java.lang.NullPointerException

Subject.java
--------------

import java.util.*;

public class Subject
{
      private String myCode, myName;
      private Vector students;
      
      public Subject()
      {
            setStudents();
      }
      
      public Subject(String code, String name)
      {
          setCode(code);
          setName(name);      
      }
      
      public void setCode(String code)
      {
            this.myCode = code;
      }
      
      public void setName(String name)
      {
          this.myName = name;      
      }
      
      public String getCode()
      {
          return this.myCode;       
      }
      
      public String getName()
      {
            return this.myName;
      }
      
      public void setStudents()
      {
            if(students==null)
                students = new Vector();
      }
      
      public Vector getStudents()
      {
            return students;
      }
      
      public boolean addStudents(Students s)
      {
        return getStudents().add(s);
      }
      
      public String toString()
      {
            return getName();
      }
}



Students.java
----------------

public class Students
{
      String myName, myID;
      double myMark;
      
      public Students(String name, String id, double mark)
      {
            myName = name;
            myID = id;
            myMark = mark;
      }
}



TestSubject.java
-------------------

import java.util.*;

public class TestSubject
{
      public static void main(String[] args)
      {
            Subject mySubject = new Subject("CS101","Programming");
            mySubject.addStudents(new Students("Mary","A111",30.4));
            Vector studentsList = mySubject.getStudents();
            Collections.sort(studentsList);
            
            Iterator iter = studentsList.iterator();
            while(iter.hasNext())
            {
                  System.out.println(iter.next());
            }
      }
}


I am getting an error:

Exception in thread "main" java.lang.NullPointerException
        at Subject.addStudents(Subject.java:52)
        at TestSubject.main(TestSubject.java:8)
Press any key to continue...


How do I solve this problem?


Your help is kindly appreciated.


Regards

Eugene
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