Avoid/Eliminate Duplicate Code

[kicker]

I am trying to avoid duplicate code. How can I get this program to calculate one set of credit hours / letter grades, prompt the user (y/n) for more class info, keep calculating and then display the results when the user indicates they have no more class info to input. The first program is my most current attempt but I see myself having to duplicate the code with slight modifications to account fro the additional user data. If you look down below the first one you will see the mess I made before, but at least that one gives the desired results.

import javax.swing.*;

public class StudentGPA {
		
	static String userInput, letterGrade, anotherClass;
	static int creditHours;
	static double totalCreditHours, gradeScore, gradePoints, totalGradePoints, gradePointAverage;
	
public static void main (String[] args) {
		
	userInput = JOptionPane.showInputDialog("Enter credit hours for the class.");
	creditHours = Integer.parseInt(userInput);
	validateUserInput(userInput);
	totalCreditHours = creditHours;
	letterGrade = JOptionPane.showInputDialog("Enter letter grade for the class.");
	letterGrade = letterGrade.toUpperCase();
	validateLetterGrade(letterGrade);
	calculateGradeScore(letterGrade);
	gradePoints = creditHours * gradeScore;
	totalGradePoints = gradePoints;
	gradePointAverage = totalGradePoints / totalCreditHours;
	anotherClass = JOptionPane.showInputDialog("Do you have more classes to input?");
	anotherClass = anotherClass.toUpperCase();
	validateAnotherClass(anotherClass);
					
	displayGPA();
	System.exit(0);	
		
}
	
public static double calculateGradeScore(String letterGrade){
		
	if (letterGrade.equals("A"))
	gradeScore = 4.0;
	else
	if (letterGrade.equals("B"))
	gradeScore = 3.0;
	else 
	if (letterGrade.equals("C"))
	gradeScore = 2.0;
	else
	if (letterGrade.equals("D"))
	gradeScore = 1.0;
	else
	if (letterGrade.equals("F"))
	gradeScore = 0.0;
					
	return gradeScore;
	}

public static void validateUserInput (String userInput) {
	while (creditHours<1 || creditHours>6) {
		JOptionPane.showMessageDialog(null,
		"Credit hours must be between 1 & 6.");
		userInput = JOptionPane.showInputDialog("Enter credit hours for the class.");
		creditHours = Integer.parseInt(userInput);
		}
	}
	
public static void validateLetterGrade (String letterGrade) {
	while (!letterGrade.equals("A") && !letterGrade.equals("B") 
	&& !letterGrade.equals("C") && !letterGrade.equals("D") 
	&& !letterGrade.equals("F")){
		JOptionPane.showMessageDialog(null,
		"You entered an invalid letter grade.");
		letterGrade = JOptionPane.showInputDialog("Enter letter grade for the class.");
		letterGrade = letterGrade.toUpperCase();
		}
}
	
public static void validateAnotherClass (String anotherClass){
	while (!anotherClass.equals("Y") && !letterGrade.equals("N")){
	JOptionPane.showMessageDialog(null,
	"Please enter y or n.");
	anotherClass = JOptionPane.showInputDialog("Do you have more classes to input?");
	anotherClass = anotherClass.toUpperCase();
		}
}

public static void displayGPA() {
	JOptionPane.showMessageDialog(null, "GPA = "+totalGradePoints+"/"+totalCreditHours+" = "+gradePointAverage);
	}
}

import javax.swing.*;

public class StudentGPA {
	//declare variables
	static String userInput, letterGrade, anotherClass;
	static int creditHours;
	static double totalCreditHours, gradeScore, gradePoints, totalGradePoints, gradePointAverage;
	//start main method
	public static void main (String[] args) {
		
	//get credit hours
	userInput = JOptionPane.showInputDialog("Enter credit hours for the class.");
	creditHours = Integer.parseInt(userInput);
	//validate credit hours
	validateUserInput(userInput);
	totalCreditHours = creditHours;
	
	//get letter grade
	letterGrade = JOptionPane.showInputDialog("Enter letter grade for the class.");
	letterGrade = letterGrade.toUpperCase();
	//validate letter grade
	validateLetterGrade(letterGrade);
	
	calculateGradeScore(letterGrade);
	gradePoints = creditHours * gradeScore;
	totalGradePoints = gradePoints;
	
