Known Direct Subclasses
Known Indirect Subclasses

This class implements a hash table, 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 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 Map interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, Hashtable is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.

See Also

Public Constructor Summary

Hashtable(int initialCapacity, float loadFactor)
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).
Hashtable()
Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
Hashtable(Map<? extends K, ? extends V> t)
Constructs a new hashtable with the same mappings as the given Map.

Public Method Summary

synchronized void
clear()
Clears this hashtable so that it contains no keys.
synchronized Object
clone()
Creates a shallow copy of this hashtable.
synchronized V
compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)
Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
synchronized V
computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)
If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.
synchronized V
computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)
If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
synchronized boolean
contains(Object value)
Tests if some key maps into the specified value in this hashtable.
synchronized boolean
containsKey(Object key)
Tests if the specified object is a key in this hashtable.
boolean
containsValue(Object value)
Returns true if this hashtable maps one or more keys to this value.
synchronized Enumeration<V>
elements()
Returns an enumeration of the values in this hashtable.
Set<Entry<K, V>>
entrySet()
Returns a Set view of the mappings contained in this map.
synchronized boolean
equals(Object o)
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
synchronized void
forEach(BiConsumer<? super K, ? super V> action)
Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
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.
synchronized V
getOrDefault(Object key, V defaultValue)
Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
synchronized int
hashCode()
Returns the hash code value for this Map as per the definition in the Map interface.
synchronized boolean
isEmpty()
Tests if this hashtable maps no keys to values.
Set<K>
keySet()
Returns a Set view of the keys contained in this map.
synchronized Enumeration<K>
keys()
Returns an enumeration of the keys in this hashtable.
synchronized V
merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
synchronized V
put(K key, V value)
Maps the specified key to the specified value in this hashtable.
synchronized void
putAll(Map<? extends K, ? extends V> t)
Copies all of the mappings from the specified map to this hashtable.
synchronized V
putIfAbsent(K key, V value)
If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.
synchronized boolean
remove(Object key, Object value)
Removes the entry for the specified key only if it is currently mapped to the specified value.
synchronized V
remove(Object key)
Removes the key (and its corresponding value) from this hashtable.
synchronized boolean
replace(K key, V oldValue, V newValue)
Replaces the entry for the specified key only if currently mapped to the specified value.
synchronized V
replace(K key, V value)
Replaces the entry for the specified key only if it is currently mapped to some value.
synchronized void
replaceAll(BiFunction<? super K, ? super V, ? extends V> function)
Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
synchronized int
size()
Returns the number of keys in this hashtable.
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).
Collection<V>
values()
Returns a Collection view of the values contained in this map.

Protected Method Summary

void
rehash()
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently.

Inherited Method Summary

Public Constructors

public Hashtable (int initialCapacity, float loadFactor)

Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.

Parameters
initialCapacity the initial capacity of the hashtable.
loadFactor the load factor of the hashtable.
Throws
IllegalArgumentException if the initial capacity is less than zero, or if the load factor is nonpositive.

public Hashtable (int initialCapacity)

Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).

Parameters
initialCapacity the initial capacity of the hashtable.
Throws
IllegalArgumentException if the initial capacity is less than zero.

public Hashtable ()

Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).

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).

Parameters
t the map whose mappings are to be placed in this map.
Throws
NullPointerException if the specified map is null.

Public Methods

public synchronized void clear ()

Clears this hashtable so that it contains no keys.

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.

Returns
  • a clone of the hashtable

public synchronized V compute (K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)

Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). For example, to either create or append a String msg to a value mapping: 

 map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
 (Method merge() is often simpler to use for such purposes.)

If the remapping function returns null, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
key key with which the specified value is to be associated
remappingFunction the remapping function to compute a value
Returns
  • the new value associated with the specified key, or null if none

public synchronized V computeIfAbsent (K key, Function<? super K, ? extends V> mappingFunction)

If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

If the mapping function returns null, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in: 

 map.computeIfAbsent(key, k -> new Value(f(k)));

Or to implement a multi-value map, Map<K,Collection<V>>, supporting multiple values per key: 

 map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);

The mapping function should not modify this map during computation.

Parameters
key key with which the specified value is to be associated
mappingFunction the mapping function to compute a value
Returns
  • the current (existing or computed) value associated with the specified key, or null if the computed value is null

public synchronized V computeIfPresent (K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)

If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
key key with which the specified value is to be associated
remappingFunction the remapping function to compute a value
Returns
  • the new value associated with the specified key, or null if none

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 Map interface in the collections framework).

