Passing Values from ComboBox for Calculations

Hello Experts! In my program, I want the figures from the combobox to fill in the text boxes to be used in calculations. The combobox is filled in by reading from an external file (data.txt) which contains:
7,5.35
15,5.5
20,5.65
30,5.75
Right now when I select from the combobox and hit Calculate, the program acts like there is no input and gives the user an error. If upon selection, the program inserted the numbers into the textboxes, then the program would calculate as if the user input figures in the textbox and hit Calculate (which it does correctly). Can you show me how to do this? Here is my code (I know my graph is not populating the graph window right now...still working on that, might open a new question if needed):

import java.awt.*;
import java.awt.event.*;
import java.awt.font.*;
import java.awt.geom.*;
import java.io.*;
import java.text.*;
import javax.swing.*;
import javax.swing.border.*;
import javax.swing.JComboBox;

public class MC extends JFrame implements ActionListener {
    JTextField principalTxt, termTxt, rateTxt, monthlyPaymentTxt;
    JTextArea displayArea;
    JButton calculateButton, graphButton, amortizeButton, clearButton, exitButton;
    JComboBox options;

    //formatting variables
    DecimalFormat df = new DecimalFormat("\u00A4#,##0.00"); //currency
    DecimalFormat pf = new DecimalFormat("#,##0.00%");      //percentages
    DecimalFormat mi = new DecimalFormat("#,##0.000%");     //percentages

    // Can these 6 be removed and declared as local variables?
    int term = 0;
    double principal = 0;
    double rate = 0;
    double monthlyPayment = 0;
    double interest = 0;
    int notePeriod = 0;

    int[] termArray;
    double[] rateArray;
    private float[] principalAmts;
    private float[] interestAmts;

    public MC() {
        super ("Mortgage Payment Calculator by S Kemen");
        setSize(550, 500);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        load();
        setVisible(true);

        // Initialize components.
        JPanel row1 = new JPanel();
        JLabel mortgageLabel = new JLabel("MORTGAGE PAYMENT CALCULATOR", JLabel.CENTER);

        JPanel row2 = new JPanel(new GridLayout(1, 2));
        JLabel principalLabel = new JLabel("Mortgage Principal $",JLabel.LEFT);
        principalTxt = new JTextField(10);

        JPanel row3 = new JPanel(new GridLayout(1, 2));
        JLabel termLabel = new JLabel("Mortgage Term (Yrs)",JLabel.LEFT);
        termTxt = new JTextField(10);

        JPanel row4 = new JPanel(new GridLayout(1, 2));
        JLabel rateLabel = new JLabel("Interest Rate (%)", JLabel.LEFT);
        rateTxt = new JTextField(10);

        JPanel row5 = new JPanel(new GridLayout(1, 2));
        JLabel presetLabel = new JLabel("Preset Term and Interest Rate:", JLabel.LEFT);

        JPanel row6 = new JPanel(new GridLayout(1,2));
        options = new JComboBox();
        MessageFormat mf = new MessageFormat("{0} years at {1,number,#.##}%");
        JLabel choiceLabel = new JLabel(" (choose rate)");
        for (int i = 0; i < termArray.length; i++) {
            options.addItem(mf.format(new Object[] { new Integer(termArray [i]),
                                                     new Double(rateArray[i])}));
        }

        JPanel row7 = new JPanel(new GridLayout(1, 2));
        JLabel monthlyPaymentLabel = new JLabel("Monthly Payment $", JLabel.LEFT);
        monthlyPaymentTxt = new JTextField(10);

        //create buttons
        JPanel button = new JPanel(new FlowLayout(FlowLayout.CENTER, 10, 10));
        graphButton = new JButton("Display Graph");
        amortizeButton = new JButton("Amortize Payments");
        clearButton = new JButton("Clear");
        exitButton = new JButton("Exit");
        calculateButton = new JButton("Calculate");

