增强for循环 怎么判断是否是最后一个元素

int[] j;String str="";for(int i:j){str+=i+",";}str=str.substring(0, str.length()-1);int l =teacher_ids.length;int i;for(i=0;i<l;i++){ids+=teacher_ids[i]+",";}ids=ids.substring(0, ids.length()-1);