java Map 遍历速度最优解(转)

java Map 遍历速度最优解

出自:http://blog.csdn.net/hotjavaweb/article/details/7086459

第一种:
Map map = new HashMap();
Iterator iter = map.entrySet().iterator();
while (iter.hasNext()) {
Map.Entry entry = (Map.Entry) iter.next();
Object key = entry.getKey();
Object val = entry.getValue();
}
效率高,以后一定要使用此种方式!
第二种:
Map map = new HashMap();
Iterator iter = map.keySet().iterator();
while (iter.hasNext()) {
Object key = iter.next();
Object val = map.get(key);
}
效率低,以后尽量少使用!
HashMap的遍历有两种常用的方法,那就是使用keyset及entryset来进行遍历,但两者的遍历速度是有差别的,下面请看实例:
public class HashMapTest {
public static void main(String[] args) ...{
HashMap hashmap = new HashMap();
for (int i = 0; i < 1000; i ) ...{
hashmap.put("" i, "thanks");
}
long bs = Calendar.getInstance().getTimeInMillis();
Iterator iterator = hashmap.keySet().iterator();
while (iterator.hasNext()) ...{
System.out.print(hashmap.get(iterator.next()));
}
System.out.println();
System.out.println(Calendar.getInstance().getTimeInMillis() - bs);
listHashMap();
}
public static void listHashMap() ...{
java.util.HashMap hashmap = new java.util.HashMap();
for (int i = 0; i < 1000; i ) ...{
hashmap.put("" i, "thanks");
}
long bs = Calendar.getInstance().getTimeInMillis();
java.util.Iterator it = hashmap.entrySet().iterator();
while (it.hasNext()) ...{
java.util.Map.Entry entry = (java.util.Map.Entry) it.next();
// entry.getKey() 返回与此项对应的键
// entry.getValue() 返回与此项对应的值
System.out.print(entry.getValue());
}
System.out.println();
System.out.println(Calendar.getInstance().getTimeInMillis() - bs);
}
}
对于keySet其实是遍历了2次,一次是转为iterator,一次就从hashmap中取出key所对于的value。而entryset只是遍历了第一次,他把key和value都放到了entry中,所以就快了。
注:Hashtable的遍历方法和以上的差不多!

 

进行实例分析一下下:

以下通过程序来简单实践一下HashMap的的遍历
如果要保持HashMap的遍历顺序和原插入顺序一致,可以使用LinkedHashMap,使用方法和HashMap一样,改一下声明即可:LinkedHashMap myMap = new LinkedHashMap(); 当然需要导入:java.util.LinkedHashMap
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
public class MapList {


public static void main(String[] args) {
  // TODO Auto-generated method stub
  HashMap myMap = new HashMap();
  myMap.put("hello", "你好");
  myMap.put("bye", "再见");
  myMap.put("thanks", "谢谢");
  myMap.put("ok", "好的");
  System.out.println("--------------------遍历key和value----------------------");
  for(Iterator iter = myMap.entrySet().iterator();iter.hasNext();){
            Map.Entry element = (Map.Entry)iter.next();
            Object strKey = element.getKey();
            Object strObj = element.getValue();
            System.out.println("myMap.get(\""+strKey+"\")="+strObj);
  }
  System.out.println();
  System.out.println("--------------------遍历整个HashMap----------------------");
  Collection objs = myMap.entrySet();
  for (Iterator iterator=objs.iterator(); iterator.hasNext();){
   Object obj = iterator.next();
   System.out.println(obj);
  }
  System.out.println();
  System.out.println("--------------------遍历HashMap的key----------------------");
  Collection keys = myMap.keySet();
  for (Iterator iterator=keys.iterator(); iterator.hasNext();){
   Object key = iterator.next();
   System.out.println(key);
  }
  System.out.println();
  System.out.println("--------------------遍历HashMap的value----------------------");
  Collection values = myMap.values();
  for (Iterator iterator=values.iterator(); iterator.hasNext();){
   Object value = iterator.next();
   System.out.println(value);
  }
}
}
运行结果:
--------------------遍历key和value----------------------
myMap.get("hello")=你好
myMap.get("thanks")=谢谢
myMap.get("ok")=好的
myMap.get("bye")=再见
--------------------遍历整个HashMap----------------------
hello=你好
thanks=谢谢
ok=好的
bye=再见
--------------------遍历HashMap的key----------------------
hello
thanks
ok
bye
--------------------遍历HashMap的value----------------------
你好
谢谢
好的
再见

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