        //create textarea to display Amortize output
        displayArea = new JTextArea(10, 45);
        JScrollPane scroll = new JScrollPane(displayArea);

        Container pane = getContentPane();
        // Set layout before adding components.
        pane.setLayout(new BoxLayout( pane, BoxLayout.Y_AXIS));

        Border rowborder = new EmptyBorder( 3, 10, 3, 10 );

        pane.add(row1);
        row1.add(mortgageLabel);
        row1.setMaximumSize( new Dimension( 10000, row1.getMinimumSize().height));
        row1.setBorder( rowborder);

        pane.add(row2);
        row2.add(principalLabel);
        row2.add(principalTxt);
        row2.setMaximumSize( new Dimension( 10000, row2.getMinimumSize().height));
        row2.setBorder( rowborder);

        pane.add(row3);
        row3.add(termLabel);
        row3.add(termTxt);
        row3.setMaximumSize( new Dimension( 10000, row3.getMinimumSize().height));
        row3.setBorder( rowborder);

        pane.add(row4);
        row4.add(rateLabel);
        row4.add(rateTxt);
        row4.setMaximumSize( new Dimension( 10000, row4.getMinimumSize().height));
        row4.setBorder( rowborder);

        pane.add(row5);
        row5.add(presetLabel);
        row5.setMaximumSize( new Dimension( 10000, row4.getMinimumSize().height));
        row5.setBorder( rowborder);

        pane.add(row6);
        row6.add(choiceLabel);
        row6.setMaximumSize( new Dimension( 10000, row4.getMinimumSize().height));
        row6.setBorder( rowborder);

        JPanel radioPanel = new JPanel();
        radioPanel.setLayout(new FlowLayout(FlowLayout.LEFT, 4, 4 ));
        radioPanel.add(options);
        pane.add(radioPanel);
        radioPanel.setMaximumSize( new Dimension( 10000,
                                         radioPanel.getMinimumSize().height));
        radioPanel.setBorder( rowborder);


        pane.add(row7);
        row7.add(monthlyPaymentLabel);
        row7.add(monthlyPaymentTxt);
        monthlyPaymentTxt.setEnabled(false);  //set payment amount uneditable
        monthlyPaymentTxt.setFocusable(false);
        row7.setMaximumSize( new Dimension( 10000, row5.getMinimumSize().height));
        row7.setBorder( rowborder);

        //Add buttons
        button.add(calculateButton);
        button.add(clearButton);
        button.add(exitButton);
        button.add(amortizeButton);
        button.add(graphButton);
        pane.add(button);
        button.setMaximumSize( new Dimension( 10000, button.getMinimumSize().height));

        //scroll text pane for Amortize output
        scroll.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));
        pane.add(scroll);
        setContentPane(pane);

        //add listeners for buttons and combo box
        options.addActionListener(this);
        clearButton.addActionListener(this);
        exitButton.addActionListener(this);
        calculateButton.addActionListener(this);
        amortizeButton.addActionListener(this);
        graphButton.addActionListener(this);

    }

    public void actionPerformed(ActionEvent event) {
        Object command = event.getSource();

        if(command == calculateButton) { //function for Calculate button
            try {
                principal = Double.parseDouble(principalTxt.getText());
            } catch(NumberFormatException e) {
                //catch null pointer exception if Principal is null
                JOptionPane.showMessageDialog(null, "Invaild Entry!  Please Try Again",
                                              "ERROR", JOptionPane.ERROR_MESSAGE);
            }
            try {
                //Get input from Term text box, checked first before combo box
                term = Integer.parseInt(termTxt.getText());
                rate = Double.parseDouble(rateTxt.getText()); //Input from Rate text box
            } catch(NumberFormatException e)  {
                //If Term and Rate are null, buttons are checked for input values
                //Set rate and term based on which item in the combobox is selected


