Why throwing exception is not working when parameter is null?

Question

I created a method which remove the students from a HashMAP. it should throw an exception when the id is null. Did somebody know why it is not working?

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");
        }
    }

public class Application { 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();


}

}

And it seems like you'd want that before you try to use id. — Fred Larson
– Fred Larson, Commented Jun 10, 2020 at 18:03
1) Please, add code where students is created/read, currently it's impossible to understand what's in that variable. 2) containsKey(id == null) is actually the same as containsKey(True), if students is just a set of id's, you can replace it with containsKey(null) — Rayan Ral
– Rayan Ral, Commented Jun 10, 2020 at 18:27

Nitin Bisht · Accepted Answer · 2020-06-10 18:11:30Z

2

Try:

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");
        }

    }

Note:

Assuming if students do not contain key then throw an exception. Hence added else statement.
Also, checked passed id initially. if it is not found then throw an exception. You can modify it as per your requirement.

answered Jun 10, 2020 at 18:11

Nitin Bisht

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2 Comments

Arvind Kumar Avinash Over a year ago

Horrible suggestion! Why should NPE be thrown for a missing student? NPE should strictly be thrown for null references. You should suggest to create a custom business exception and throw the same instead of misusing NPE.

Nitin Bisht Over a year ago

@ArvindKumarAvinash you are right. We can do a lot with above code. However Motive was to solve the problem and guide the user to move ahead as per his/her written code

Tim Bender · Accepted Answer · 2020-06-10 18:06:54Z

1

You should check the correctness of input parameters before using them. This follows a typical guard pattern.

my_function(some params) {
  if (precondition not met) { //the guard
    error handling;
  }
  business logic;
}

answered Jun 10, 2020 at 18:06

Tim Bender

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Comments

Mr. Developer · Accepted Answer · 2023-01-25 09:00:03Z

0

As your Student.id is in String format, you should check for null by using equals().
Like this:

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 (id.equals("null")) {
        throw new NullPointerException("The student does not exist");
    }
}

answered Jan 25, 2023 at 9:00

Mr. Developer

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