Sort objects in ArrayList by date?

Question

You can make your object comparable:

public static class MyObject implements Comparable<MyObject> {

  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<MyObject>() {
  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<MyObject> {

  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<MyObject>() {
  public int compare(MyObject o1, MyObject o2) {
      if (o1.getDateTime() == null || o2.getDateTime() == null)
        return 0;
      return o1.getDateTime().compareTo(o2.getDateTime());
  }
});

Since Java 8 the List interface provides the sort method. Combined with lambda expression the easiest solution would be

// sort DateTime typed list
list.sort((d1,d2) -> d1.compareTo(d2));
// or an object which has an DateTime attribute
list.sort((o1,o2) -> o1.getDateTime().compareTo(o2.getDateTime()));
// or like mentioned by Tunaki
list.sort(Comparator.comparing(o -> o.getDateTime()));

Reverse sorting

Java 8 comes also with some handy methods for reverse sorting.

//requested by lily
list.sort(Comparator.comparing(o -> o.getDateTime()).reversed());

You can use Collections.sort method. It's a static method. You pass it the list and a comparator. It uses a modified mergesort algorithm over the list. That's why you must pass it a comparator to do the pair comparisons.

Collections.sort(myList, new Comparator<MyObject> {
   public int compare(MyObject o1, MyObject o2) {
      DateTime a = o1.getDateTime();
      DateTime b = o2.getDateTime();
      if (a.lt(b)) 
        return -1;
      else if (a.lteq(b)) // it's equals
         return 0;
      else
         return 1;
   }
});

Note that if myList is of a comparable type (one that implements Comparable interface) (like Date, Integer or String) you can omit the comparator and the natural ordering will be used.

list.sort(Comparator.comparing(o -> o.getDateTime()));

The best answer IMHO from Tunaki using Java 8 lambda

Given MyObject that has a DateTime member with a getDateTime() method, you can sort an ArrayList that contains MyObject elements by the DateTime objects like this:

Collections.sort(myList, new Comparator<MyObject>() {
    public int compare(MyObject o1, MyObject o2) {
        return o1.getDateTime().lt(o2.getDateTime()) ? -1 : 1;
    }
});

This is how I solved:

Collections.sort(MyList, (o1, o2) -> o1.getLastModified().compareTo(o2.getLastModified()));

Hope it help you.

Future viewers, I think this is the simplest solution, if your model contains a string type date ("2020-01-01 10:00:00" for example), then just write the following line to sort the data by date descending from newest to the oldest:

Collections.sort(messages, (o1, o2) -> o2.getMessageDate().compareTo(o1.getMessageDate()));