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
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 MapHashtable is synchronized.
java.lang.Object.equals(java.lang.Object)java.lang.Object.hashCode()rehash()CollectionMapHashMapTreeMapinitialCapacity the initial capacity of the hashtable.loadFactor the load factor of the hashtable.java.lang.IllegalArgumentException if the initial capacity is less
than zero, or if the load factor is nonpositive.initialCapacity the initial capacity of the hashtable.java.lang.IllegalArgumentException if the initial capacity is less
than zero.t the map whose mappings are to be placed in this map.java.lang.NullPointerException if the specified map is null.Enumerationelements()keySet()MapEnumerationkeys()values()Map containsKey method.
Note that this method is identical in functionality to
containsValue, (which is part of the
Map
value a value to search fortrue if and only if some key maps to the
value argument in this hashtable as
determined by the equals method;
false otherwise.java.lang.NullPointerException if the value is nullNote that this method is identical in functionality to contains(java.lang.Object)Map
value value whose presence in this hashtable is to be testedjava.lang.NullPointerException if the value is nullkey possible keytrue if and only if the specified object
is a key in this hashtable, as determined by the
equals method; false otherwise.java.lang.NullPointerException if the key is nullcontains(java.lang.Object)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.)
key the key whose associated value is to be returnednull if this map contains no mapping for the keyjava.lang.NullPointerException if the specified key is nullput(java.lang.Object,java.lang.Object)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 keyvalue the valuenull if it did not have onejava.lang.NullPointerException if the key or value is
nulljava.lang.Object.equals(java.lang.Object)get(java.lang.Object)key the key that needs to be removednull if the key did not have a mappingjava.lang.NullPointerException if the key is nullt mappings to be stored in this mapjava.lang.NullPointerException if the specified map is nullSetSetCollectiono object to be compared for equality with this hashtableMap.equals(java.lang.Object)Map.hashCode()This differs from the regular put method in several ways. No checking for rehashing is necessary since the number of elements initially in the table is known. The modCount is not incremented because we are creating a new instance. Also, no return value is needed.