Rational Numbers GUI; Problem Displaying Answer

Question

I have to make a GUI that adds, subtracts, divides, and multiplies rational numbers. You input them as fractions. I think most of it is right, it does compile, but I don't know how to get my answer to display. I tried simple stuff like just adding it into the setText part at the end, but is there something that I'm missing like convert it to a string or something, I don't really know, I'm still very new to all this.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class RatDemo extends JFrame implements ActionListener {
           private JTextField jtfR1, jtfR2, jtfResult;
           private JButton jbtAdd, jbtSub, jbtDiv, jbtMul, jbtClr, jbtEx;
           public int numerator, denominator;
     public RatDemo(int numerator, int denominator)
            this.numerator = numerator;
            this.denominator = denominator;
             reduce();
     private void reduce()
             int divisor = gcd((numerator < 0)?-1*numerator:numerator,
                  (denominator < 0)?-1*denominator:denominator);
             if (divisor != 0)
                 numerator /= divisor;
                 denominator /= divisor;
     private int gcd(int a, int b)
           if (b == 0) return a; else return gcd(b,a%b);
     public int getNumerator()
         return numerator;
     public void print1(RatDemo RR)
          System.out.print( RR.numerator + "/" + RR.denominator);
     public int getDenominator()
         return denominator;
     public static RatDemo Add(RatDemo r1, RatDemo r2)
               int newNum = r1.numerator * r2.denominator + r1.denominator * r2.numerator ;
               int newDenom = r1.denominator * r2.denominator;
               return new RatDemo(newNum,newDenom);
     public static RatDemo Subtract(RatDemo r1, RatDemo r2)
          int newNum = r1.numerator*r2.denominator-r1.denominator*r2.numerator;
          int newDenom = r1.denominator*r2.denominator;
            return new RatDemo(newNum, newDenom);
     public static RatDemo Multiply(RatDemo r1, RatDemo r2)
               int newNum = r1.numerator * r2.numerator;
               int newDenom = r1.denominator * r2.denominator;
               return new RatDemo(newNum,newDenom);
     public static RatDemo Divide(RatDemo r1, RatDemo r2)
               int newNum = r1.numerator * r2.denominator;
               int newDenom = r1.denominator * r2.numerator;
               return new RatDemo(newNum,newDenom);
     public static void main(String [] args)
           RatDemo frame = new RatDemo();
           RatDemo r1 = new RatDemo(3, 17);
           RatDemo r2 = new RatDemo(5, 9);;
          RatDemo r3 = new RatDemo();
            frame.setTitle(" Rational Numbers ");
            frame.setSize(500,140);
            frame.setVisible(true);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     public RatDemo()
         JPanel p1 = new JPanel();
         p1.setLayout(new FlowLayout());
         p1.add( new JLabel("Rational 1"));
         p1.add( jtfR1 = new JTextField(6));
         p1.add( new JLabel("Rational 2"));
         p1.add( jtfR2 = new JTextField(6));
         JPanel p2 = new JPanel();
         p2.setLayout( new FlowLayout());
         p2.add ( new JLabel("Results "));
         p2.add( jtfResult = new JTextField(30));
         jtfResult.setEditable(false);
         JPanel p3 = new JPanel();
         p3.setLayout(new FlowLayout());
         p3.add( jbtAdd = new JButton("Add"));
         p3.add( jbtSub = new JButton("Subtract"));
         p3.add( jbtDiv = new JButton("Divide"));
         p3.add( jbtMul = new JButton("Multiply"));
         p3.add( jbtClr = new JButton("Clear"));
         p3.add( jbtEx = new JButton("Exit"));
            getContentPane().setLayout(new BorderLayout());
         getContentPane().add(p1, BorderLayout.NORTH);
         getContentPane().add(p2, BorderLayout.CENTER);
         getContentPane().add(p3, BorderLayout.SOUTH);
             jbtAdd.addActionListener(this);
            jbtSub.addActionListener(this);
            jbtMul.addActionListener(this);
            jbtDiv.addActionListener(this);
            jbtClr.addActionListener(this);
            jbtEx.addActionListener(this);
     public void actionPerformed( ActionEvent e)
             String actionCommand = e.getActionCommand();
             if (e.getSource() instanceof JButton)
                  if ("Add".equals(actionCommand))
                       calculate('+');
                  else if ("Subtract".equals(actionCommand))
                       calculate('-');
                  else if ("Multiply".equals(actionCommand))
                        calculate('*');
                  else if ("Divide".equals(actionCommand))
                        calculate('/');
                  else if ("Clear".equals(actionCommand))
                        jtfR1.setText(" ");
                        jtfR2.setText(" ");
                        jtfResult.setText(" ");
                  else if ("Exit".equals(actionCommand))
                        System.exit(0);
     private void calculate(char operator)
            switch(operator)
                  case '+':
                    jtfResult.setText("The sum of the rational numbers is ");
                    break;
                  case '-':
                    jtfResult.setText("The difference of the rational numbers is ");
                   break;
                  case '*':
                    jtfResult.setText("The product of the rational numbers is ");
                   break;
                  case '/':
                    jtfResult.setText("The quotient of the rational numbers is ");
                   break;
                  default:
                   jtfResult.setText("Invalid Output");
}

Answer 1

If I understand correctly, in your calculate method you just concatenate the result onto the end of the string.
System.out.println("Hello " + 3);  // Hello 3
System.out.println("Hello " + 4 + 2);  // Hello 42
System.out.println("Hello " + ( 2 + 3)); // Hello 5