                //If no option is selected and combo box null,
                // this is an actual error.
                JOptionPane.showMessageDialog(null, "Invaild Entry! " +
                                              "Please Try Again",
                                              "ERROR", JOptionPane.ERROR_MESSAGE);
            }
            interest = rate / 100 / 12;  //Monthly interst rate
            notePeriod = term * 12;      //Number of months over which loan is amortized

            //calculation formula
            monthlyPayment = (principal * interest) /
                                   (1 - Math.pow(1 + interest, -notePeriod));

            monthlyPaymentTxt.setText("" + df.format(monthlyPayment));
        }

        if(command == clearButton) { //Function for Clear button
            principalTxt.setText("");
            monthlyPaymentTxt.setText("");
            rateTxt.setText("");
            termTxt.setText("");
            displayArea.setText("");
        }

        if(command == exitButton) { //Function for Exit button
             System.exit(0);
        }
        if (command == amortizeButton) { //Function for Amortize button
            //Amoritization variables
            double loanBalance = notePeriod * monthlyPayment;
            // You need to use the member variables with these two names
            // so you can send them to your GraphPanel. Instantiate them
            // here and fill the elements in this loop. The member
            // variables remain null and choke up GraphPanel.
            double interestPaid = 0;       //Interest paid on the loan
            double monthlyPrincipal = 0;   //Principal in each monthly payment
            //running total of principal after payment
            double principalBalance = principal;
            //String for output
            String titles = "Month\t Principal\t\tInterest\t\tBalance\n";

            displayArea.setText(titles);
            displayArea.append("");        //Inserts a blank line

            interestAmts  = new float[term*12];
            principalAmts = new float[term*12];

            //This loop is used to calculate and display the payment schedule information
            for(int counter = 0; counter < term * 12 - 0; counter++) { //start outer loop
                //start inner loop
                interestPaid = principalBalance * interest;
                interestAmts[counter] = (float)interestPaid;
                monthlyPrincipal = monthlyPayment - interestPaid;
                principalAmts[counter] = (float)monthlyPrincipal;
                loanBalance = loanBalance - monthlyPayment;
                principalBalance = principalBalance - monthlyPrincipal;

                //formatting for output
                displayArea.setCaretPosition(0);
                displayArea.append((counter +1) + ")\t" +
                                    df.format(monthlyPrincipal)+"\t\t" +
                                    df.format(interestPaid)+"\t\t" +
                                    df.format(principalBalance)+"\n");
            }
        }
        if (command == graphButton) { //Function for Graph button
            JDialog d = new JDialog(this, false);
            d.getContentPane().add(new GraphPanel(principalAmts, interestAmts));
            d.setSize(800,600);
            d.setLocation(200,100);
            d.setVisible(true);
        }
    }

    //reading from sequential file
    private void load() {
        Reader fis;
        try {
            fis = new InputStreamReader(getClass().getResourceAsStream("data.txt"));
            BufferedReader br = new BufferedReader( fis );
            termArray = new int[4];
            rateArray = new double[4];
            String line;
            int i = 0;
            while((line = br.readLine()) != null) {
                String[] s = line.split(",");
                termArray[ i ] = Integer.parseInt(s[0].trim());
                rateArray[ i ] = Double.parseDouble(s[1].trim());
                i++;
            }

            br.close();
        } catch ( Exception e1 ) {
            e1.printStackTrace(  );
        }
    }

class GraphPanel extends JPanel
{
    final int
        HPAD = 60,
        VPAD = 40;
    int[] data;
    Font font;
    float[] principalAmts;
    float[] interestAmts;

    public GraphPanel(float[] principalAmts, float[] interestAmts)
    {

        font = new Font("lucida sans regular", Font.PLAIN, 16);
        setBackground(Color.white);
    }

    protected void paintComponent(Graphics g)
       {
           super.paintComponent(g);

