Sorting in Java

Java is an object-oriented language and not a functional one, but there are aspects of Java that resemble functional programming. The Comparator interface is one such example.

(a) Create a file called Sorting.java. Copy and paste the following program into it:

import java.util.*;
class Sorting
{
  public static void main(String[] args)
  {
    int listSize = 30;
    int magnitude = 500;
    Random rand = new Random(12345);
    ArrayList<Integer> numbers = new ArrayList<Integer>(listSize);
    for (int i=0; i < listSize; i++)
      numbers.add(rand.nextInt() % magnitude);
    Collections.sort(numbers);
    for (Integer num : numbers)
      System.out.print(num + " ");
    System.out.println();
  }
}

Look through the documentation for the class Collections. Specifically, find the method sort, and notice that there is another version of it that takes a Comparator object.

(b) Add a class to the top of the file called BackwardOrdering that implements the Comparator interface such that integers are ordered in reverse from their usual ordering. The following line of code should then sort the integers in reverse order:

Collections.sort(numbers, new BackwardOrdering());

Add this line of code to the bottom of your program, then add a copy of the loop to print out the numbers again.

(c) Add a class to the top of the file called AlternatingOrdering that implements the Comparator interface such even that positive numbers are ordered as usual, but negative numbers are ordered in reverse. All positive numbers (and zero) are larger than all negative numbers. Add more code to the main method to sort the numbers by this wacky method using Collections.sort, then print out the results.

(d) How does this resemble functional programming? What's the connection? (Indicate your answer in program comments.)

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Submit your code in a file called Sorting.java.