NullPointerException with an array of LinkedList

Question

Hello Everybody.
I'm new to Java and I am trying to use a quite complicated data structure but I am missing something since it does not work.
Here comes my code:
import java.util.LinkedList;
class ObjectToHold {
public int i;
ObjectToHold (int j) {
i=j;
public class ModifiedArray {
static final int ARRAY_DIMENSION = 79;
private LinkedList[] myInternalArray;
ModifiedArray() {
LinkedList[] myInternalArray = new LinkedList[ARRAY_DIMENSION];
for (int i=0;i<ARRAY_DIMENSION;i++) {
myInternalArray[i] = new LinkedList();
void insert (int slot, ObjectToHold a) {
((LinkedList)myInternalArray[slot]).addLast(a);
public static void main(String[] args) {
ModifiedArray myModifiedArray = new ModifiedArray();
for (int j=0; j<ARRAY_DIMENSION;j++) {
myModifiedArray.insert(j,new ObjectToHold(j));
The class ModifiedArray contains a private array of LinkedList objects. I wrote an "insert" method to put objects of type ObjectToHold (which is a very simple one!) into the array.
While compiling the code, everything works fine.
On the other hand, when I run the compiled code I get the following run time Exception:
Java.lang.NullPointerException
at ModifiedArray.insert ....
Modified.Array.main ...
The problem must be in the way I use the reference to myInternalArray.
I also tried
myInternalArray[slot].addLast(a);
instead of
((LinkedList)myInternalArray[slot]).addLast(a);
but I got the same result.
Can anybody help me or do I have to ask in some other forum?
Thank you very much
Tommaso

Answer 1

You defined myInternalArray as local to the constructor as well as an instance variable.
Try..
import java.util.LinkedList;
class ObjectToHold {
  public int i;
  ObjectToHold (int j) {
    i=j;
public class ModifiedArray {
  static final int ARRAY_DIMENSION = 79;
  private LinkedList[] myInternalArray;
  ModifiedArray() {
    myInternalArray = new LinkedList[ARRAY_DIMENSION];
    for (int j=0; j<ARRAY_DIMENSION; j++) {
      myInternalArray[j] = new LinkedList();
  void insert (int slot, ObjectToHold a) {
    ((LinkedList)myInternalArray[slot]).addLast(a);
  public static void main(String[] args) {
    ModifiedArray myModifiedArray = new ModifiedArray();
    for (int j=0; j<ARRAY_DIMENSION;j++) {
      myModifiedArray.insert(j,new ObjectToHold(j));
}