This class
implements the Map interface using an internal
hashtable. It supports all optional Map methods,
allows key and value objects of any types, and allows
null to be used as a key or a value. Because
HashMap is based on a hashtable data structure,
the get( ) and put( ) methods
are very efficient. HashMap is much like the
Hashtable class, except that the
HashMap methods are not
synchronized (and are therefore faster), and
HashMap allows null to be used
as a key or a value. If you are working in a multithreaded
environment, or if compatibility with previous versions of Java is a
concern, use Hashtable. Otherwise, use
HashMap.
If you know in advance approximately how many mappings a
HashMap will contain, you can improve efficiency
by specifying initialCapacity when you
call the HashMap( ) constructor. The
initialCapacity argument times the
loadFactor argument should be greater than
the number of mappings the HashMap will contain. A
good value for loadFactor is 0.75; this is
also the default value. See Map for details on the
methods of HashMap. See also
TReeMap and HashSet.
Figure 16-24. java.util.HashMap<K,V>
public class
HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {
// Public Constructors
public
HashMap ( );
public
HashMap (int
initialCapacity );
public
HashMap (Map<? extends K,? extends V>
m );
public
HashMap (int
initialCapacity , float
loadFactor );
// Methods Implementing Map
public void
clear ( );
public boolean
containsKey (Object
key );
public boolean
containsValue (Object
value );
public Set<Map.Entry<K,V>>
entrySet ( );
public V
get (Object
key );
public boolean
isEmpty ( ); default:true
public Set<K>
keySet ( );
public V
put (K
key , V
value );
public void
putAll (Map<? extends K,? extends V>
m );
public V
remove (Object
key );
public int
size ( );
public Collection<V>
values ( );
// Public Methods Overriding AbstractMap
public Object
clone ( );
}