Hashtable (Java Platform SE 6) (original) (raw)

java.util

Class Hashtable<K,V>

java.lang.Object java.util.Dictionary<K,V> java.util.Hashtable<K,V>

All Implemented Interfaces:

Serializable, Cloneable, Map<K,V>

Direct Known Subclasses:

Properties, UIDefaults

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 mostHashtable 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 theHashtable 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 ownremove 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.

Since:

JDK1.0

See Also:

Object.equals(java.lang.Object), Object.hashCode(), rehash(), Collection, Map, HashMap, TreeMap, Serialized Form

Hashtable

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](../../java/lang/IllegalArgumentException.html "class in java.lang") - if the initial capacity is less than zero, or if the load factor is nonpositive.

Hashtable

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](../../java/lang/IllegalArgumentException.html "class in java.lang") - if the initial capacity is less than zero.

Hashtable

public Hashtable()

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

Hashtable

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](../../java/lang/NullPointerException.html "class in java.lang") - if the specified map is null.

Since:

1.2

size

public int size()

Returns the number of keys in this hashtable.

Specified by:

[size](../../java/util/Map.html#size%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Specified by:

[size](../../java/util/Dictionary.html#size%28%29) in class [Dictionary](../../java/util/Dictionary.html "class in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Returns:

the number of keys in this hashtable.

isEmpty

public boolean isEmpty()

Tests if this hashtable maps no keys to values.

Specified by:

[isEmpty](../../java/util/Map.html#isEmpty%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Specified by:

[isEmpty](../../java/util/Dictionary.html#isEmpty%28%29) in class [Dictionary](../../java/util/Dictionary.html "class in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Returns:

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

keys

public Enumeration<K> keys()

Returns an enumeration of the keys in this hashtable.

Specified by:

[keys](../../java/util/Dictionary.html#keys%28%29) in class [Dictionary](../../java/util/Dictionary.html "class in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Returns:

an enumeration of the keys in this hashtable.

See Also:

Enumeration, elements(), keySet(), Map

elements

public 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.

Specified by:

[elements](../../java/util/Dictionary.html#elements%28%29) in class [Dictionary](../../java/util/Dictionary.html "class in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Returns:

an enumeration of the values in this hashtable.

See Also:

Enumeration, keys(), values(), Map

contains

public 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 tocontainsValue, (which is part of theMap interface in the collections framework).

Parameters:

value - a value to search for

Returns:

true if and only if some key maps to thevalue argument in this hashtable as determined by the equals method;false otherwise.

Throws:

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the value is null

containsValue

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).

Specified by:

[containsValue](../../java/util/Map.html#containsValue%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

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](../../java/lang/NullPointerException.html "class in java.lang") - if the value is null

Since:

1.2

containsKey

public boolean containsKey(Object key)

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

Specified by:

[containsKey](../../java/util/Map.html#containsKey%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Parameters:

key - possible key

Returns:

true if and only if the specified object is a key in this hashtable, as determined by theequals method; false otherwise.

Throws:

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the key is null

See Also:

contains(Object)

get

public 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 keyk to a value v such that (key.equals(k)), then this method returns v; otherwise it returnsnull. (There can be at most one such mapping.)

Specified by:

[get](../../java/util/Map.html#get%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Specified by:

[get](../../java/util/Dictionary.html#get%28java.lang.Object%29) in class [Dictionary](../../java/util/Dictionary.html "class in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Parameters:

key - the key whose associated value is to be returned

Returns:

the value to which the specified key is mapped, ornull if this map contains no mapping for the key

Throws:

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified key is null

See Also:

[put(Object, Object)](../../java/util/Hashtable.html#put%28K, V%29)

rehash

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.

put

public V put(K key, V value)

Maps the specified key to the specifiedvalue 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.

Specified by:

[put](../../java/util/Map.html#put%28K, V%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Specified by:

[put](../../java/util/Dictionary.html#put%28K, V%29) in class [Dictionary](../../java/util/Dictionary.html "class in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

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](../../java/lang/NullPointerException.html "class in java.lang") - if the key or value isnull

See Also:

Object.equals(Object), get(Object)

remove

public 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.

Specified by:

[remove](../../java/util/Map.html#remove%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Specified by:

[remove](../../java/util/Dictionary.html#remove%28java.lang.Object%29) in class [Dictionary](../../java/util/Dictionary.html "class in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in 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](../../java/lang/NullPointerException.html "class in java.lang") - if the key is null

putAll

public 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.

Specified by:

[putAll](../../java/util/Map.html#putAll%28java.util.Map%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Parameters:

t - mappings to be stored in this map

Throws:

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified map is null

Since:

1.2

clear

public void clear()

Clears this hashtable so that it contains no keys.

Specified by:

[clear](../../java/util/Map.html#clear%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

clone

public 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.

Overrides:

[clone](../../java/lang/Object.html#clone%28%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Returns:

a clone of the hashtable

See Also:

Cloneable

toString

public 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.

Overrides:

[toString](../../java/lang/Object.html#toString%28%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Returns:

a string representation of this hashtable

keySet

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 theIterator.remove, Set.remove,removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:

[keySet](../../java/util/Map.html#keySet%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Returns:

a set view of the keys contained in this map

Since:

1.2

entrySet

public Set<Map.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 thesetValue 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 andclear operations. It does not support theadd or addAll operations.

Specified by:

[entrySet](../../java/util/Map.html#entrySet%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Returns:

a set view of the mappings contained in this map

Since:

1.2

values

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.

Specified by:

[values](../../java/util/Map.html#values%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Returns:

a collection view of the values contained in this map

Since:

1.2

equals

public boolean equals(Object o)

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

Specified by:

[equals](../../java/util/Map.html#equals%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Overrides:

[equals](../../java/lang/Object.html#equals%28java.lang.Object%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Parameters:

o - object to be compared for equality with this hashtable

Returns:

true if the specified Object is equal to this Map

Since:

1.2

See Also:

Map.equals(Object)

hashCode

public int hashCode()

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

Specified by:

[hashCode](../../java/util/Map.html#hashCode%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/Hashtable.html "type parameter in Hashtable"),[V](../../java/util/Hashtable.html "type parameter in Hashtable")>

Overrides:

[hashCode](../../java/lang/Object.html#hashCode%28%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Returns:

a hash code value for this object.

Since:

1.2

See Also:

Map.hashCode()

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