Convert HashMap to HashTable

How will you convert a HashMap having null values and/or keys to a hash table ?

import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class HashTableHashMap
{

  public static void main(String[] args) {
      Map map = new HashMap();
      map.put("a",null);
      map.put("b","B");
      map.put(null,"D");
      System.out.println(map);
      Hashtable ht = new Hashtable();
      Set set = map.entrySet();
      Iterator itr = set.iterator();
      while(itr.hasNext()){
          Map.Entry entry =  (Map.Entry)itr.next();
          Object key = entry.getKey();
          Object val = entry.getValue();
          if(key==null){
              key = ""+null; // Or whatever you want
            }
          if(val==null){
              val = ""+null; // Or whatever you want
            }
          ht.put(key,val);
      }
      System.out.println(ht);
  }
}