HashMap遍历

Map虽然实现了Collection接口，但是并不能直接遍历。如果想遍历Map，需要使用entrySet

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/**  
*Entry: java.util.Entry  
*hashmap类型为HashMap  
*/  
for(Iterator iter=hashmap.entrySet().iterator(); iter.hasNext();){   
    Entry entry = (Entry)iter.next();   
    entry.getKey();     //返回与此项对应的键   
    entry.getValue();   //返回与此项对应的值   
}  
/**
*Entry: java.util.Entry
*hashmap类型为HashMap
*/
for(Iterator iter=hashmap.entrySet().iterator(); iter.hasNext();){
 Entry entry = (Entry)iter.next();
 entry.getKey();  //返回与此项对应的键
 entry.getValue(); //返回与此项对应的值
}

另外一种方式是使用keySet进行遍历，看别人的文章和程序运行结果来看，似乎要慢一些。因为对于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。

首选应该是上面的方法。

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import java.util.HashMap;   
import java.util.Iterator;   
import java.util.Calendar;   
  
public class HashMapTest {   
  
public static void main(String[] args) {   
HashMap hashmap = new HashMap();   
for(int i=0;i<1000;i++){   
hashmap.put(""+i,"hello");   
}   
  
long bs = Calendar.getInstance().getTimeInMillis();   
Iterator iterator = hashmap.keySet().iterator();   
//String value = "";   
while(iterator.hasNext()) {   
//value = hashmap.get(iterator.next());   
System.out.println(hashmap.get(iterator.next()));   
}   
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,"hello");   
}   
long bs = Calendar.getInstance().getTimeInMillis();   
//Set set = hashmap.entrySet() ;   
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.println(entry.getValue());   
}   
System.out.println(Calendar.getInstance().getTimeInMillis() - bs);   
}   
  
}  
import java.util.HashMap;
import java.util.Iterator;
import java.util.Calendar;

public class HashMapTest {

public static void main(String[] args) {
HashMap hashmap = new HashMap();
for(int i=0;i<1000;i++){
hashmap.put(""+i,"hello");
}

long bs = Calendar.getInstance().getTimeInMillis();
Iterator iterator = hashmap.keySet().iterator();
//String value = "";
while(iterator.hasNext()) {
//value = hashmap.get(iterator.next());
System.out.println(hashmap.get(iterator.next()));
}
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,"hello");
}
long bs = Calendar.getInstance().getTimeInMillis();
//Set set = hashmap.entrySet() ;
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.println(entry.getValue());
}
System.out.println(Calendar.getInstance().getTimeInMillis() - bs);
}

}

 
HashSet hs;
  Iterator it = hs.iterator();
  Object obj = null;
  while(it.hasNext())
  {
  obj = it.next();
  }





二.遍历HashSet

Set set = new HashSet();

  for(int i=0;i<100;i++)
  {
   set .add("123");
  }

for(Iterator it=set.iterator();it.hasNext();)
  {
   System.out.println(it.next());
  }

三.遍历Hashtable(同步、线程安全的)

Hashtable table = new Hashtable();
  table.put(1, "1");
  table.put(2, "1");
  table.put(3, "1");
  //遍历key
  Enumeration e = table.keys();

  while( e. hasMoreElements() ){

  System.out.println( e.nextElement() );

  }
  //遍历value
  e = table.elements();

  while( e. hasMoreElements() ){

  System.out.println( e.nextElement() );

  }