Using collections.sort() with compareTo, not sorting

Question

So currently I\'m working on an assignment where I have to two classes, one is named Fysiker and the other Human. Fysiker is simply an extension of the Human class. Human has tw

Answer 1

In fact the method defined in the subclass :

public int compareTo(Fysiker o){

doesn't override the method in the base class :

public int compareTo(Human o){

You could define the subclass with the same signature to override effectively :

public int compareTo(Human o){

and using instanceof to make the comparison according to the real type.
But it would be not a good idea either. Indeed, Fysiker would know how to compare Human and Fysiker but Human would know how to compare only Humans.

The Comparable.compareTo() contract states that :

The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.

Comparable should not try to be interoperable between classes as it may violate the transitivity comparison principle.

I think that in your case, as alternative, you should use a Comparator to sort elements.

You have two ways.

1) If the list contains only Fysiker instances, declare a List of Fysiker and create a Comparator :

List fysiker = new ArrayList();
...
Collections.sort(fysiker);

It limits the type of the elements that the List may accept but it is wished in this specific case.

2) If the list contains both Human and Fysiker instances, declare a List of Human and create a Comparator :

List humans = new ArrayList();
...
Collections.sort(fysiker);

In the Comparator implementation, you should check the type of the instances and compare them according to :

public class ComparatorHumanAndFysiker implements Comparator{

      public int compare(Human o1, Human o2){

           if (o1 instanceof Fysiker && o2 instanceof Fysiker){
              Fysiker f1 = (Fysiker) o1;
              Fysiker f2 = (Fysiker) o2;
              // specific comparison
              return ...;
           }

            // else mixed type comparison or human comparison              
            return o1.age - o2.age;                         
        }
 }