import java.util.Scanner;   // Needed in order to make use of objects
                            // from the Scanner class

/* Java program that prompts the user to enter two integers, say k and m,
** and that reports the sum of the integers in the interval [k,m].
** The point is to illustrate how to 
** --create a Scanner object (i.e., one derived from the java.util.Scanner
**   class) that can read input entered at the keyboard
** --read integer values (using the Scanner's nextInt() method)
** --declare and call methods with parameters
** --declare, call, and make use of the value returned by a functional method
*/

public class SumOfRangeApp {

   public static void main(String[] args)
   {
      // Establish a Scanner object that can read input entered at keyboard
      Scanner keyboard = new Scanner(System.in);

      // Prompt user to enter a number and then read it into a variable
      System.out.print("Enter lower bound:");
      int lowBound = keyboard.nextInt();

      // Prompt user to enter a number and then read it into a variable
      System.out.print("Enter upper bound:");
      int highBound = keyboard.nextInt();

      // Call functional method to compute the desired sum and store the
      // result in a variable.
      int rangeSum = sumOfRange(lowBound, highBound);

      // Print a message that reports the input values and the output
      // computed therefrom.
      System.out.println("Sum of integers in range " + lowBound +
                         ".." + highBound + " is " + rangeSum);
   }


   /* Functional method that returns the sum of the integers in the interval
   ** specified by the caller (via the method's parameters) (i.e., [low,high]).
   ** In the case that the interval is empty (i.e., low > high), that sum is
   ** zero, of course.
   */
   public static int sumOfRange(int low, int high)
   {
      int sum = 0;
      for (int i=low; i <= high; i=i+1) {
         // loop invariant: sum = low + (low+1) + ... + (i-1)
         sum = sum + i;
      }
      return sum;
   }

}