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java.lang.Object
|
+--java.util.Dictionary
|
+--java.util.Hashtable
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 efficiency: its capacity and its load
factor. The load factor should be between 0.0 and 1.0. When
the number of entries in the hashtable exceeds the product of the
load factor and the current capacity, the capacity is increased by
calling the rehash method. Larger load factors use
memory more efficiently, at the expense of larger expected time
per lookup.
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 numbers = new Hashtable();
numbers.put("one", new Integer(1));
numbers.put("two", new Integer(2));
numbers.put("three", new Integer(3));
To retrieve a number, use the following code:
Integer n = (Integer)numbers.get("two");
if (n != null) {
System.out.println("two = " + n);
}
As of JDK1.2, this class has been retrofitted to implement Map, so that it becomes a part of Java's collection framework. Unlike the new collection implementations, Vector is synchronized.
The Iterators returned by the iterator and listIterator methods of the Collections returned by all of Hashtable'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 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. The Enumerations returned by Hashtable's keys and values methods are not fail-fast.
| Inner Class Summary | |
| static | Hashtable.Entry
Hashtable collision list. |
| Hashtable.EntrySet
|
|
| Hashtable.Enumerator
A hashtable enumerator class. |
|
| Hashtable.KeySet
|
|
| Hashtable.ValueCollection
|
|
| Field Summary | |
| int | count
The total number of entries in the hash table. |
| static int | ENTRIES
|
| java.util.Set | entrySet
|
| java.util.Set | keySet
|
| static int | KEYS
|
| float | loadFactor
The load factor for the hashtable. |
| int | modCount
The 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. |
| static long | serialVersionUID
use serialVersionUID from JDK 1.0.2 for interoperability |
| Hashtable.Entry[] | table
The hash table data. |
| int | threshold
The table is rehashed when its size exceeds this threshold. |
| java.util.Collection | values
|
| static int | VALUES
|
| 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, which is 0.75. |
|
| Hashtable()
Constructs a new, empty hashtable with a default capacity and load factor, which is 0.75. |
|
| Hashtable(java.util.Map t)
Constructs a new hashtable with the same mappings as the given Map. |
|
| Method Summary | |
| void | clear()
Clears this hashtable so that it contains no keys. |
| java.lang.Object | clone()
Creates a shallow copy of this hashtable. |
| boolean | contains(java.lang.Object value)
Tests if some key maps into the specified value in this hashtable. |
| boolean | containsKey(java.lang.Object key)
Tests if the specified object is a key in this hashtable. |
| boolean | containsValue(java.lang.Object value)
Returns true if this Hashtable maps one or more keys to this value. |
| java.util.Enumeration | elements()
Returns an enumeration of the values in this hashtable. |
| java.util.Set | entrySet()
Returns a Set view of the entries contained in this Hashtable. |
| boolean | equals(java.lang.Object o)
Compares the specified Object with this Map for equality, as per the definition in the Map interface. |
| java.lang.Object | get(java.lang.Object key)
Returns the value to which the specified key is mapped in this hashtable. |
| int | hashCode()
Returns the hash code value for this Map as per the definition in the Map interface. |
| boolean | isEmpty()
Tests if this hashtable maps no keys to values. |
| java.util.Set | keySet()
Returns a Set view of the keys contained in this Hashtable. |
| java.util.Enumeration | keys()
Returns an enumeration of the keys in this hashtable. |
| void | putAll(java.util.Map 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. |
| java.lang.Object | put(java.lang.Object key,
java.lang.Object value)
Maps the specified key to the specified
value in this hashtable. |
| void | readObject(java.io.ObjectInputStream s)
Reconstitute the Hashtable from a stream (i.e. |
| void | rehash()
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. |
| java.lang.Object | remove(java.lang.Object key)
Removes the key (and its corresponding value) from this hashtable. |
| int | size()
Returns the number of keys in this hashtable. |
| java.lang.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). |
| java.util.Collection | values()
Returns a Collection view of the values contained in this Hashtable. |
| void | writeObject(java.io.ObjectOutputStream s)
Save the state of the Hashtable to a stream (i.e. |
| Methods inherited from class java.util.Dictionary | |
| elements, get, isEmpty, keys, put, remove, size | |
| Methods inherited from class java.lang.Object | |
| Field Detail |
private transient Hashtable.Entry[] table
private transient int count
private int threshold
private float loadFactor
private transient int modCount
private static final long serialVersionUID
private transient java.util.Set keySet
private transient java.util.Set entrySet
private transient java.util.Collection values
private static final int KEYS
private static final int VALUES
private static final int ENTRIES
| Constructor Detail |
public Hashtable(int initialCapacity,
float loadFactor)
initialCapacity
- the initial capacity of the hashtable.
loadFactor
- a number between 0.0 and 1.0.public Hashtable(int initialCapacity)
initialCapacity
- the initial capacity of the hashtable.public Hashtable()
public Hashtable(java.util.Map t)
| Method Detail |
public int size()
public boolean isEmpty()
true if this hashtable maps no keys to values;
false otherwise.public java.util.Enumeration keys()
public java.util.Enumeration elements()
public boolean contains(java.lang.Object value)
containsKey
method.
Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the collections framework).
value
- a value to search for.true if and only if some key maps to the
value argument in this hashtable as
determined by the equals method;
false otherwise.null.public boolean containsValue(java.lang.Object value)
Note that this method is identical in functionality to contains (which predates the Map interface).
value
- value whose presence in this Hashtable is to be tested.public boolean containsKey(java.lang.Object key)
key
- possible key.true if and only if the specified object
is a key in this hashtable, as determined by the
equals method; false otherwise.public java.lang.Object get(java.lang.Object key)
key
- a key in the hashtable.null if the key is not mapped to any value in
this hashtable.protected void rehash()
public java.lang.Object put(java.lang.Object key,
java.lang.Object value)
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.
key
- the hashtable key.
value
- the value.null if it did not have one.null.public java.lang.Object remove(java.lang.Object key)
key
- the key that needs to be removed.null if the key did not have a mapping.public void putAll(java.util.Map t)
public void clear()
public java.lang.Object clone()
public java.lang.String toString()
Overrides to toString method of Object.
public java.util.Set keySet()
public java.util.Set entrySet()
public java.util.Collection values()
public boolean equals(java.lang.Object o)
public int hashCode()
private void writeObject(java.io.ObjectOutputStream s)
throws IOException
private void readObject(java.io.ObjectInputStream s)
throws IOException,
java.lang.ClassNotFoundException
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