Java Comparable

import java.util.*;

public class zadanie6 {

    public static void main(String[] args) {

        TreeSet<Integer> i = new TreeSet<Integer>();
        TreeSet<Dog> d = new TreeSet<Dog>();
        
        d.add(new Dog(1));
        d.add(new Dog(2));
        d.add(new Dog(1));

        i.add(1);
        i.add(2);
        i.add(1);
        
        System.out.println(d.size()+" "+i.size());
    }

}

class Dog{
    int size;

    public Dog(int s) {
        size = s;
    }
    
}

 Exception in thread "main" java.lang.ClassCastException: Testy.Dog cannot be cast to java.lang.Comparable
    at java.util.TreeMap.compare(Unknown Source)
    at java.util.TreeMap.put(Unknown Source)
    at java.util.TreeSet.add(Unknown Source)
    at Testy.zadanie6.main(zadanie6.java:13)