Higher order function

There are quite a number of ways to build higher order function. Let's take comparator as an example.

Single use class

   class PersonComparator implements Comparator<Person> {
        @Override
        public int compare(Person a, Person b){
            return a.getName().compareTo(b.getName());
        }
    }
    PriorityQueue<Person> pq = new PriorityQueue<>(new PersonComparator()); 

Anonymous class

    PriorityQueue<Person> pq = new PriorityQueue<>(new Comparator<Person>() { 
        @Override
        public int compare(Person a, Person b){
            return a.getName().compareTo(b.getName());
        }
    });

Lambda: both method reference and lambda closure works fine

Closure

    PriorityQueue<Person> pq = new PriorityQueue<>((x,y) -> x.getName().compareTo(y.getName()));

Method reference: apply to any object

    // pre-requisite: You need to have a getter method for that attribute that you wish to compare
    PriorityQueue<Person> pq = new PriorityQueue<>(Comparator.comparing(Person::getName)); 

    pq.add(new Person("charlie"));
    pq.add(new Person("alpha"));
    pq.add(new Person("delta"));
    pq.add(new Person("bravo"));

    while(!pq.isEmpty()) {
        System.out.println(pq.poll());
    }