java.lang.NullPointerException

Question

I have Implemented this code as shown below but i've seen this error many times:
Exception in thread "main"
java.lang.NullPointerException
at ArrayMethods.addElement(ArrayMethods.java:24)
code :
// QUEUE IMPLEMENTATION
public class Queue{
// INIT QUEUE ARRAY
private int[] array;
// GET THE CLASS FOR WORK WITH ARRAYS;
ArrayMethods am = new ArrayMethods();
// ENQUEUE
public int[] enqueue(int elem){
// Something goes here...
array = am.addElement(array,elem);
return array;
}
// DEQUEUE
public String dequeue(){
// Something goes here...
if(array.length == 0) {
return "Array is empty";
}
else {
array=am.removeElement(array,array.length);
return "Dequeue!";
}
}
// PRINT ALL ELEMENTS
public int[] elements(){
if(array.length == 0){
return array;
}
else {
return array;
}
}
}

Answer 1

Here is the ArrayMethods class:
// ARRAY FUNCTIONS
public class ArrayMethods /*implements ArrayMethods*/ {
public static int[] addElement(int[] array, int elem){
// Something goes here...
// Init output array
int[] arr = new int[array.length +1];
// Copy the elements of the array
// to an output array and the extra one
for(int i = 0,k = 0; i < array.length; i++){
arr[k++] = array[i];
// Check if array filled for add new element
if (arr.length-1 == array.length){
arr[k] = elem;
}
else{
continue;
}
}
// Retrun to the output
return arr;
}
}