Question

I have a user give me a random array of objects, I want to do some error checking and basically I want the null objects to be at the end of the array, so that the middle of the array is made up of only non-null objects (sorting of the objects doesn’t matter).

Here is what I have, it isn’t working.
Can anyone please help.

private void properArray(){
    int i = 0;
    int j;
    int cap = theHeap.length;
    for(; i < (cap-1); i++){
        if (theHeap[i] == null){
            j = i + 1;
            while(j < (cap-1)){
                if(theHeap[j] != null){
                    theHeap[i] = theHeap[j];
                    theHeap[j] = null;
                }
                j++;  
            }
        }
    } 
}

Answer 1

Here’s a simpler way how you can sort such an array:

Arrays.sort(theHeap, new Comparator() {
  public int compare(Object o1, Object o2) {
    // nulls are "greater" than non-nulls
    if (o1 == null && o2 != null) return 1;
    // non-nulls are "smaller" than nulls
    if (o1 != null && o2 == null) return -1;
    // in all other comparisons, we don't care
    return 0;
  }
});

Or with Java 8:

Arrays.sort(theHeap, (o1, o2) -> (o1 == null && o2 != null) ?  1
                               : (o1 != null && o2 == null) ? -1
                               :                               0);

If you have Apache Commons Collections on your classpath, you can write this with even less code:

Arrays.sort(theHeap, new NullComparator());

As Ted mentions, this performs in O(n log n) and creates a clone of your array for sorting… It is thus not the fastest solution…