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java.lang.Object
java.util.Dictionary<K,V>
java.util.Hashtable<K,V>
This implementation of Hashtable uses a hash-bucket approach. That is: linear probing and rehashing is avoided; instead, each hashed value maps to a simple linked-list which, in the best case, only has one node. Assuming a large enough table, low enough load factor, and / or well implemented hashCode() methods, Hashtable should provide O(1) insertion, deletion, and searching of keys. Hashtable is O(n) in the worst case for all of these (if all keys hash to the same bucket).
This is a JDK-1.2 compliant implementation of Hashtable. As such, it belongs, partially, to the Collections framework (in that it implements Map). For backwards compatibility, it inherits from the obsolete and utterly useless Dictionary class.
Being a hybrid of old and new, Hashtable has methods which provide redundant capability, but with subtle and even crucial differences. For example, one can iterate over various aspects of a Hashtable with either an Iterator (which is the JDK-1.2 way of doing things) or with an Enumeration. The latter can end up in an undefined state if the Hashtable changes while the Enumeration is open.
Unlike HashMap, Hashtable does not accept `null' as a key value. Also, all accesses are synchronized: in a single thread environment, this is expensive, but in a multi-thread environment, this saves you the effort of extra synchronization. However, the old-style enumerators are not synchronized, because they can lead to unspecified behavior even if they were synchronized. You have been warned.
The iterators are fail-fast, meaning that any structural
modification, except for remove()
called on the iterator
itself, cause the iterator to throw a
ConcurrentModificationException
rather than exhibit
non-deterministic behavior.
HashMap
, TreeMap
, IdentityHashMap
, LinkedHashMap
, Serialized FormConstructor Summary | |
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Method Summary | |
Set |
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void |
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Object |
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boolean | |
boolean |
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boolean |
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Enumeration |
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boolean | |
V | |
int |
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boolean |
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Set |
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Enumeration |
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V |
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void |
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protected void |
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V | |
int |
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String |
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Collection |
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Methods inherited from class java.util.Dictionary<K,V> | |
elements , get , isEmpty , keys , put , remove , size |
Methods inherited from class java.lang.Object | |
clone , equals , extends Object> getClass , finalize , hashCode , notify , notifyAll , toString , wait , wait , wait |
public Hashtable()
Construct a new Hashtable with the default capacity (11) and the default load factor (0.75).
public Hashtable(extends K, V> m)
Construct a new Hashtable from the given Map, with initial capacity the greater of the size ofm
or the default of 11.Every element in Map m will be put into this new Hashtable.
- Parameters:
m
- a Map whose key / value pairs will be put into the new Hashtable. NOTE: key / value pairs are not cloned in this constructor.
- Throws:
NullPointerException
- if m is null, or if m contains a mapping to or from `null'.
- Since:
- 1.2
public Hashtable(int initialCapacity)
Construct a new Hashtable with a specific inital capacity and default load factor of 0.75.
- Parameters:
initialCapacity
- the initial capacity of this Hashtable (>= 0)
- Throws:
IllegalArgumentException
- if (initialCapacity < 0)
public Hashtable(int initialCapacity, float loadFactor)
Construct a new Hashtable with a specific initial capacity and load factor.
- Parameters:
initialCapacity
- the initial capacity (>= 0)loadFactor
- the load factor (> 0, not NaN)
- Throws:
IllegalArgumentException
- if (initialCapacity < 0) || ! (loadFactor > 0.0)
public SetV>> entrySet ()
Returns a "set view" of this Hashtable's entries. The set is backed by the hashtable, so changes in one show up in the other. The set supports element removal, but not element addition. The set is properly synchronized on the original hashtable. Sun has not documented the proper interaction of null with this set, but has inconsistent behavior in the JDK. Therefore, in this implementation, contains, remove, containsAll, retainAll, removeAll, and equals just ignore a null entry, or an entry with a null key or value, rather than throwing aNullPointerException
. However, calling entry.setValue(null) will fail.Note that the iterators for all three views, from keySet(), entrySet(), and values(), traverse the hashtable in the same sequence.
- Specified by:
- V>> entrySet in interface Map<K,V>
- Returns:
- a set view of the entries
- Since:
- 1.2
public Object clone()
Returns a shallow clone of this Hashtable. The Map itself is cloned, but its contents are not. This is O(n).
- Returns:
- the clone
public boolean contains(Object value)
Returns true if this Hashtable contains a valueo
, such thato.equals(value)
. This is the same ascontainsValue()
, and is O(n).
- Parameters:
value
- the value to search for in this Hashtable
- Returns:
- true if at least one key maps to the value
- Throws:
NullPointerException
- ifvalue
is null
- See Also:
containsValue(Object)
,containsKey(Object)
public boolean containsKey(Object key)
Returns true if the supplied objectequals()
a key in this Hashtable.
- Specified by:
- containsKey in interface Map<K,V>
- Parameters:
key
- the key to search for in this Hashtable
- Returns:
- true if the key is in the table
- Throws:
NullPointerException
- if key is null
- See Also:
containsValue(Object)
public boolean containsValue(Object value)
Returns true if this Hashtable contains a valueo
, such thato.equals(value)
. This is the new API for the oldcontains()
.
