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());
}
});
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.