Most concise way to get max(A.x) in a Collection<A>

Question

I have a class Person defined with those attributes :

private String name;
private String surname;
private Date birthday;

I want to get the max of the birthdays in a Collection<Person>.

In Java 7, without external libraries, I would do it like this :

public Date maxAgeJava7(Collection<Person> persons) {
  Date max = null;
  for (Person person : persons) {
    if (max == null || person.getBirthday().after(max)) {
      max = person.getBirthday();
    }
  }
  return max;
}

Now, In Java 8, it is possible to write it like this :

public Date maxAgeJava8(Collection<Person> persons) {
  return persons.stream().map(Person::getBirthday).max((Date d1, Date d2)-> d1.compareTo(d2)).get();
}

This is far better BUT I don't like this part :

(Date d1, Date d2)-> d1.compareTo(d2)

As Date implements Comparable, is there a way to tell Java to use the compareTo() method, without having to explicitly write it?

Answer 1

max requires a Comparator as parameter.

I think the shortest thing you can do is to call the method compareTo using method reference.

public Date maxAgeJava8(Collection<Person> persons) {
    return persons.stream().map(Person::getBirthday).max(Date::compareTo).get();
}

Also note that you don't need to use a Stream to accomplish this. You could simply use Collections.max providing a custom comparator:

public Date maxAgeJava8(Collection<Person> persons) {
    return Collections.max(persons, Comparator.comparing(Person::getBirthday)).getBirthday();
}