Java's Comparator

Java's Comparator<T> is an interface that defines a compare(T o1, T o2) method with two arguments that represent compared objects.

The compare method works similarly to the Comparable.compareTo() method:

• return $0$ when two objects o1 and o2 are equal.
• return a value $> 0$ when the first object is greater than the second one.
• return a value $< 0$ when the first object is less than the second one.

So to give a higher "order" to a student with fewer prior submissions, we must create the following Comparator:

private static class LessSubmissionHigherPriority 
  implements Comparator<Submission> {

  @Override
  public int compare(Submission s1, Submission s2) {
    return s1.getNumPriorSubmission() - s2.getNumPriorSubmission();
  }
}

We can (just to have fun) create a Comparator to reverse the natural ordering of submissions:

private static class ReverseNaturalOrdering 
  implements Comparator<Submission> {

  @Override
  public int compare(Submission s1, Submission s2) {
    // notice s2 is compared to s1, the order matters!
    return s2.compareTo(s1);
  }
}

The following Comparator would meet the natural ordering of submissions:

private static class NaturalOrdering 
  implements Comparator<Submission> {

  @Override
  public int compare(Submission s1, Submission s2) {
    return s1.compareTo(s2);
  }
}

I encourage you to use the "demo" (Main.java in the demo package of the starter code) to experiment with Comparators.