关于HashTable的遍历方法解析

要遍历一个Hashtable，api中提供了如下几个方法可供我们遍历：

　　keys() - returns an Enumeration of the keys of this Hashtable

　　keySet() - returns a Set of the keys

　　entrySet() - returns a Set of the mappings

　　elements() - returns an Enumeration of the values of this Hashtable

4种方法，那种更好呢，写段代码来比较一下吧：

import java.util.Enumeration;

import java.util.Hashtable;

import java.util.Iterator;

import java.util.Map.Entry;

public class HashtableTest
{

     public static void main(String[]
args) {

           long start
= 0;

           long end
= 0;

          

          Hashtable<String, String> table = new Hashtable<String,
String>();

           for( int i
= 0; i < 1000000; i++) {

              table.put( "key:" +
i, "value:" + i);

          }

          

           //1、使用keys()

          start = System. currentTimeMillis();

          Enumeration<String> en1 = table.keys();

           while(en1.hasMoreElements())
{

              en1.nextElement();

          }

          end = System. currentTimeMillis();

          System. out.println( "Enumeration
keys costs " + (end - start) + " milliseconds" );

          

           //2、使用elements()

          start = System. currentTimeMillis();

          Enumeration<String> en2 = table.elements();

           while(en2.hasMoreElements())
{

              en2.nextElement();

          }

          end = System. currentTimeMillis();

          System. out.println( "Enumeration
elements costs " + (end - start) + " milliseconds" );

          

           //3、使用keySet()

          start = System. currentTimeMillis();

          Iterator<String> it1 = table.keySet().iterator();

           while(it1.hasNext())
{

              it1.next();

          }

          end = System. currentTimeMillis();

          System. out.println( "Iterator
keySet costs " + (end - start) + " milliseconds" );

          

           //4、使用entrySet()

          start = System. currentTimeMillis();

          Iterator<Entry<String, String>> it2 = table.entrySet().iterator();

           while(it2.hasNext())
{

              it2.next();

          }

          end = System. currentTimeMillis();

          System. out.println( "Iterator
entrySet costs " + (end - start) + " milliseconds" );

     }

}

运行结果如下：

Enumeration keys costs 16 milliseconds

Enumeration elements costs 15 milliseconds

Iterator keySet costs 17 milliseconds

Iterator entrySet costs 16 milliseconds

通过运行结果显示，在如今jdk的版本下，两者差距不是很大，不过使用Enumeration去遍历会稍微快一些。