Why?该怎么处理

Why?
public   String   selectReconstruction(String[]   roads){
String   result= " ";
for   (int   i   =   0;   i   <   roads.length;   i++)   {
StringTokenizer   st     =   new   StringTokenizer(roads[i], "   ");
String   id   =   st.nextToken();
String   s   =   st.nextToken();
String   f   =   st.nextToken();
int   cost=st.hasMoreElements()?Integer.parseInt(st.nextToken()):0;
vertics.add(s);
vertics.add(f);
ids.put(s,   s);
ids.put(f,   f);
Edge   e1   =   new   Edge(id,s,f,cost);
edges.add(e1);
}

循环第一次正常，第二次就发生ClassCastException,why?

[解决办法]
装箱时出错？
帮顶。。。
[解决办法]
哪一个语句抛出的异常？!