Hashtable类中的四种遍历方法对比

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。