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I am writing a java code to add a student, remove a student and print the whole database of a class' grades. After running some times I have encountered 2 issues which I would like to find out where I am going wrong and what I can do to change my code.

1) Not sure why I am getting a null pointer exception error in my code, this is a code to help remove a student from a database and shift the students after into the position before. The issue is on this one line: if (student[l].equals(studenttoremove))

2) If I add a student in method newstudent, then remove a student in the method removestudent it will show up with nulls after printing the names to show which possible students to remove. How can I remove the nulls after it?

 //studentgradedatabase
import java.util.Scanner;
public class studentgradedatabase 
{
    String newstudent;
    int newgrade;
    Scanner in = new Scanner(System.in);
    String[]student = new String[10];
    int[]grade = new int[10];
    int numberofstudents = 0;
    int matchstudent = 0;
    int nullcounter = 0;

    void newstudent(String a, int b)
    {
        newstudent = a.toUpperCase();
        newgrade = b;
        if (numberofstudents == 10 || numberofstudents > 10)
        {
            System.out.print("Maximum Number of Students exceeded, Please Delete a Student First");
        }
        else
        {
            student[numberofstudents] = newstudent;
            grade[numberofstudents] = newgrade;
            numberofstudents = numberofstudents+1;
        }
    }
    void removestudent()
    {
        String studenttoremove;
        for (int i=0;i<student.length-nullcounter;i++)
        {
            System.out.print(student[i] + " ");
        }
        System.out.println(" ");
        System.out.println("Input the name of the student you want to remove: ");
        studenttoremove = in.next().toUpperCase();
        for (int l=0;l<student.length;l++)
        {
            if (student[l].equals(studenttoremove))
            {
                matchstudent = l;
            }
            else
            {
                System.out.print("Student name not found");
            }
        }
        for (int y=matchstudent; y<student.length-1; y++)
        {
            student[y] = student[y+1];
            grade[y] = grade[y+1];
        }
        for (int x=student.length-1; x<student.length;x++)
        {
            student[x] = null;
            grade[x] = 0;
        }
        System.out.print("Removed Student");
    }
    void printdatabase()
    {
        for (int i=0;i<student.length;i++)
        {
            if (student[i] == null)
            {
                nullcounter = nullcounter+1;
            }
        }
        for (int j=0;j<student.length-nullcounter;j++)
        {
            System.out.print(student[j] + " ");
        }
        System.out.println("");
        for (int j=0;j<grade.length-nullcounter;j++)
        {
            System.out.print(grade[j] + " ");
        }
    }
}
Jasper YIP
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  • 2
  • I've read that one post on nullpointer exceptions and I don't see why I'm still getting it since I have referenced both variables (student and studenttoremove) before the actual line itself? Am I missing something important? – Jasper YIP Nov 30 '17 at 00:33
  • NPE occurs when you perform an operation on a reference pointing to null. try checking various variables involved in the line you said is giving the error. – Harshil Sharma Nov 30 '17 at 00:33
  • `student[l]` is the likely null, perhaps because you aren't using `nullcounter` when deciding how far to loop. – D M Nov 30 '17 at 00:48

0 Answers0