Implementing Arrays

Question

Hi,
I am an novice Java programmer and i am having some trouble with arrays, I have written a program that calculates grades, but it does not use arrays. I would like to use arays in my program and at the same time have it support up to 50 people. Please help me in this matter, thanks,
Martin
import javax.swing.JOptionPane;
import java.text.DecimalFormat;
public class GradeWatch9{//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) with validations
          boolean firstnamecheck=false;
          firstname = JOptionPane.showInputDialog("Enter The Student's First Name: ");
          for ( int j=0; j<firstname.length(); j++ )
               if(!Character.isLetter(firstname.charAt(j)))
                    firstnamecheck=false;
                    firstname=JOptionPane.showInputDialog("Invalid Entry: \n You have entered an integer. \nPlease Re-enter Sudent's first name: ");
          boolean secondnamecheck=false;
          secondname = JOptionPane.showInputDialog("Enter The Student's Last Name: ");
               for ( int j=0; j<secondname.length(); j++ )
               if(!Character.isLetter(secondname.charAt(j)))
                    secondnamecheck=false;
                    secondname=JOptionPane.showInputDialog ("Invalid Entry: \n You have entered an integer. \nPlease Re-enter Sudent's last name: ");
          boolean studentidcheck=false;
          do
                    studentid = JOptionPane.showInputDialog("Student Id is the Student's Social Security Number.\nEnter The Student's ID #: ");
          if (studentid.length()!=9)
                    JOptionPane.showMessageDialog(
                         null,
                         "INVALID ENTRY: PLEASE ENTER 9 DIGITS WITHOUT DASHES.",
                         "ERROR", JOptionPane.ERROR_MESSAGE);
          else {
               for ( int j=0; j < studentid.length(); j++ ){
                              if(!Character.isDigit(studentid.charAt(j)))
                                        JOptionPane.showMessageDialog(
                              null,
                              "INVALID ENTRY: YOU HAVE ENTERED LETTERS.",
                              "ERROR", JOptionPane.ERROR_MESSAGE);
                              else {
                                   studentid_int= Integer.parseInt(studentid);
                                   if      (studentid_int<0)
                                        JOptionPane.showMessageDialog(
                              null,
                              "INVALID ENTRY: YOU HAVE ENTERED A NEGATIVE NUMBER.",
                              "ERROR",
                              JOptionPane.ERROR_MESSAGE);
                                   else studentidcheck=true;
               } while (studentidcheck==false);
          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

using arrays is pretty simple. You may want to clean up your code a bit, and place declerations outside of the main method, and create some smaller methods that dow rok, rather then having main carry the whole load. try something like this:
public class GradeWatch9
Student [] studentFile=new Student[50]; /*makes an array with 50 cells
*here I used another class, Student, which you will have to write. the Student class could be as small as two data fields, String name and int grade.
  public static void main ( String args[] )
     for(int i=0;i<studentFile.length;i++)
         String name=getNextName();  //method you write to get the name
         studentFile=new Student(name);
int grade=getGrade(); /*method to get and calculate grades*/
studentFile[i].setGrade(grade);
getNextName()
//gets the students name
getGrade()
//gets and calculates grade
public class Student
int grade
String name
Student(String n)
name=n;
public void setGrade(int g)
grade=g;
see if that helps a bit :)