[轉]HashTable vs HashMap

HashTable和HashMap的不同主要有四種:
                                                                               
一、HashTable 是繼承java.util.Dictionary
   HashMap   是繼承java.util.AbstractMap
                                                                               
Dictionary 是舊的class，
【DOC】NOTE: This class is obsolete.
New implementations should implement the Map interface, rather than
extending this class.
                                                                               
java.util.AbstractMap 已經implements Map Interface，因此繼承它的class
                                                                               
只需要實作它的method就可以實作出需要的Map (例如get,put,entrySet method)
可參：http://java.sun.com/j2se/1.4.2/docs/api/java/util/AbstractMap.html
                                                                              
java.util.Dictionary 並沒有去Implements Map Interface，HashTable是另外去
implements Map interface，也因此HashTable能被算在Java Util Framework中。
                                                                              
【DOC】" 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"                                                                             
                                                                               另外，Dictionary只提供最基本的abstract method
                                                                              
【DOC】"The Dictionary class is the abstract parent of any class,                                                                             such as Hashtable, which maps keys to values"
                                                                              
二、HashMap 的Key跟Value都可以是null，HashTable則不行
                                                                              
也就是在HashMap裡可以這麼寫：
HashMap m = new HashMap();
m.put("1",null);
m.put(null, new Integer(2));
                                                                              
三、最大的不同是 HashTable的method是 Synchronized，而HashMap則沒有
                                                                              
【DOC】"Unlike the new collection implementations, Hashtable is synchronized."
                                                                              
因此有兩個情形要考慮：
                                                                              
(1)沒有Multithread的情形下，使用HashMap即可，因為HashTable有作Synchronization，相對地它的Performance是劣於HashMap的
                                                                              
(2)有Multithread情形時，如果要使用HashMap，要作外部的Synchronization                                                                              
【DOC】"If multiple threads access this map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally."
                                                                              
方法有很多種，API DOC裡提到可以對包含這個Map的Object作Synchronized
Map m = Collections.synchronizedMap(new HashMap(...));

四、HashMap拿掉了HashTable的contains(Object value)因為這個method容易造成混淆倒底是contains key還是value？？ 這個method改用containsValue(Object value)取代掉了。

ref:http://psalter.spaces.live.com/blog/cns!F7AA90C7BB6EC803!124.entry