Package rma.util

Class RMAHashtable

java.lang.Object
java.util.Dictionary
rma.util.RMAHashtable
All Implemented Interfaces:
Serializable, Cloneable
public class RMAHashtable extends Dictionary implements 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.

An instance of RMAHashtable 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 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. 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.

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 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 the Java 2 platform v1.2, this class has been retrofitted to implement Map, so that it becomes a part of Java's collection framework. Unlike the new collection implementations, Hashtable 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.

Since:
JDK1.0
See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    RMAHashtable()
    Constructs a new, empty hashtable with a default capacity and load factor, which is 0.75.
    RMAHashtable(int initialCapacity)
    Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is 0.75.
    RMAHashtable(int initialCapacity, float loadFactor)
    Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
    RMAHashtable(Map t)
    Constructs a new hashtable with the same mappings as the given Map.

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    clear()
    Clears this hashtable so that it contains no keys.
    Object
    clone()
    Creates a shallow copy of this hashtable.
    boolean
    contains(Object value)
    Tests if some key maps into the specified value in this hashtable.
    boolean
    containsKey(Object key)
    Tests if the specified object is a key in this hashtable.
    boolean
    containsValue(Object value)
    Returns true if this RMAHashtable maps one or more keys to this value.
    Enumeration
    elements()
    Returns an enumeration of the values in this hashtable.
    Object
    get(Object key)
    Returns the value to which the specified key is mapped in this hashtable.
    boolean
    isEmpty()
    Tests if this hashtable maps no keys to values.
    Enumeration
    keys()
    Returns an enumeration of the keys in this hashtable.
    Object
    put(Object key, Object value)
    Maps the specified key to the specified value in this hashtable.
    void
    putAll(Map t)
    Copies all of the mappings from the specified Map to this RMAHashtable These mappings will replace any mappings that this RMAHashtable had for any of the keys currently in the specified Map.
    protected void
    rehash()
    Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently.
    Object
    remove(Object key)
    Removes the key (and its corresponding value) from this hashtable.
    int
    size()
    Returns the number of keys in this hashtable.

    Methods inherited from class java.lang.Object

    equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Constructor Details

    • RMAHashtable

      public RMAHashtable(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 - if the initial capacity is less than zero, or if the load factor is nonpositive.

    • RMAHashtable

      public RMAHashtable(int initialCapacity)
      Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is 0.75.
      Parameters:
      initialCapacity - the initial capacity of the hashtable.
      Throws:
      IllegalArgumentException - if the initial capacity is less than zero.

    • RMAHashtable

      public RMAHashtable()
      Constructs a new, empty hashtable with a default capacity and load factor, which is 0.75.

    • RMAHashtable

      public RMAHashtable(Map t)
      Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with a capacity of twice the number of entries in the given Map or 11 (whichever is greater), and a default load factor, which is 0.75.
      Parameters:
      t - the map whose mappings are to be placed in this map.
      Since:
      1.2

  • Method Details

    • size

      public int size()
      Returns the number of keys in this hashtable.
      Specified by:
      size in class Dictionary
      Returns:
      the number of keys in this hashtable.

    • isEmpty

      public boolean isEmpty()
      Tests if this hashtable maps no keys to values.
      Specified by:
      isEmpty in class Dictionary
      Returns:
      true if this hashtable maps no keys to values; false otherwise.

    • keys

      public Enumeration keys()
      Returns an enumeration of the keys in this hashtable.
      Specified by:
      keys in class Dictionary
      Returns:
      an enumeration of the keys in this hashtable.
      See Also:

    • elements

      public Enumeration 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 in class Dictionary
      Returns:
      an enumeration of the values in this hashtable.
      See Also:

    • 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 to containsValue, (which is part of the Map interface in the collections framework).

      Parameters:
      value - a value to search for.
      Returns:
      true if and only if some key maps to the value argument in this hashtable as determined by the equals method; false otherwise.
      Throws:
      NullPointerException - if the value is null.
      See Also:

    • containsValue

      public boolean containsValue(Object value)
      Returns true if this RMAHashtable maps one or more keys to this value.

      Note that this method is identical in functionality to contains (which predates the Map interface).

      Parameters:
      value - value whose presence in this RMAHashtable is to be tested.
      Returns:
      true if this map maps one or more keys to the specified value.
      Since:
      1.2
      See Also:

    • containsKey

      public boolean containsKey(Object key)
      Tests if the specified object is a key in this hashtable.
      Parameters:
      key - possible key.
      Returns:
      true if and only if the specified object is a key in this hashtable, as determined by the equals method; false otherwise.
      See Also:

    • get

      public Object get(Object key)
      Returns the value to which the specified key is mapped in this hashtable.
      Specified by:
      get in class Dictionary
      Parameters:
      key - a key in the hashtable.
      Returns:
      the value to which the key is mapped in this hashtable; null if the key is not mapped to any value in this hashtable.
      See Also:

    • 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 Object put(Object key, Object value)
      Maps the specified 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.

      Specified by:
      put in class Dictionary
      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 - if the key or value is null.
      See Also:

    • remove

      public Object 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 in class Dictionary
      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.

    • putAll

      public void putAll(Map t)
      Copies all of the mappings from the specified Map to this RMAHashtable These mappings will replace any mappings that this RMAHashtable had for any of the keys currently in the specified Map.
      Parameters:
      t - Mappings to be stored in this map.
      Since:
      1.2

    • clear

      public void clear()
      Clears this hashtable so that it contains no keys.

    • 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 in class Object
      Returns:
      a clone of the hashtable.