對 set 的遍歷  
  
1.迭代遍歷：  
Set<String> set = new HashSet<String>();  
Iterator<String> it = set.iterator();  
while (it.hasNext()) {  
  String str = it.next();  
  System.out.println(str);  
}  
  
2.for迴圈遍歷：  
for (String str : set) {  
      System.out.println(str);  
}  
  
  
優點還體現在泛型 假如 set中存放的是Object  
  
Set<Object> set = new HashSet<Object>();  
for迴圈遍歷：  
for (Object obj: set) {  
      if(obj instanceof Integer){  
                int aa= (Integer)obj;  
             }else if(obj instanceof String){  
               String aa = (String)obj  
             }  
              ........  
}