Error sorting list using Collections.sort

Question

I'm getting a NullPointerException when i try to sort a list with by doing the Collections.sort(list2)

the list contains the following strings

[BOOTH 4, ENP ROOM, BOOTH 6, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, BOOTH 10, BOOTH 7, BOOTH 3, BOOTH 1, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, BOOTH 1, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, BOOTH 10, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, C&T Waiting Area, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null]

Do i need to use comparator for this list?

"I'm getting an exception" is not a useful error report. Stack traces are there for a reason. — skaffman
– skaffman, Commented Dec 8, 2010 at 10:02
@user521180 which exception?some stack trace or code would actually help you — ant
– ant, Commented Dec 8, 2010 at 10:03

daveb · Accepted Answer · 2010-12-08 10:04:33Z

4

You could use a comparator that can handle null values. E.g.

private static final class NullsFirstComparator implements Comparator<String> {
  public int compare(String lhs, String rhs) {
    if (lhs == rhs)
      return 0;
    if (lhs == null)
      return -1;
    if (rhs == null)
      return 1;
    return lhs.compareTo(rhs);
  }
}

Which will sort the null values first (they are less than everything)

answered Dec 8, 2010 at 10:04

daveb

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Boris Pavlović · Accepted Answer · 2010-12-08 10:03:06Z

2

Remove null values before sorting. They can't be sorted

answered Dec 8, 2010 at 10:03

Boris Pavlović

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2 Comments

Stephen C Over a year ago

Strictly, they can be sorted, but only if you provide a Comparator object that does something sensible with null references.

Boris Pavlović Over a year ago

@Stephen C - Yes, you are right. Couldn't see any comparators in the invocation: Collections.sort(list2)

Vinay Lodha · Accepted Answer · 2010-12-08 10:17:01Z

1

You can also use NullComparator in Apache commons. It provide lot of useful utilities for writing comparator

answered Dec 8, 2010 at 10:17

Vinay Lodha

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