在foreach循环中使用remove报ConcurrentModificationException异常原因

   int dindex=0;
                    int did=getInt("请输入需要删除的学生学号：");
                    for (Student student:list) {
                        if(student.id==did){
                            list.remove(student);
                        }
                    }

int dindex=-1;
                    int did=getInt("请输入需要删除的学生学号：");
                    for (Student student:list) {
                        if(student.id==did){
                            dindex=list.indexOf(student);
                        }
                    }
                    if(dindex!=-1)
                        list.remove(dindex);
                    else
                        System.out.println("没有此学生");