How to sort List of objects by some property

I have simple class

public class ActiveAlarm {
    public long timeStarted;
    public long timeEnded;
    private String name = "";
    private String description = "";
    private String event;
    private boolean live = false;
}

and List<ActiveAlarm> con. How to sort in ascending order by timeStarted, then by timeEnded? Can anybody help? I know in C++ with generic algorithm and overload operator <, but I am new to Java.

Solution 1:

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;
  }
}

Solution 2:

Using Comparator

For Example:

class Score {

    private String name;
    private List<Integer> scores;
    // +accessor methods
}

    Collections.sort(scores, new Comparator<Score>() {

        public int compare(Score o1, Score o2) {
            // compare two instance of `Score` and return `int` as result.
            return o2.getScores().get(0).compareTo(o1.getScores().get(0));
        }
    });

With Java 8 onwards, you can simply use lambda expression to represent Comparator instance.

Collections.sort(scores, (s1, s2) -> { /* compute and return int */ });