Implementing Arrays - Problem!

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

You should use a List instead, it will be more maintainable if you want this program to work for more students, without having to edit the souce:
List students = new ArrayList(); // Create a List
// To add students
students.add(student);
// To iterate through them
Iterator it = students.iterator();
while(it.hasNext)
Student s = (Student)it.next();
// For direct access
Student s = (Student)students.get(index);
class Student extends Object
private String firstName;
private String secondName;
private String id;
private String midtermGrade;
private String finalGrade;
public Student()
// Add a setter/getter for each variable
also override toString()
now with this bean, you could open an option pane passing a JList that was create with the passed in List