- Specified by:
- containsValue in interface Map<K,V>
- Parameters:
value
- the value to search for in this Hashtable
- Returns:
- true if at least one key maps to the value
- Throws:
NullPointerException
- ifvalue
is null
- Since:
- 1.2
- See Also:
contains(Object)
,containsKey(Object)
public Enumerationelements()
Return an enumeration of the values of this table. There's no point in synchronizing this, as you have already been warned that the enumeration is not specified to be thread-safe.
- Overrides:
- elements in interface Dictionary<K,V>
- Returns:
- the values
public boolean equals(Object o)
Returns true if this Hashtable equals the supplied Objecto
. As specified by Map, this is:(o instanceof Map) && entrySet().equals(((Map) o).entrySet());
- Parameters:
o
- the object to compare to
- Returns:
- true if o is an equal map
- Since:
- 1.2
public V get(Object key)
Return the value in this Hashtable associated with the supplied key, ornull
if the key maps to nothing.
- Overrides:
- get in interface Dictionary<K,V>
- Parameters:
key
- the key for which to fetch an associated value
- Returns:
- what the key maps to, if present
- Throws:
NullPointerException
- if key is null
- See Also:
put(Object, Object)
,containsKey(Object)
public int hashCode()
Returns the hashCode for this Hashtable. As specified by Map, this is the sum of the hashCodes of all of its Map.Entry objects
- Returns:
- the sum of the hashcodes of the entries
- Since:
- 1.2
public boolean isEmpty()
Returns true if there are no key-value mappings currently in this table.
- Overrides:
- isEmpty in interface Dictionary<K,V>
- Returns:
size() == 0
public SetkeySet()
Returns a "set view" of this Hashtable's keys. The set is backed by the hashtable, so changes in one show up in the other. The set supports element removal, but not element addition. The set is properly synchronized on the original hashtable. Sun has not documented the proper interaction of null with this set, but has inconsistent behavior in the JDK. Therefore, in this implementation, contains, remove, containsAll, retainAll, removeAll, and equals just ignore a null key rather than throwing aNullPointerException
.
- Returns:
- a set view of the keys
- Since:
- 1.2
- See Also:
values()
,entrySet()
public Enumerationkeys()
Return an enumeration of the keys of this table. There's no point in synchronizing this, as you have already been warned that the enumeration is not specified to be thread-safe.
- Overrides:
- keys in interface Dictionary<K,V>
- Returns:
- the keys
- See Also:
elements()
,keySet()
public V put(K key, V value)
Puts the supplied value into the Map, mapped by the supplied key. Neither parameter may be null. The value may be retrieved by any object whichequals()
this key.
- Overrides:
- put in interface Dictionary<K,V>
- Parameters:
key
- the key used to locate the valuevalue
- the value to be stored in the table
- Returns:
- the prior mapping of the key, or null if there was none
- Throws:
NullPointerException
- if key or value is null
- See Also:
get(Object)
,Object.equals(Object)
public void putAll(extends K, V> m)
Copies all elements of the given map into this hashtable. However, no mapping can contain null as key or value. If this table already has a mapping for a key, the new mapping replaces the current one.
- Parameters:
m
- the map to be hashed into this
- Throws:
NullPointerException
- if m is null, or contains null keys or values
protected void rehash()
Increases the size of the Hashtable and rehashes all keys to new array indices; this is called when the addition of a new value would cause size() > threshold. Note that the existing Entry objects are reused in the new hash table.This is not specified, but the new size is twice the current size plus one; this number is not always prime, unfortunately. This implementation is not synchronized, as it is only invoked from synchronized methods.
public V remove(Object key)
Removes from the table and returns the value which is mapped by the supplied key. If the key maps to nothing, then the table remains unchanged, andnull
is returned.
- Overrides:
- remove in interface Dictionary<K,V>
- Parameters:
key
- the key used to locate the value to remove
- Returns:
- whatever the key mapped to, if present
public int size()
Returns the number of key-value mappings currently in this hashtable.
- Overrides:
- size in interface Dictionary<K,V>
- Returns:
- the size
public String toString()
Converts this Hashtable to a String, surrounded by braces, and with key/value pairs listed with an equals sign between, separated by a comma and space. For example,"{a=1, b=2}"
.NOTE: if the
toString()
method of any key or value throws an exception, this will fail for the same reason.
- Returns:
- the string representation
public Collectionvalues()
Returns a "collection view" (or "bag view") of this Hashtable's values. The collection is backed by the hashtable, so changes in one show up in the other. The collection supports element removal, but not element addition. The collection is properly synchronized on the original hashtable. Sun has not documented the proper interaction of null with this set, but has inconsistent behavior in the JDK. Therefore, in this implementation, contains, remove, containsAll, retainAll, removeAll, and equals just ignore a null value rather than throwing aNullPointerException
.
- Returns:
- a bag view of the values
- Since:
- 1.2
- See Also:
keySet()
,entrySet()