Sort objects in ArrayList by date?

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 listlist.sort((d1,d2) -> d1.compareTo(d2));// or an object which has an DateTime attributelist.sort((o1,o2) -> o1.getDateTime().compareTo(o2.getDateTime()));// or like mentioned by Tunakilist.sort(Comparator.comparing(o -> o.getDateTime()));

Reverse sorting

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

//requested by lilylist.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.