How to sort List of objects by some property

by

Either make ActiveAlarm implement Comparable<ActiveAlarm> or implement Comparator<ActiveAlarm> in a separate class. Then call:

Collections.sort(list);

or

Collections.sort(list, comparator);

In general, it’s a good idea to implement Comparable<T> if there’s a single “natural” sort order… otherwise (if you happen to want to sort in a particular order, but might equally easily want a different one) it’s better to implement Comparator<T>. This particular situation could go either way, to be honest… but I’d probably stick with the more flexible Comparator<T> option.

EDIT: Sample implementation:

public class AlarmByTimesComparer implements Comparator<ActiveAlarm> {
  @Override
  public int compare(ActiveAlarm x, ActiveAlarm y) {
    // TODO: Handle null x or y values
    int startComparison = compare(x.timeStarted, y.timeStarted);
    return startComparison != 0 ? startComparison
                                : compare(x.timeEnded, y.timeEnded);
  }

  // I don't know why this isn't in Long...
  private static int compare(long a, long b) {
    return a < b ? -1
         : a > b ? 1
         : 0;
  }
}

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