for(int i=0;i<3;i++){

    System.out.println("Choose your courseID:");

    String courseID = input.next();

    for(Courses cr:ssc.coursesSelect){

        //if(cr.getId()==courseID.toString()){ //为什么这样比较返回false？

        if(cr.getId().equals(courseID)){

            System.out.println("Success!");

        }

    }

}