	//prompt user for another class
	anotherClass = JOptionPane.showInputDialog("Do you have other class information?");
	anotherClass = anotherClass.toUpperCase();
	while (anotherClass.equals("Y")) {
		//get next class credit hours
		userInput = JOptionPane.showInputDialog("Enter credit hours for the next class.");
		creditHours = Integer.parseInt(userInput);
		//validate next class credit hours
		while (creditHours<0 || creditHours>6) {
		//display error message
		JOptionPane.showMessageDialog(null,
		"Credit hours must be between 1 & 6.");
		userInput = JOptionPane.showInputDialog("Enter credit hours for the next class.");
		creditHours = Integer.parseInt(userInput);
			
		}
		totalCreditHours += creditHours;
		//get letter grade for next class
		letterGrade = JOptionPane.showInputDialog("Enter letter grade for the next class.");
		letterGrade = letterGrade.toUpperCase();
		//validate letter grade
		while (!letterGrade.equals("A") && !letterGrade.equals("B") 
		&& !letterGrade.equals("C") && !letterGrade.equals("D") 
		&& !letterGrade.equals("F")){
			//display error			
		JOptionPane.showMessageDialog(null,
		"You entered an invalid letter grade.");
		letterGrade = JOptionPane.showInputDialog("Enter letter grade for the next class.");
		letterGrade = letterGrade.toUpperCase();
			
		}
		calculateGradeScore(letterGrade);
		//more claculations
		gradePoints = creditHours * gradeScore;
		totalGradePoints += gradePoints;

		//more classes
		anotherClass = JOptionPane.showInputDialog("Do you have other class information?");
		anotherClass = anotherClass.toUpperCase();
		
		}
		
		gradePointAverage = totalGradePoints / totalCreditHours;
		displayGPA();
		System.exit(0);	
		
}
	//letter grade calculations method
	public static double calculateGradeScore(String letterGrade){
		
		if (letterGrade.equals("A"))
		gradeScore = 4.0;
		else
		if (letterGrade.equals("B"))
		gradeScore = 3.0;
		else 
		if (letterGrade.equals("C"))
		gradeScore = 2.0;
		else
		if (letterGrade.equals("D"))
		gradeScore = 1.0;
		else
		if (letterGrade.equals("F"))
		gradeScore = 0.0;
						
		return gradeScore;
		}

	public static void validateUserInput (String userInput) {
		while (creditHours<0 || creditHours>6) {
			//display error message
			JOptionPane.showMessageDialog(null,
				"Credit hours must be between 1 & 6.");
			userInput = JOptionPane.showInputDialog("Enter credit hours for the class.");
			creditHours = Integer.parseInt(userInput);
			}
	}
	
	public static void validateLetterGrade (String letterGrade) {
		while (!letterGrade.equals("A") && !letterGrade.equals("B") 
		&& !letterGrade.equals("C") && !letterGrade.equals("D") 
		&& !letterGrade.equals("F")){
			//display error message
			JOptionPane.showMessageDialog(null,
				"You entered an invalid letter grade.");
			letterGrade = JOptionPane.showInputDialog("Enter letter grade for the class.");
			letterGrade = letterGrade.toUpperCase();
				}
	}
		public static void validateAnotherClass (String anotherClass){
			
		}

//display results method
	public static void displayGPA() {
		JOptionPane.showMessageDialog(null, "GPA = "+totalGradePoints+"/"
+totalCreditHours+" = "+gradePointAverage);
	}
}

[hardwired]

do {
    get user input
    calculate one set of credit hours / letter grades,
    prompt the user (y/n) for more class info
} while(userWantsToDoMore);

[kicker]

I have cleaned everything up the best I can. All the calculations, validations and outputs work. I have seen some other programs similar in nature but they use a lot of do while, else and if.

Would this program be said to contain duplicate code? Am I going about it all wrong?

