java comparator, how to sort by integer?

Question

Im trying to learn comparator in java and I have found this great example online, my question is how would this code be changed so that the pet names are ordered by age and

Answer 1

One simple way is

Comparator ageAscendingComp = ...;
Comparator ageDescendingComp = Collections.reverseOrder(ageAscendingComp);
// then call the sort method

On a side note, Dog should really not implement Comparator. It means you have to do strange things like

Collections.sort(myList, new Dog("Rex", 4));
// ^-- why is a new dog being made? What are we even sorting by?!
Collections.sort(myList, myList.get(0));
// ^-- or perhaps more confusingly

Rather you should make Compartors as separate classes.

eg.

public class DogAgeComparator implments Comparator {
    public int compareTo(Dog d1, Dog d2) {
        return d1.getAge() - d2.getAge();
    }
}

This has the added benefit that you can use the name of the class to say how the Comparator will sort the list. eg.

Collections.sort(someDogs, new DogNameComparator());
// now in name ascending order

Collections.sort(someDogs, Collections.reverseOrder(new DogAgeComparator()));
// now in age descending order

You should also not not have Dog implement Comparable. The Comparable interface is used to denote that there is some inherent and natural way to order these objects (such as for numbers and strings). Now this is not the case for Dog objects as sometimes you may wish to sort by age and sometimes you may wish to sort by name.