Sort objects in ArrayList by date?

Question

Every example I find is about doing this alphabetically, while I need my elements sorted by date.

My ArrayList contains objects on which one of the datamembers is a

Answer 1

You can make your object comparable:

public static class MyObject implements Comparable {

  private Date dateTime;

  public Date getDateTime() {
    return dateTime;
  }

  public void setDateTime(Date datetime) {
    this.dateTime = datetime;
  }

  @Override
  public int compareTo(MyObject o) {
    return getDateTime().compareTo(o.getDateTime());
  }
}

And then you sort it by calling:

Collections.sort(myList);

However sometimes you don't want to change your model, like when you want to sort on several different properties. In that case, you can create comparator on the fly:

Collections.sort(myList, new Comparator() {
  public int compare(MyObject o1, MyObject o2) {
      return o1.getDateTime().compareTo(o2.getDateTime());
  }
});

However, the above works only if you're certain that dateTime is not null at the time of comparison. It's wise to handle null as well to avoid NullPointerExceptions:

public static class MyObject implements Comparable {

  private Date dateTime;

  public Date getDateTime() {
    return dateTime;
  }

  public void setDateTime(Date datetime) {
    this.dateTime = datetime;
  }

  @Override
  public int compareTo(MyObject o) {
    if (getDateTime() == null || o.getDateTime() == null)
      return 0;
    return getDateTime().compareTo(o.getDateTime());
  }
}

Or in the second example:

Collections.sort(myList, new Comparator() {
  public int compare(MyObject o1, MyObject o2) {
      if (o1.getDateTime() == null || o2.getDateTime() == null)
        return 0;
      return o1.getDateTime().compareTo(o2.getDateTime());
  }
});