Use the following file:

SquareTester.java

/**
   This program tests the Square class.
 */
public class SquareTester
{
   public static void main(String[] args)
   {
      Square mySquare = new Square(2);

      System.out.println("Area: " + mySquare.getArea());
      System.out.println("Expected: 4");

      System.out.println("Perimeter: " + mySquare.getPerimeter());
      System.out.println("Expected: 8");

      System.out.println("Diagonal length: " + mySquare.getDiagonalLength());
      System.out.println("Expected: 2.828427125");
   }
}