Parameters
value a value to search for
Returns
  • true if and only if some key maps to the value argument in this hashtable as determined by the equals method; false otherwise.
Throws
NullPointerException if the value is null

public synchronized boolean containsKey (Object key)

Tests if the specified object is a key in this hashtable.

Parameters
key possible key
Returns
  • true if and only if the specified object is a key in this hashtable, as determined by the equals method; false otherwise.
Throws
NullPointerException if the key is null
See Also

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 Map interface).

Parameters
value value whose presence in this hashtable is to be tested
Returns
  • true if this map maps one or more keys to the specified value
Throws
NullPointerException if the value is null

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.

Returns
  • an enumeration of the values in this hashtable.
See Also

public Set<Entry<K, V>> entrySet ()

Returns a Set 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.

Returns
  • a set view of the mappings contained in this map

public synchronized boolean equals (Object o)

Compares the specified Object with this Map for equality, as per the definition in the Map interface.

Parameters
o object to be compared for equality with this hashtable
Returns
  • true if the specified Object is equal to this Map
See Also

public synchronized void forEach (BiConsumer<? super K, ? super V> action)

Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.

Parameters
action The action to be performed for each entry

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.)

Parameters
key the key whose associated value is to be returned
Returns
  • the value to which the specified key is mapped, or null if this map contains no mapping for the key
Throws
NullPointerException if the specified key is null
See Also

public synchronized V getOrDefault (Object key, V defaultValue)

Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.

Parameters
key the key whose associated value is to be returned
defaultValue the default mapping of the key
Returns
  • the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key

public synchronized int hashCode ()

Returns the hash code value for this Map as per the definition in the Map interface.

Returns
  • this object's hash code.
See Also

public synchronized boolean isEmpty ()

Tests if this hashtable maps no keys to values.

Returns
  • true if this hashtable maps no keys to values; false otherwise.

public Set<K> keySet ()

Returns a 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 (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.

Returns
  • a set view of the keys contained in this map

public synchronized Enumeration<K> keys ()

Returns an enumeration of the keys in this hashtable.

Returns
  • an enumeration of the keys in this hashtable.
See Also

public synchronized V merge (K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)

If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a String msg to a value mapping: 

 map.merge(key, msg, String::concat)

If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
key key with which the resulting value is to be associated
value the non-null value to be merged with the existing value associated with the key or, if no existing value or a null value is associated with the key, to be associated with the key
remappingFunction the remapping function to recompute a value if present
Returns
  • the new value associated with the specified key, or null if no value is associated with the key

public synchronized V put (K key, V value)

Maps the specified key to the specified value in this hashtable. Neither the key nor the value can be null.

The value can be retrieved by calling the get method with a key that is equal to the original key.

Parameters
key the hashtable key
value the value
Returns
  • the previous value of the specified key in this hashtable, or null if it did not have one
Throws
NullPointerException if the key or value is null
See Also

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.

Parameters
t mappings to be stored in this map
Throws
NullPointerException if the specified map is null

public synchronized V putIfAbsent (K key, V value)

If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.

Parameters
key key with which the specified value is to be associated
value value to be associated with the specified key
Returns
  • the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)

public synchronized boolean remove (Object key, Object value)

Removes the entry for the specified key only if it is currently mapped to the specified value.

Parameters
key key with which the specified value is associated
value value expected to be associated with the specified key
Returns
  • true if the value was removed

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.

Parameters
key the key that needs to be removed
Returns
  • the value to which the key had been mapped in this hashtable, or null if the key did not have a mapping
Throws
NullPointerException if the key is null

public synchronized boolean replace (K key, V oldValue, V newValue)

Replaces the entry for the specified key only if currently mapped to the specified value.

Parameters
key key with which the specified value is associated
oldValue value expected to be associated with the specified key
newValue value to be associated with the specified key
Returns
  • true if the value was replaced

public synchronized V replace (K key, V value)

Replaces the entry for the specified key only if it is currently mapped to some value.

Parameters
key key with which the specified value is associated
value value to be associated with the specified key
Returns
  • the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)

public synchronized void replaceAll (BiFunction<? super K, ? super V, ? extends V> function)

Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Exceptions thrown by the function are relayed to the caller.

Parameters
function the function to apply to each entry

public synchronized int size ()

Returns the number of keys in this hashtable.

Returns
  • the number of keys in this hashtable.

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.

Returns
  • a string representation of this hashtable

public Collection<V> 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 (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.

Returns
  • a collection view of the values contained in this map

Protected Methods

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.