Why do I have NullPointerException in my sort implementation?

Question

I\'m working on my assignment and I am suppose to create a sort method for my array. However, I\'m getting a null pointer error in my sort method and I\'m not sure why. This is

Answer 1

See if any elements of setArray are null. Arrays.sort() requires that each element of the array being sorted has to be non-null: "All elements in the array must implement the Comparable interface."

Answer 2

The cause of NPE is that you store null in your array. The implementation of ComparableTimSort does not check for null.

 private static void binarySort(Object[] a, int lo, int hi, int start) {
 214         assert lo <= start && start <= hi;
 215         if (start == lo)
 216             start++;
 217         for ( ; start < hi; start++) {
 218             @SuppressWarnings("unchecked")
 219             Comparable<Object> pivot = (Comparable) a[start];
 220 
 221             // Set left (and right) to the index where a[start] (pivot) belongs
 222             int left = lo;
 223             int right = start;
 224             assert left <= right;
 225             /*
 226              * Invariants:
 227              *   pivot >= all in [lo, left).
 228              *   pivot <  all in [right, start).
 229              */
 230             while (left < right) {
 231                 int mid = (left + right) >>> 1;
 232                 if (pivot.compareTo(a[mid]) < 0)
 233                     right = mid;
 234                 else
 235                     left = mid + 1;
 236             }
 237             assert left == right;
 238 

You must assure that null value is not stored in your array.

To do that you must assert that argument passed in method add is not null and create a copy of your container before sorting to valid size.

public Object[] sorted() {

    Object[] toSort = Arrays.copyOf(getSetArray(),getSize());
    Arrays.sort(toSort);
    return toSort;
}