Java 集合 同步HashMap

Map map = Collections.synchronizedMap(new HashMap());
...
//This doesn't need to be in synchronized block
Set set = map.keySet();
// Synchronizing on map, not on set
synchronized (map) {  
      // Iterator must be in synchronized block
      Iterator iterator = set.iterator(); 
      while (iterator.hasNext()){
          ...
      }
}

package beginnersbook.com;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.Iterator;
public class HashMapSyncExample {
    public static void main(String args[]) {
         HashMap<Integer, String> hmap= new HashMap<Integer, String>();
         hmap.put(2, "Anil");
         hmap.put(44, "Ajit");
         hmap.put(1, "Brad");
         hmap.put(4, "Sachin");
         hmap.put(88, "XYZ");

         Map map= Collections.synchronizedMap(hmap);
         Set set = map.entrySet();
         synchronized(map){
             Iterator i = set.iterator();
             // Display elements
             while(i.hasNext()) {
                Map.Entry me = (Map.Entry)i.next();
                System.out.print(me.getKey() + ": ");
                System.out.println(me.getValue());
             }
         }
    }
}

1: Brad
2: Anil
4: Sachin
88: XYZ
44: Ajit