Interfaces + casting problem

Question

ok, so i have a class Student that goes like
import java.util.*;
interface Copyable extends Cloneable {
     public Object clone();
     public String toString();
public class Student implements Copyable {
     private String iName = "";
     private Student iMentor = null;
     private boolean isClone = false;
     public void setName (String name) {
          iName = name;
     public Student() {}
     public Student(String name, Student mentor) {
          iName = name;
          iMentor = mentor;
        //more code here
}i also have a class Demo that contains the following:
interface CopyListener {
     public void copyAdded(Copyable obj);
public class Demo implements CopyListener {
     public void copyAdded(Copyable obj) {
          System.out.println("Added " + obj.toString());
     public static void main(String[] args) {
          Demo demo1 = new Demo();
          Demo demo2 = new Demo();
          ArrayListWrapper wrapper = new ArrayListWrapper();
          wrapper.addListener(demo1);
          wrapper.addListener(demo2);
          Student mentor = new Student("Mentor", null);
          Student std1 = new Student("Student1", mentor);
          Student std2 = new Student("Student2", mentor);
          Student std3 = new Student("Student3", mentor);
          wrapper.add(std1);
          wrapper.add(std2);
          wrapper.add(std3);
          Student std4 = new Student("Student3", mentor);
          wrapper.add(std4);
          wrapper.show();
}the last class is ArrayListWrapper.
it goes like
import java.util.*;
public class ArrayListWrapper {
     private CopyListener[] iListeners = new CopyListener[20];
     private ArrayList iArrayList = new ArrayList();
     public ArrayListWrapper() {}
     public void addListener (CopyListener listener) {
          boolean added = false;
          for (int i = 0; i < iListeners.length; i++ ) {
               if (iListeners[i] == null) {
                    iListeners[i] = listener;
                    added = true;
                    break;
     public void add(Copyable obj) {
          if(!iArrayList.contains(obj)) {
               Object objClone = obj.clone();
               if(objClone != null){
                    iArrayList.add(objClone);
                    for (int i = 0; i < iListeners.length; i++ ) {
                         iListeners.copyAdded((Copyable)objClone);
     public void show() {
          for (int i = 0; i < iArrayList.size(); i++ ) {
               iArrayList.get(i).toString();
it all compiles, but i get a runtime error:
Exception in thread "main" java.lang.NullPointerException
        at ArrayListWrapper.add(ArrayListWrapper.java:30)
        at Demo.main(Demo.java:23)line 30 in ArrayListWrapper is
iListeners[i].copyAdded((Copyable)objClone);
and line 23 in Demo
wrapper.add(std1);
which i guess are a part of one and same problem/error.
really not sure how to fix it, though

Answer 1

Well, what do you think the problem is? I've substituted j for i, for clarity.
Exception in thread "main" java.lang.NullPointerException
at ArrayListWrapper.add(ArrayListWrapper.java:30)
at Demo.main(Demo.java:23)
line 30 in ArrayListWrapper is
iListeners[j].copyAdded((Copyable)objClone);
What could possibly cause a NullPointerException on this line? Can you think of any ways to figure out what is null here at runtime? And why, oh why, are so many people incapable of the most trivial debugging?
grumble