使用Collections.sort的错误排序列表

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?

Answer 1

You could use a comparator that can handle null values. Eg

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)

Answer 2

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

Answer 3

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