Run Time Error

[Frances Hollis]

OK I got my code to compile but I am getting RTE. I would appreciate if someone could give me some help. I am probably overlooking something simple because I do not have a good understanding of using the methods for Collections yet. I am still learning how to read the APIs.

//Driver provided by Mr. Lebeau.
 
import javax.swing.*;
public class RosterTest
{
 
   public static void main(String[] args)
   {
      String input;
      Roster myRoster = new Roster();
      do
      {
         input = menu();
         if(input == null || input.equalsIgnoreCase("q"))
         {
            JOptionPane.showMessageDialog(null,"Goodbye!");
         }
         else if(input.equalsIgnoreCase("a"))
         {
            doAdd(myRoster);
         }
         else if(input.equalsIgnoreCase("f"))
         {
            doFind(myRoster);
         }
         else if(input.equalsIgnoreCase("r"))
         {
            doRemove(myRoster);
         }
         else if(input.equalsIgnoreCase("l"))
         {
            JOptionPane.showMessageDialog(null,myRoster);
         }
         else
         {
            JOptionPane.showMessageDialog(null,"Invalid - try again!");
         }
      }while(input != null && !input.equalsIgnoreCase("q"));
      System.exit(0);
   }
 
   public static String menu()
   {
      String s = "MENU\na - add\nf - find\nr - remove"
         + "\nl - list\nq - quit\nEnter a letter:";
      return JOptionPane.showInputDialog(s);
   }
 
   public static void doAdd(Roster myRoster)
   {
      String name = JOptionPane.showInputDialog(
            "Enter student name:");
      int number = Integer.parseInt(JOptionPane
         .showInputDialog("Enter student number:"));
      double gpa = Double.parseDouble(JOptionPane
         .showInputDialog("Enter student gpa:"));
      boolean added = myRoster.add(new Student(name,number,gpa));
      if(!added)
      {
         JOptionPane.showMessageDialog(null,"Student with number " +
            number + " already exists!");
      }
   }
 
   public static void doFind(Roster myRoster)
   {
      int number = Integer.parseInt(JOptionPane
         .showInputDialog("Enter student number:"));
      Student foundStu = myRoster.find(number);
      if(foundStu == null)
      {
         JOptionPane.showMessageDialog(null,"No student with number "
            + number + "!");
      }
      else
      {
         JOptionPane.showMessageDialog(null,foundStu);
      }
   }
 
   public static void doRemove(Roster myRoster)
   {
      int number = Integer.parseInt(JOptionPane
         .showInputDialog("Enter student number:"));
      Student removedStu = myRoster.remove(number);
      if(removedStu == null)
      {
         JOptionPane.showMessageDialog(null,"No student with number "
            + number);
      }
      else
      {
         JOptionPane.showMessageDialog(null,"Removed student: " + removedStu);
      }
   }
}
 

 
import java.util.*;
 
public class Roster
{
private Student s;
 
Set<Student> mySet = new TreeSet<Student>();
 
public boolean add(Student s)
{
mySet.add(s);
return true;
}
 

public Student remove(int number)
{
mySet.remove(s);
return s;
}
 
public Student find(int number)
{
if(mySet.contains(s))
{
return s;
}
else
{
return null;
}
}
 
public String toString()
{
String stu = "";
for(Student s: mySet);
{
stu = stu + "\n" + s;
}
return stu;
 
}
 
}
 
public class Student implements Comparable<Student>
{
private String name;//The student name;
private int number;//The student number.
private double gpa;//The student grade point average.
private int value;
 
public Student(String nam, int num, double g)
{
name = nam;
number = num;
gpa = g;
}
 
public int getNumber()
{
       return number;
    }
 
    public int compareTo(Student other)
    {
return this.value - other.value;
}
 
public boolean equals(int number)
{
if (this.getNumber() == number)
{
return true;
}
else
{
return false;
}
}
public String toString()
{
return name + ", " + number + ", " + gpa;
}
}    

Exception in thread "main" java.lang.NullPointerException
at java.util.TreeMap.compare(TreeMap.java:1093)
at java.util.TreeMap.getEntry(TreeMap.java:347)
at java.util.TreeMap.containsKey(TreeMap.java:204)
at java.util.TreeSet.contains(TreeSet.java:196)
at Roster.find(Roster.java:31)
at RosterTest.doFind(RosterTest.java:69)
at RosterTest.main(RosterTest.java:24)
Press any key to continue . . .

[fred rosenberger]

a NullPointerExeption comes when you have a reference to an object, but no actual object has been created.

i'm guessing your trying to read something out of your Roster before you put something into it?

[Prabhu Venkatachalam]

   
public int compareTo(Student other)
{
  return this.value - other.value;
}

In your Student class, you are implementing compareTo method from Comparable interface. In that method, without checking whether supplied object is null or not, you are calling the method.

Try changing this like below,

    
 
public int compareTo(Student other)
{
  if(other instanceof Student)
    return this.value - other.value;
  else
    return false;
}