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FlexDoc/Javadoc 2.0 Demo Java Doc |
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; if the Hashtable is structurally modified at any time after the enumeration is created then the results of enumerating are undefined.
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.
Nested Class Summary |
Nested classes/interfaces inherited from interface java.util.Map |
Constructor Summary |
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Constructs a new, empty hashtable with a default initial capacity (11)
and load factor (0.75).
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Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity
and default load factor (0.75).
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Hashtable(int initialCapacity, float loadFactor)
Constructs a new, empty hashtable with the specified initial
capacity and the specified load factor.
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Constructs a new hashtable with the same mappings as the given
Map.
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Method Summary |
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void |
clear()
Clears this hashtable so that it contains no keys.
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clone()
Creates a shallow copy of this hashtable.
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Attempts to compute a mapping for the specified key and its current
mapped value (or null if there is no current mapping).
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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.
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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.
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boolean |
Tests if some key maps into the specified value in this hashtable.
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boolean |
containsKey(Object key)
Tests if the specified object is a key in this hashtable.
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boolean |
containsValue(Object value)
Returns true if this hashtable maps one or more keys to this value.
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elements()
Returns an enumeration of the values in this hashtable.
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entrySet()
Returns a Set view of the mappings contained in this map.
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boolean |
Compares the specified Object with this Map for equality,
as per the definition in the Map interface.
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void |
Performs the given action for each entry in this map until all entries
have been processed or the action throws an exception.
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Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.
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Returns the value to which the specified key is mapped, or
defaultValue if this map contains no mapping for the key.
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int |
hashCode()
Returns the hash code value for this Map as per the definition in the
Map interface.
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boolean |
isEmpty()
Tests if this hashtable maps no keys to values.
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keys()
Returns an enumeration of the keys in this hashtable.
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keySet()
Returns a Set view of the keys contained in this map.
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If the specified key is not already associated with a value or is
associated with null, associates it with the given non-null value.
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Maps the specified key to the specified
value in this hashtable.
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void |
Copies all of the mappings from the specified map to this hashtable.
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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.
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protected void |
rehash()
Increases the capacity of and internally reorganizes this
hashtable, in order to accommodate and access its entries more
efficiently.
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Removes the key (and its corresponding value) from this
hashtable.
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boolean |
Removes the entry for the specified key only if it is currently
mapped to the specified value.
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Replaces the entry for the specified key only if it is
currently mapped to some value.
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boolean |
Replaces the entry for the specified key only if currently
mapped to the specified value.
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void |
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.
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int |
size()
Returns the number of keys in this hashtable.
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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). |
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values()
Returns a Collection view of the values contained in this map.
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Methods inherited from class java.lang.Object |
Methods inherited from interface java.util.Map |
public Hashtable |
(int initialCapacity, float loadFactor) |
public Hashtable |
(int initialCapacity) |
public Hashtable |
() |
public Hashtable |
public int size |
() |
public boolean isEmpty |
() |
() |
() |
public boolean contains |
(Object value) |
Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the collections framework).
public boolean containsValue |
(Object value) |
Note that this method is identical in functionality to contains (which predates the Map interface).
public boolean containsKey |
(Object key) |
public V get |
(Object 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.)
protected void rehash |
() |
public V put |
The value can be retrieved by calling the get method with a key that is equal to the original key.
public V remove |
(Object key) |
public void putAll |
public void clear |
() |
public Object clone |
() |
public String toString |
() |
,
" (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.
() |
() |
() |
public boolean equals |
(Object o) |
public int hashCode |
() |
public V getOrDefault |
public void forEach |
public void replaceAll |
public V putIfAbsent |
public boolean remove |
public boolean replace |
public V replace |
public V computeIfAbsent |
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.
This method will, on a best-effort basis, throw a ConcurrentModificationException if the mapping function modified this map during computation.
public V computeIfPresent |
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.
This method will, on a best-effort basis, throw a ConcurrentModificationException if the remapping function modified this map during computation.
public V compute |
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.
This method will, on a best-effort basis, throw a ConcurrentModificationException if the remapping function modified this map during computation.
public V merge |
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.
This method will, on a best-effort basis, throw a ConcurrentModificationException if the remapping function modified this map during computation.
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FlexDoc/Javadoc 2.0 Demo Java Doc |