GradeCalculator.java

import java.util.Scanner;

public class GradeCalculator {

    public static void main(String[] args) {
        try (Scanner input = new Scanner(System.in)) {
            while (true) {
                System.out.print("Please enter your grade: ");
                double grade = input.nextDouble();

                if (grade > 100 || grade < 0) {
                    System.out.println("The grade you entered is an invalid grade. Please try again...");
                    continue;
                }

                System.out.println("Your letter grade is: " + calculateGradeOnlyMath(grade));
            }
        }
    }

    private static char calculateGradeOnlyMath(double grade) {
        // Boolean mapping (100, 0) -> (1, 0)
        // Map values 100 >= grade >= 60 to 1.
        // Map values 60 > grade >= 0 to 0.
        double booleanMap = Math.floor(grade / 60);

        // A linear delta (line) that passes (60, 2) & (90, 5).
        double linearDelta = (Math.min(grade, 99.99999999999999) / 10) - 4;

        // Maps values like so:
        // 100 >= grade >= 90: 5
        // 90 > grade >= 80: 4
        // 80 > grade >= 70: 3
        // 70 > grade >= 60: 2
        // 60 > x >= 0: 0
        int normalized = (int) (booleanMap * linearDelta);

        // Get the value for the ascii character by subtracting
        // the normalized delta.
        int asciiValue = 70 - normalized;
        return (char) asciiValue;
    }

}