Iterator class

[Dillinger4]

Does anyone know the proper way to loop through a collection?
This is a code snippet from a program i am working on and im not sure why i cant use an Iterator on a TreeMap.

 Map stringMap = new TreeMap();

   for(Iterator i = stringMap.iterator(); i.hasNext();){            
         if(stringMap.containsKey(key)){
             return stringMap.get(i.next());     
         }
      i.next();
   }

I can use it on a LinkedList with no problem.

    import java.util.*;
 
     public class Test{
        public static void main(String[] args){
          String greeting = "Hello";
          String departing = "GoodBye";
 
         List l = new LinkedList();
         l.add(greeting); 
         l.add(departing);
      
      for(Iterator i = l.iterator(); i.hasNext();){            
         System.out.println(i.next());     
      }