Chapter 13.6, Problem 13.6.5CP

What is wrong in the following code?

public class Test {

  public static void main(String[] args) {

  Person[] persons = {new Person(3), new Person(4), new Person(1)};

  java.util.Arrays.sort(persons);

  }

}

class Person {

  private int id;

  Person(int id) {

  this.id = id;

  }

}