           Graphics2D g2 = (Graphics2D)g;
           g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                               RenderingHints.VALUE_ANTIALIAS_ON);
           g2.setFont(font);
           FontRenderContext frc = g2.getFontRenderContext();
           int w = getWidth();
           int h = getHeight();
           // scales
           float xInc = (w - HPAD - VPAD) / (interestAmts.length - 1);//11f;  //distance between each point
           float yInc = (h - 2*VPAD) / 10f;
           int[] dataVals = getDataVals();        //min and max values for y-axis
           float yScale = dataVals[2] / 10f;

           // ordinate (y - axis)
           g2.draw(new Line2D.Double(HPAD, VPAD, HPAD, h - VPAD));
           // plot tic marks
           float x1 = HPAD, y1 = VPAD, x2 = HPAD - 3, y2;
           for(int j = 0; j < 10; j++)
           {
               g2.draw(new Line2D.Double(x1, y1, x2, y1));
               y1 += yInc;
           }
           // labels
           String text; LineMetrics lm;
           float xs, ys, textWidth, height;
           for(int j = 0; j <= 10; j++)
           {
               text = String.valueOf(dataVals[1] - (int)(j * yScale));
               textWidth = (float)font.getStringBounds(text, frc).getWidth();
               lm = font.getLineMetrics(text, frc);
               height = lm.getAscent();
               xs = HPAD - textWidth - 7;
               ys = VPAD + (j * yInc) + height/2;
               g2.drawString(text, xs, ys);
                     if(j == 0) g2.drawString("Principal", xs, ys - 15);
           }

           // abcissa (x - axis)
           g2.draw(new Line2D.Double(HPAD, h - VPAD, w - VPAD, h - VPAD));
           // tic marks
           x1 = HPAD; y1 = h - VPAD; y2 = y1 + 3;
           for(int j = 0; j < interestAmts.length; j++)
           {
               g2.draw(new Line2D.Double(x1, y1, x1, y2));
               x1 += xInc;
           }
           // labels
           ys = h - VPAD;
           for(int j = 0; j < interestAmts.length; j++)
           {
               text = String.valueOf(j + 1);
               textWidth = (float)font.getStringBounds(text, frc).getWidth();
               lm = font.getLineMetrics(text, frc);
               height = lm.getHeight();
               xs = HPAD + j * xInc - textWidth/2;
               g2.drawString(text, xs, ys + height);
                     if( j == (interestAmts.length - 1)) g2.drawString("Year", (int)w /2 , ys + height + 15);
           }

           // plot data
             float yy2 = 0, yy1 = 0, xx2 = 0, xx1;
           x1 = HPAD;
             xx1 = HPAD;
           yScale = (float)(h - 2*VPAD) / dataVals[2];

           for(int j = 0; j < interestAmts.length; j++)
           {
             g.setColor(Color.blue);
             y1 = VPAD + (h - 2*VPAD) - (principalAmts[j] - dataVals[0]) * yScale;
                 g2.fillOval((int)x1, (int)y1 - 2, 5, 5);

               if(j > 0)
               g2.draw(new Line2D.Double(x1, y1, x2, y2));
               x2 = x1;
               y2 = y1;
               x1 += xInc;

             g.setColor(Color.red);
             yy1 = VPAD + (h - 2*VPAD) - (interestAmts[j] - dataVals[0]) * yScale;
                 g2.fillOval((int)xx1, (int)yy1 - 1, 5, 5);
                  if(j > 0)
               g2.draw(new Line2D.Double(xx1, yy1, xx2, yy2));
               xx2 = xx1;
               yy2 = yy1;
               xx1 += xInc;

           }
    }

    private int[] getDataVals()
    {
        int max = Integer.MIN_VALUE;
        int min = Integer.MAX_VALUE;
          int j = interestAmts.length -1;
          max = (int)principalAmts[j];
          min = (int)interestAmts[j];
        int span = max - min;
        return new int[] { min, max, span };
    }
}

    public static void main(String[] args) {
        MC smc = new MC();
    }
 }