Sort ArrayList of custom Objects by property

Question

I read about sorting ArrayLists using a Comparator but in all of the examples people used compareTo which according to some research is a method for Strings.

I wanted to sort an ArrayList of custom objects by one of their properties: a Date object (getStartDay()). Normally I compare them by item1.getStartDate().before(item2.getStartDate()) so I was wondering whether I could write something like:

public class CustomComparator {     public boolean compare(Object object1, Object object2) {         return object1.getStartDate().before(object2.getStartDate());     } }  public class RandomName {     ...     Collections.sort(Database.arrayList, new CustomComparator);     ... } 

Answer 1

Since Date implements Comparable, it has a compareTo method just like String does.

So your custom Comparator could look like this:

public class CustomComparator implements Comparator<MyObject> {     @Override     public int compare(MyObject o1, MyObject o2) {         return o1.getStartDate().compareTo(o2.getStartDate());     } } 

The compare() method must return an int, so you couldn't directly return a boolean like you were planning to anyway.

Your sorting code would be just about like you wrote:

Collections.sort(Database.arrayList, new CustomComparator()); 

A slightly shorter way to write all this, if you don't need to reuse your comparator, is to write it as an inline anonymous class:

Collections.sort(Database.arrayList, new Comparator<MyObject>() {     @Override     public int compare(MyObject o1, MyObject o2) {         return o1.getStartDate().compareTo(o2.getStartDate());     } }); 

---

Since java-8

You can now write the last example in a shorter form by using a lambda expression for the Comparator:

Collections.sort(Database.arrayList,                          (o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate())); 

And List has a sort(Comparator) method, so you can shorten this even further:

Database.arrayList.sort((o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate())); 

This is such a common idiom that there's a built-in method to generate a Comparator for a class with a Comparable key:

Database.arrayList.sort(Comparator.comparing(MyObject::getStartDate)); 

All of these are equivalent forms.