Error with Arithmetic Operations on Fractions (multiply,divide,add,sub

I've been working on an assingment that requires us to get fractions to multiply,divide,subtract, and add. I'm pretty that part of my code works, but when I go to run it I get an error that I don't understand how to fix. Can anyone explain what the error means and what should I do to solve it. I've tried making everything equal to num and den, but I think that is causing the problem.

[import java.util.*;
import java.util.Scanner;
public class TextLab05 {


	/**
	 * @param args
	 */
		// TODO Auto-generated method stub
		static int num1, den1;   // numerator and denominator of the 1st rational number
		static int num2, den2;   // numerator and denominator of the 2nd rational number

		
		public static void main (String args[])
		{
			enterData();

			Rational r1 = new Rational(num1,den1);
			Rational r2 = new Rational(num2,den2);
			Rational r3 = new Rational();

			r3.multiply(r1,r2);
			System.out.println("\n\n" + r1.getOriginal() + " * " + r2.getOriginal() + "  =  " + r3.getRational());
			r3.divide(r1,r2);
			System.out.println("\n" + r1.getOriginal() + " / " + r2.getOriginal() + "  =  " + r3.getRational());
		}
			public static void enterData()
			{
				Scanner input = new Scanner(System.in);
				System.out.print("\nEnter the 1st numerator ----> ");
				num1 = input.nextInt();
				System.out.print("\nEnter the 1st denominator --> ");
				den1 = input.nextInt();
				System.out.println("\nEnter the 2nd numerator ----> ");
				num2 = input.nextInt();
				System.out.println("\nEnter the 2nd denominator --> ");
				den2= input.nextInt();
			}
}



		class Rational
		{
			
			private int firstNum;	// entered numerator
			private int firstDen;	// entered denominator
			private int num;		// reduced numerator
			private int den;		// reduced denominator
            int fractionNum;
            int fractionDen;
//Rational
            public Rational(int n1,int d1)
            {
            	this.firstNum=n1;
            	this.firstDen=d1;
            }
            public Rational ()
            {
            	
            }
            
//			getNum
			public int getNum()
			{
				num=firstNum;
			    return num;
			}
//			getDen
            public int getDen()
            {
            	den=firstDen;
            	return den;
            }
//			getDecimal
            public double getDecimal()
            {
            return (double)num/den;
            	
            }
//			getRational
            public String getRational()
            {
             return fractionNum+"/"+fractionDen;
            }
            	
//			getOriginal
             public String getOriginal()
             {
            	return firstNum+"/"+firstDen;
             }
//			reduce
             public void reduce()
             {
            int GCF;
         	GCF= getGCF(num,den);
         	fractionNum=num/GCF;
         	fractionDen= den/GCF;
             }

			public void displayData()
			{
				System.out.println();
				System.out.println(getNum() + "/" + getDen() + " equals " + getDecimal());
				System.out.println();
				System.out.println("and reduces to " + getRational());
				System.out.println();
			}
			
			private int getGCF(int n1,int n2)
			{
				int rem = 0;
				int gcf = 0;
				do
				{
					rem = n1 % n2;
					if (rem == 0)
						gcf = n2;
					else
					{
						n1 = n2;
						n2 = rem;
					}
				}
				while (rem != 0);
				return gcf;
			}
			{

		}
         public void multiply(Rational r1,Rational r2)
         { 
             num=r1.getNum()*r2.getNum();
             den=r1.getDen()*r2.getDen();
             reduce();
         }
        	
         public void divide (Rational r1,Rational r2)
         {
        	 num=r1.getNum()*r2.getDen();
        	 den=r1.getDen()*r2.getNum();
        	 reduce();
         }
         public void add (Rational r1, Rational r2)
         {
        	 
         }
         public void subtract (Rational r1, Rational r2)
         {
        	 
         }
{

There is an error about something being divisible by zero. The fractionnum and den should be delete and everything is equal to num and den.