Java Programming

Hi guys, long time lurker for information and tips in java.

I'm making a java fraction calculator in my HS CSCI class, and I when I showed my work to my teacher he says the calculator does not show accurate calculations. He wouldn't tell me what was wrong, though, and I was wondering if you guys could help me pinpoint the problem and how I can fix them? Thanks.

import javax.swing.JOptionPane; 
  

public class Lab08bst 

{
public static void main (String args[])
{   
  String strNum1 = JOptionPane.showInputDialog("Enter Numerator 1"); 
  String strDen1 = JOptionPane.showInputDialog("Enter Denominator 1");
  String strNum2 = JOptionPane.showInputDialog("Enter Numerator 2"); 
  String strDen2 = JOptionPane.showInputDialog("Enter Denominator 2");
  
  int num1 = Integer.parseInt(strNum1);
  int den1 = Integer.parseInt(strDen1);
  int num2 = Integer.parseInt(strNum2);
  int den2 = Integer.parseInt(strDen2); 
  
  Rational r1 = new Rational(num1,den1);
  Rational r2 = new Rational(num2,den2);
  Rational r3 = new Rational(1,1);
  Rational r4 = new Rational(1,1);
  Rational r5 = new Rational(1,1);
  Rational r6 = new Rational(1,1);
  
  r3.product(r1,r2);
  r4.divide(r1,r2);
  r5.add(r1,r2);
  r6.subtract(r1,r2); 
    
  
  JOptionPane.showMessageDialog(null,r1.getRational() + " * " + r2.getRational() + " = " + r3.getRational() + "\n" + r1.getRational() + " / " + r2.getRational() + " = " + r4.getRational() + "\n" + r1.getRational() + " + " + r2.getRational() + " = " + r5.getRational()+ "\n" + r1.getRational() + " - " + r2.getRational() + " = " + r6.getRational());
    
    
  System.exit(0); 
  
} 
  
}


				



class Rational
{
	private int num;
	private int den;
	private int firstNum;
	private int firstDen;	
 
  
  
	public Rational(int n, int d)
	{
	num = n;
	den = d;
	firstNum = n;
	firstDen = d;
	reduce();

	}
	public int getNum()
	{
		
		return num;
	}
		
		public int getDen()
{
		return den;
}
private int getGCF(int n, int d)
{
	int rem = 0;
	int gcf = 0;
	
	do
	{
	rem = n % d;
	 
if (rem == 0)
	gcf = d;
else
	{
		n = d;
		d = rem;
		
	}
	
} while(rem != 0);
	return gcf;
	}
void reduce()
	
	{
		int gcf = getGCF(firstNum, firstDen);
		num = firstNum / gcf;
		den = firstDen / gcf;
	}
void product(Rational a, Rational b)
	{
		int gcf = getGCF(firstNum, firstDen);
		num = a.num * b.num;
		den = a.den * b.den;
	}

void divide(Rational a, Rational b)
	{
		int gcf = getGCF(firstNum, firstDen);
		num = a.num / b.num;
		den = a.den / b.den;
	}
	
void add(Rational a, Rational b)
	{
		int gcf = getGCF(firstNum, firstDen);
		num = a.num + b.num;
		den = a.den + b.den;
	}
void subtract(Rational a, Rational b)
	{
		int gcf = getGCF(firstNum, firstDen);
		num = a.num - b.num;
		den = a.den - b.den;
	}
	
public double getDecimal()
	{
		return (double)num / den;
	}
public String getRational()
	{
		return num + "/" + den;
	}
	
public String getOriginal()
	{
		return firstNum + "/" + firstDen;
	}
 
 
 
}