How do i test my code in terminal without the main method?

Question

import java.lang.String;

public class Convert{
   /* Purpose: converts a given numerical grade to a letter grade             *
     * Input  : a number                                                       *
     * output : the letter grade (F, D-, D, ..., A+) corresponding to the      *
     *          input grade if the input is valid, "Invalid" otherwise         */
    public static String convertToLetter(double grade){
        String letter ="";

        if (grade >= 90 && grade <=100){
            letter="A+";
        }
        else if(grade >= 85 && grade <=90){
            letter="A";
        }
        else if(grade >= 80 && grade <=85){
            letter="A-";
        }
        else if(grade >= 77 && grade <=80){
            letter="B+";
        }
        else if(grade >= 73 && grade <=77){
            letter="B";
        }
        else if(grade >= 70 && grade <=73){
            letter="B-";
        }
        else if(grade >= 67 && grade <=70){
            letter="B-";
        }
        else if(grade >= 63 && grade <=67){
            letter="C+";
        }
        else if(grade >= 60 && grade <=63){
            letter="C-";
        }
        else if(grade >= 57 && grade <=60){
            letter="D+";
        }
        else if(grade >= 53 && grade <=57){
            letter="D";
        }
        else if(grade >= 50 && grade <=53){
            letter="D-";
        }
        else if(grade >= 0 && grade <=50){
            letter="F";
        }
        else{
            letter="Invalid";
        }

        return letter;

    }

    public static int convertToGradePoint(String letterGrade) {
        /* Purpose: converts a given letter grade to its equivalent grade point *
         * Input : A valid letter grade in the range D- to A+ (no F's) *
         * output : The grade point corresponding to the input letter grade *
         * Use 0 for F and -1 for Invalid input *
         * Note : you MUST use a switch/case for this method
         */

        int gradepoint =0;
        switch(letterGrade){
            case "A+": gradepoint=12;
                break;
            case "A": gradepoint=11;
                break;
            case "A-": gradepoint=10;
                break;
            case "B+": gradepoint=9;
                break;
            case "B": gradepoint=8;
                break;
            case "B-": gradepoint=7;
                break;
            case "C+": gradepoint=6;
                break;
            case "C": gradepoint=5;
                break;
            case "C-": gradepoint=4;
                break;
            case "D+": gradepoint=3;
                break;
            case "D": gradepoint=2;
                break;
            case "D-": gradepoint=1;
                break;
            default: gradepoint = -1;
                break;
        }
        return gradepoint;
    }
}

I keep getting when "Error: Main method not found in class Convert, please define the main method as: public static void main(String[] args) or a JavaFX application class must extend javafx.application.Application"

Answer 1

How do i test my code in terminal without the main method?

You don't. Either add a main() method to your class, or create a test class that contains a main() method:

public class TestConvert {
   public static void main(String[] args){
      double grade = 88.0;
      String letter = Convert.convertToLetter( grade );
      // output result here...


   }
}

Instead of a fixed grade, use a loop top check multiple grades.