为什么当参数为null时抛出异常不起作用？

Question

我创建了一个从HashMap中删除学生的方法。当id为NULL时，它会抛出一个异常。有人知道它为什么不起作用吗？

public void deleteStudent(String firstName, String lastName, String phoneNumber, String birthDate, PersonGender gender, String id)  {
        Student student = new Student(firstName, lastName, phoneNumber, birthDate, gender, id);

        if (students.containsKey(id)) {
            students.remove(id);
        }
        if (students.containsKey(id == null)) {
              throw new NullPointerException("The student does not exist");
        }
    }

公共类应用程序{public static void main(String[]args){

    StudentRepository myStudent = new StudentRepository();
    myStudent.addStudent("St","Rt","0742", "1993.03.04", PersonGender.MALE, "1930303");
    myStudent.addStudent("Sr","Ro","0742", "1994.03.04", PersonGender.MALE, "1940304");
    myStudent.addStudent("Se","Rb","0742", "1995.03.04", PersonGender.MALE, "1950305");
    myStudent.addStudent("Sm","Re","0742", "1996.03.04", PersonGender.MALE, "1950306");
    myStudent.deleteStudent("Str","Rob","0742", "1992.03.04", PersonGender.MALE, "null");
    myStudent.addStudent("Sr","Ro","0742", "1994.03.04", PersonGender.MALE, "1940304");
    myStudent.displayStudents();


}

}

Answer 1

尝试：

public void deleteStudent(String firstName, String lastName, String phoneNumber, String birthDate, PersonGender gender, String id)  {
        Student student = new Student(firstName, lastName, phoneNumber, birthDate, gender, id);

        if (id == null) {
              throw new NullPointerException("ID is null");
        }

        if (students.containsKey(id)) {
            students.remove(id);
        }
        else {
            throw new NullPointerException("The student does not exist");
        }

    }

注：

1. 假设students不包含键，则引发异常。因此添加了else语句。
2. 同时，首先检查通过ID。如果找不到，则抛出异常。您可以根据需要对其进行修改。