Sorting using a comparable class

Question

I'm just not sure how to approach this problem. This is the error message I'm getting:

Exception in thread "main" java.lang.NullPointerException
    at java.util.Date.getMillisOf(Date.java:939)
    at java.util.Date.compareTo(Date.java:959)
    at FirstOccComparator.compare(FirstOccComparator.java:11)
    at FirstOccComparator.compare(FirstOccComparator.java:1)
    at java.util.Arrays.mergeSort(Arrays.java:1270)
    at java.util.Arrays.mergeSort(Arrays.java:1281)
    at java.util.Arrays.mergeSort(Arrays.java:1281)
    at java.util.Arrays.mergeSort(Arrays.java:1281)
    at java.util.Arrays.mergeSort(Arrays.java:1281)
    at java.util.Arrays.mergeSort(Arrays.java:1281)
    at java.util.Arrays.mergeSort(Arrays.java:1281)
    at java.util.Arrays.mergeSort(Arrays.java:1281)
    at java.util.Arrays.mergeSort(Arrays.java:1281)
    at java.util.Arrays.sort(Arrays.java:1210)
    at Planner.sort(Planner.java:62)
    at Test.main(Test.java:81)

Test @ line 81:

p.sort( new FirstOccComparator() );

where p is a is planner class I made.

Planner.sort @ line 62:

public void sort(Comparator<AbstractEvent> c) {
        Arrays.sort(schedule, c);
    } 

This is my FirstOccComparator class: http://pastebin.com/4FZv4nXf (posted on pastebin because it was too wide and was hard to format here). In this class hasMoreOccurrences() return true/false if there are more reccurrences of the event. nextOccurrence() returns a Date of the next occurrence.

I'm pretty sure what I'm missing here is very simple, I'm still new at interfaces and comparator classes.

Thanks for the help!

Answer 1

There's a null Date object in code you haven't shared...

Just a side note, you can reduce the size of your comparator drastically. Essentially what you're doing is saying

if (x<0)
  result = -1;
else if (x==0)
  result = 0;
else if (x>0)
  result = 1;

Why not just say result = x; Or in your particular example:

public int compare(AbstractEvent event1, AbstractEvent event2) {        
  int result = 0;       
  if (event1.hasMoreOccurrences() && event2.hasMoreOccurrences())
    result = event1.nextOccurrence().compareTo(event2.nextOccurrence());
  return result;
}

Which can again be shortened (if such is your style) to one line:

public int compare(AbstractEvent event1, AbstractEvent event2) {        
  return (event1.hasMoreOccurrences() && event2.hasMoreOccurrences()) ? event1.nextOccurrence().compareTo(event2.nextOccurrence()) : 0;
}