class Contact implements Comparable< Contact>{
        int age;
        public Contact(int age){
            this.age = age;
        }
        public int compareTo(Contact c){
            // ordering: 1, 2, 3
            return age - c.age;
        }
    }

    public static List< Contact> findKSmallestItem(List< Contact> list, int k){
        List< Contact> ret = new ArrayList<>();
        PriorityQueue queue = new PriorityQueue();
        for(Contact c : list){
            queue.add(c);
        }
        while(k > 0){
            Contact c = (Contact)queue.remove();
            ret.add(c);
            k--;
        }
        return ret;
    }
    

    class Person{
        Integer age;
        public Person(Integer age){
            this.age = age;
        }
    }

    PiorityQueue Queue = new PiorityQueue<>((a, b) -> a.age - b.age); 
    queue.add(new Person(1));
    queue.add(new Person(2));
    queue.add(new Person(0));
    while(0 < queue.size()){
        Person pe = queue.remove();
        p(pe.age); 
    }

    

Source Code