Validating Char. & Strings

Question

Hello,
I am a novice programmer and my program does not check for strings and numeric characters. Below is my program, please give me some feedback, as soon as possible. Thanks a lot
Martin
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
public class GradeWatch6{//start of class definition GradeWatch
     public static void main ( String args[] ) {//start of main method
          //string declarations
          String firstname="";
          String secondname;
          String studentid;
          String midtermgrade;
          String finalgrade;
          String listings="";
          String option;//variable used to receive user's option <-1>//<0>
          //character declarations
          char actualgrade;//letter grade assigned to total points earned
          char lettergrade=0;//assignment of a letter grade
          //integer declarations
          int loop=0;//(this will be converted fr. option//will compare result)
          int studentid_int;
          int midterm_grade=0;
          int final_grade=0;
          double midterm_gradevalue=0;//point calculations
          double final_gradevalue=0;//point calculations
          double total_gradevalue=0;//total points possible out of 100
          do
          //input dialogs (5)
          boolean firstnamecheck=true;
          while (firstnamecheck=false){
          firstname = JOptionPane.showInputDialog("Enter The Student's First Name: ");
          if (!Character.isLetter(firstname.charAt(0)))
               firstnamecheck=false;
          for (int j=1;j<firstname.length();j++)
               if (!Character.isDigit(firstname.charAt(j)))
                         firstnamecheck=false;
                         System.out.println(""+firstnamecheck);//printing on black screen
                         secondname= JOptionPane.showInputDialog ( "Invalid Entry!" );
          secondname = JOptionPane.showInputDialog("Enter The Student's Last Name: ");
          studentid = JOptionPane.showInputDialog("Student Id is the Student's Social Security Number.\nEnter The Student's ID #: ");
          studentid_int= Integer.parseInt(studentid);
          /*Validates student id so that it can only receive a 9 digit number,
          no negative values, and only alows integer input/
          while ( (studentid.length()!=9) ||(studentid_int<0) )
                    studentid= JOptionPane.showInputDialog ( "Invalid Entry! \n Student Id is the Student's Social Security Number.\nPlease Re-Enter Student Id: ");
                    studentid_int= Integer.parseInt(studentid);
          midtermgrade = JOptionPane.showInputDialog("This is worth 40% of Student's Grade.\nEnter Mid-term Grade: ");
          midterm_grade = Integer.parseInt( midtermgrade);
          while ( (midterm_grade < 0) || (midterm_grade > 100) )
                    midtermgrade=JOptionPane.showInputDialog ("Invalid Entry!\nA grade entered must be between (0-100).\nPleae Re-Enter Mid-term Grade: ");
                    midterm_grade = Integer.parseInt( midtermgrade);
          finalgrade = JOptionPane.showInputDialog("This is worth 60% of Student's Grade.\nEnter Final Grade:");
          final_grade= Integer.parseInt( finalgrade);
          while ( (final_grade<0) || (final_grade > 100) )
                    finalgrade= JOptionPane.showInputDialog ("Invalid Entry!\nA grade entered must be between (0-100).\nPleae Re-Enter the Final's Grade: ");
                    final_grade= Integer.parseInt( finalgrade);
          DecimalFormat twoDigits = new DecimalFormat( "0.00");
          //calculations for grades
               //midterm is worth 40% of grade
          midterm_gradevalue = midterm_grade * .4;
               //final is worth 60% of grade
          final_gradevalue = final_grade * .6;
               //calculates the final grade for the student in class
          total_gradevalue = midterm_gradevalue + final_gradevalue;
          //if else statement for grade allotment
          if (total_gradevalue>=90)
               lettergrade='A';
          else if (total_gradevalue>=80)
               lettergrade='B';
          else if (total_gradevalue>=70)
               lettergrade='C';     
          else if (total_gradevalue>=60)
               lettergrade='D';     
          else if (total_gradevalue>=50)
               lettergrade='F';          
          //Shows Output of Information Entered (5) of one student               
          JOptionPane.showMessageDialog
               (null, "Student: " + secondname + " , " + firstname +
                    "\nMid-Term Points Earned: " + midterm_gradevalue +
                    "\nFinal Points Earned: " + final_gradevalue +
                    "\nTotal Points Earned: " + total_gradevalue +
                    "\nGrade Assigned: "+ lettergrade,
                    "\n STUDENT ID #: " + studentid,
                    JOptionPane.PLAIN_MESSAGE);
          /*Askes user whether or not to input an additional student*/
          option = JOptionPane.showInputDialog("Enter < 0 > to input a student. \nEnter < -1 > to quit. ");
          //converts string input into integer loop     
          loop = Integer.parseInt( option );
     listings +="Student ID #: " + studentid +
               "\n [ " + secondname + ", " + firstname + " ] " +
               "\n Midterm Points Earned: " + midterm_gradevalue +
               "\n Final Points Earned: " + final_gradevalue +
               "\n Total Points Earned: " + total_gradevalue +
               "\n**********Grade Assigned: "+ lettergrade + "\n";
          } while ( loop !=-1);     
          JOptionPane.showMessageDialog(
               null, listings,
               "Class Results: ",
               JOptionPane.PLAIN_MESSAGE);
          System.exit ( 0 );
     }//end of programming method
}//end of class definition

Answer 1

You didn't indent your code, so that makes it hard to read, and there's a lot of it that has nothing to do with what your question might be. But you want feedback? Well you are "validating" the student's first name in a strange way. A letter followed by any characters that aren't digits -- for example, "a++?" would pass. And that validation will fail if an empty string is provided, because "charAt(0)" assumes there is at least one character. I would just check it isn't blank. And I would provide a meaningful error message. "Invalid Entry" is unhelpful as it doesn't tell the user what is wrong with the input.