/*
 *  File: Validate.java
 *  Author: Java, Java, Java
 *  Description: This class computes the average of an
 *    unlimited number of grades input by the user. The
 *    user terminates the input by typing the sentinel
 *    value 9999. The user's inputs are validated, so that
 *    only legal grades (0..100) are averaged.
 */

import java.io.*;
public class Validate {
    private BufferedReader input = new BufferedReader  // Handles console input
        (new InputStreamReader(System.in));

    /**
     *  convertStringTodouble() converts a String of the form "54.87" into
     *  its corresponding double value (54.87).
     *  @param s -- stores the string to be converted
     *  @return A double is returned
     */
    private double convertStringTodouble(String s) {
        Double doubleObject = Double.valueOf(s);
        return doubleObject.doubleValue();
    }

    /**
     *  getAndValidateGrade() handles all the input required by the program. It inputs
     *   a grade as a String, converts it to double, and then validates that it is
     *   either between 0 and 100 or equal to 9999 (the sentinel).
     *  @return a double representing the sentinel (9999) or a grade between 0 and 100
     *  Precondition: None
     *  Postcondition: Either result == 9999 or 0 <= result <= 100
     */
    private double getAndValidateGrade() throws IOException {
        double grade = 0;
        do {
            System.out.print("Input a grade (e.g., 85.3) ");
            System.out.print("or 9999 to indicate the end of the list >> ");
            String inputString  = input.readLine();
            grade = convertStringTodouble(inputString) ;
            if ((grade != 9999) && ((grade < 0) || (grade > 100)))
                System.out.println("Error: grade must be between 0 and 100\n");  // Input error
            else
                System.out.println("You input " + grade + "\n");                 // OK input
        } while ((grade != 9999) && ((grade < 0) || (grade > 100)));
        return grade;
    } // getAndValidateGrade()

    /**
     *  inputAndAverageGrades() handles the program's main processing task.
     *   It repeatedly inputs a grade from the user, adding to the running
     *   total. When the input loop is exited, it calculates and returns the average.
     *  @return a double representing the computed average
     */
    public double inputAndAverageGrades() throws IOException {
        double runningTotal = 0;
        int count = 0;
        double grade = getAndValidateGrade();             // Initialize: priming input
        while (grade != 9999) {                           // Loop test: sentinel
            runningTotal += grade;
            count++;
            grade = getAndValidateGrade();                // Update: get next grade
        } // while

        if (count > 0)                                    // Guard against divide-by-zero
            return runningTotal / count;                      // Return the average
        else
            return 0;                                         // Special (error) return value
    } // inputAndAverageGrades()

    /**
     *  main() -- Creates an instance of the Validate object to compute
     *   the average of the user's test grades.
     */
    public static void main( String argv[] ) throws IOException {
        System.out.println("This program calculates average grade."); // Explain program
        Validate avg = new Validate();
        double average = avg.inputAndAverageGrades();
        if (average == 0)                                           // Error case
            System.out.println("You didn't enter any grades.");
        else
            System.out.println("Your average is " + average );
    } // main()
} // Validate