The reason I ask is that I got this as a comment from my professor. It is kind of cryptic to me

BTW, avoid code duplication. by avoiding the same code of the loop body
being done before the loop too.

import javax.swing.*;
import java.text.*;

public class StudentGPA {
		
		static String userInput, letterGrade, anotherClass;
		static int creditHours;
		static double totalCreditHours, gradeScore, gradePoints, totalGradePoints, gradePointAverage;
	
	public static void main (String[] args) {
		
		
		userInput = JOptionPane.showInputDialog("Enter credit hours for the class.");
		creditHours = Integer.parseInt(userInput);
		validateUserInput(userInput);
		totalCreditHours = creditHours;
		
		letterGrade = JOptionPane.showInputDialog("Enter letter grade for the class.");
		letterGrade = letterGrade.toUpperCase();
		validateLetterGrade(letterGrade);
		calculateGradeScore(letterGrade);
		gradePoints = creditHours * gradeScore;
		totalGradePoints = gradePoints;
		
		anotherClass = JOptionPane.showInputDialog("Do you have more classes to input?");
		anotherClass = anotherClass.toUpperCase();
		validateAnotherClass(anotherClass);
		
		while (anotherClass.equals("Y")){
			userInput = JOptionPane.showInputDialog("Enter credit hours for the next class.");
			creditHours = Integer.parseInt(userInput);
			validateUserInput(userInput);
			totalCreditHours += creditHours;
		
			letterGrade = JOptionPane.showInputDialog("Enter letter grade for the next class.");
			letterGrade = letterGrade.toUpperCase();
			validateLetterGrade(letterGrade);
			calculateGradeScore(letterGrade);
			gradePoints = creditHours * gradeScore;
			totalGradePoints += gradePoints;
		
			anotherClass = JOptionPane.showInputDialog("Do you have more classes to input?");
			anotherClass = anotherClass.toUpperCase();
			validateAnotherClass(anotherClass);
			
		}
		
		gradePointAverage = totalGradePoints / totalCreditHours;
		displayGPA();
		System.exit(0);	
		
}
	
	public static double calculateGradeScore(String letterGrade){
		
		if (letterGrade.equals("A"))
		gradeScore = 4.0;
		else
		if (letterGrade.equals("B"))
		gradeScore = 3.0;
		else 
		if (letterGrade.equals("C"))
		gradeScore = 2.0;
		else
		if (letterGrade.equals("D"))
		gradeScore = 1.0;
		else
		if (letterGrade.equals("F"))
		gradeScore = 0.0;
						
		return gradeScore;
		}

	public static void validateUserInput (String userInput) {
		while (creditHours<1 || creditHours>6) {
			JOptionPane.showMessageDialog(null,
				"Credit hours must be between 1 & 6.");
			userInput = JOptionPane.showInputDialog("Enter credit hours for the class.");
			creditHours = Integer.parseInt(userInput);
			}
	}
	
	public static void validateLetterGrade (String letterGrade) {
		while (!letterGrade.equals("A") && !letterGrade.equals("B") 
		&& !letterGrade.equals("C") && !letterGrade.equals("D") 
		&& !letterGrade.equals("F")){
			JOptionPane.showMessageDialog(null,
				"You entered an invalid letter grade.");
			letterGrade = JOptionPane.showInputDialog("Enter letter grade for the class.");
			letterGrade = letterGrade.toUpperCase();
				}
	}
	
	public static void validateAnotherClass (String anotherClass){
		while (!anotherClass.equals("Y") && !anotherClass.equals("N")){
			JOptionPane.showMessageDialog(null,
			"Please enter y or n.");
			anotherClass = JOptionPane.showInputDialog("Do you have more classes to input?");
			anotherClass = anotherClass.toUpperCase();
				}
	}
	
	public static void displayGPA() {
		DecimalFormat df = new DecimalFormat("####.00");
		JOptionPane.showMessageDialog(null, "GPA = "+(int)totalGradePoints+"/"+(int)totalCreditHours+" = "+(df.format(gradePointAverage)));
	}
}

[Norm]

Your code asks the same question outside and again inside the loop:
Enter ...A
Enter ...B
Do ...?
>>> begin loop <<
Enter ...A
Enter ...B
Do ...?
>>> end loop <<<
Can you think how to only ask a question in one place instead of two?

Have you looked at the do{}while loop?

[kicker]

I have tried to apply do{} while, but being a novice, when I start playing around all it does is break the existing program.

On a scale of 1 to 10 how close am I with the existing code?
1 being start over and 10 being just place the do{}while tags in the right spots and build it.

[Norm]

all it does is break the existing program.

Only thing to say is: keep trying.

[kicker]

I figured out why my previous attempts at using a do{}while didn't work!

while (anotherClass.equals("Y")); <-- This works.

while (anotherClass == "Y")); <-- This does not work.

Thank you for encouraging me and not giving it to me!

I still have an issue with one of my validations but I will figure it out.

Thanks again!