1.java

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class App {
  public static class Person {
    private String name;
    private int weight;

    public Person(String name, int weight) {
      this.name = name;
      this.weight = weight;
    }

    public int getWeight() {
      return this.weight;
    }
  }

  public static class PersonWeightComparator implements Comparator<Person> {
    public int compare(Person a, Person b) {
      return a.getWeight() - b.getWeight();
    }
  }

  public static void main(String[] args) {
    List<Person> people = new ArrayList<Person>();
    people.add(new Person("a", 120));
    people.add(new Person("b", 200));
    people.add(new Person("c", 180));

    Comparator<Person> comparator = new PersonWeightComparator();
    Collections.sort(people, comparator);
    for (Person person : people) {                // 120, 180, 200
      System.out.println(person.getWeight());
    }
  }
}

2.java

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class App {
  public static class StringLengthComparator implements Comparator<String> {
    public int compare(String a, String b) {
      return a.length() - b.length();
    }
  }

  public static void main(String[] args) {
    List<String> strings = new ArrayList<String>();
    strings.add("word");
    strings.add("a");
    strings.add("the");

    Comparator<String> comparator = new StringLengthComparator();
    String longest = Collections.max(strings, comparator);
    System.out.println(longest);                // word
  }
}

3.scala

import scala.util.Sorting

object App {
  case class User(id: Int, name: String)

  object User {
    implicit object IdOrdering extends Ordering[User] {
      def compare(a: User, b: User): Int = {
        a.id.compare(b.id)
      }
    }

    object NameOrdering extends Ordering[User] {
      def compare(a: User, b: User): Int = {
        a.name.compare(b.name)
      }
    }
  }

  def main(args: Array[String]) {
    val users = Array(User(1, "b"), User(3, "a"), User(2, "c"))

    Sorting.quickSort(users)
    users.foreach(println) // User(1, "b"), User(2, "c"), User(3, "a")

    Sorting.quickSort(users)(User.NameOrdering)
    users.foreach(println) // User(3, "a"), User(1, "b"), User(2, "c")
  }
}