认识Hashtable的本质

public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, Serializable

Direct Known Subclasses:
    Properties, UIDefaults

This class implements a hashtable, which maps keys to values. Any non-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. 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.

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.

以上是Oracle标准文档，拜读了一遍。 下面就其使用做一小小归纳：
1. 迭代（iterate）
// 1
    Hashtable ht = new Hashtable();
    ht.put("one",new Integer(1));
    ht.put("two",new Integer(2));

    Enumeration en = ht.elements();
    while(en.hasMoreElements()){
      // System.out.println((Integer)en.nextElement());
    }

// 2
Hashtable ht = new Hashtable();
ht.put("one",new Integer(1));
ht.put("two",new Integer(2));

Iterator it=hr.keySet().Iterator();
while(it.hasNext()){
   // System.out.println(hr.get(it.next()));
}

// 3
   Set s = ht.entrySet();
   for (Object o : s.toArray()){
     // System.out.println(o);
   }
     
2. 关于Hashtable与HashMap区别

   第一个不同是Hashtable是基于陈旧的Dictionary类的，HashMap是Java 1.2引进的Map接口的一个实现。

　　第二，最重要的是Hashtable的方法是同步的，而HashMap的方法不是。这就意味着，虽然你可以不用采取任何特殊的行为就可以在一个多线程的应用程序中用一个Hashtable，但你必须同样地为一个HashMap提供外同步。一个方便的方法就是利用Collections类的静态的synchronizedMap()方法，它创建一个线程安全的 Map对象，并把它作为一个封装的对象来返回。这个对象的方法可以让你同步访问潜在的HashMap。这么做的结果就是当你不需要同步时，你不能切断 Hashtable中的同步（比如在一个单线程的应用程序中），而且同步增加了很多处理费用。

　　第三点，只有HashMap可以让你将空值作为一个表的条目的key或value。HashMap中只有一条记录可以是一个空的key，但任意数量的条目可以是空的value。这就是说，如果在表中没有发现搜索键，或者如果发现了搜索键，但它是一个空的值，那么get()将返回null。如果有必要，用containKey()方法来区别这两种情况。

　　一些资料建议，当需要同步时，用Hashtable，反之用HashMap。但是，因为在需要时，HashMap可以被同步，HashMap的功能比 Hashtable的功能更多，而且它不是基于一个陈旧的类的，所以有人认为，在各种情况下，HashMap都优先于Hashtable。

　　关于Properties

　　有时侯，你可能想用一个hashtable来映射key的字符串到value的字符串。DOS、Windows和Unix中的环境字符串就有一些例子，如key的字符串PATH被映射到value的字符串C:\WINDOWS;C:\WINDOWS\SYSTEM。Hashtables是表示这些的一个简单的方法，但Java提供了另外一种方法：Java.util.Properties类是Hashtable的一个子类，设计用于String keys和values。Properties对象的用法同Hashtable的用法相象，但是类增加了两个节省时间的方法，你应该知道。

　　Store()方法把一个Properties对象的内容以一种可读的形式保存到一个文件中。Load()方法正好相反，用来读取文件，并设定Properties对象来包含keys和values。

　　注意，因为Properties扩展了Hashtable，可以用超类的put()方法来添加不是String对象的keys和values，但这是不可取的。另外，如果你将store()用于一个不包含String对象的Properties对象，store()将失败。作为put()和get()的替代，你应该用setProperty()和getProperty()，它们用String参数。

参考资料：
http://download.oracle.com/javase/6/docs/api/java/util/Hashtable.html
http://www.baihugu.com/blog/c2010/173.html
http://blog.csdn.net/longronglin/archive/2006/10/29/1355878.aspx