HELP!!! NumberFormatException!!!

Question

Can you find anything wrong this code that causes a NumberFormatException? I'm a beginner, so I haven't got to exception handling yet. I'm working from Deitel and Deitel's book "Java 2: How to Program". I don't care what number I put in (with or without a decimal point), the exception is always there. I don't see anything wrong with the code. Do you? Please feel free to copy this code and use it on your own computer, and let me know what works for you. Your help will be greatly appreciated. Here I've posted the code and below it is the runtime error message.
// Exercise 12.13
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Temperature extends JFrame
private JLabel enterTemp, convertTemp;
private JTextField tempField, resultsField;
private JComboBox scales;
private JRadioButton scaleButtons[];
private ButtonGroup rbGroup;
private JPanel top, middle, bottom;
private Container c;
private String scaleNames[] = {"Fahrenheit", "Centigrade", "Kelvin"};
private RadioButtonHandler handler;
public Temperature()
super("Temperature Conversion App");
c = getContentPane();
c.setLayout(new GridLayout(3,1));
setupTop();
setupMiddle();
setupBottom();
setSize(500,300);
show();
public void setupTop()
top = new JPanel();
top.setLayout(new FlowLayout());
enterTemp = new JLabel("Please enter a temperature:");
top.add(enterTemp);
tempField = new JTextField(5);
top.add(tempField);
scales = new JComboBox(scaleNames);
scales.setMaximumRowCount(1);
top.add(scales);
c.add(top);
public void setupMiddle()
middle = new JPanel();
middle.setLayout(new FlowLayout());
convertTemp = new JLabel("I wanna convert this to:");
middle.add(convertTemp);
handler = new RadioButtonHandler();
scaleButtons = new JRadioButton[3];
rbGroup = new ButtonGroup();
for (int i=0; i < scaleButtons.length; i++)
scaleButtons[i] = new JRadioButton(scaleNames, false);
scaleButtons[i].addItemListener(handler);
rbGroup.add(scaleButtons[i]);
middle.add(scaleButtons[i]);
c.add(middle);
public void setupBottom()
bottom = new JPanel();
bottom.setLayout(new FlowLayout());
resultsField = new JTextField(50);
resultsField.setEditable(false);
bottom.add(resultsField);
c.add(bottom);
private class RadioButtonHandler implements ItemListener
public void itemStateChanged(ItemEvent e)
if (e.getSource()==scaleButtons[0])
     if (scales.getSelectedIndex()==0)
     resultsField.setText("You already have the Fahrenheit temp!");
     else if (scales.getSelectedIndex()==1)
convertF2C(enterTemp.getText());
     else if (scales.getSelectedIndex()==2)
     convertF2K(enterTemp.getText());
} // end if
if (e.getSource()==scaleButtons[1])
     if (scales.getSelectedIndex()==0)
     convertC2F(enterTemp.getText());
     else if (scales.getSelectedIndex()==1)
     resultsField.setText("You already have the Centigrade temp!");
     else if (scales.getSelectedIndex()==2)
     convertC2K(enterTemp.getText());
} // end if
if (e.getSource()==scaleButtons[2])
     if (scales.getSelectedIndex()==0)
     convertK2F(enterTemp.getText());
     else if (scales.getSelectedIndex()==1)
     convertK2C(enterTemp.getText());
     else if (scales.getSelectedIndex()==2)
     resultsField.setText("You already have the Kelvin temp!");
} // end itemStateChanged
} // end RadioButtonHandler
public void convertF2C(String input)
double temp = Double.parseDouble(input);
resultsField.setText(temp + " degrees Fahrenheit is "+((5/9)*(temp-32)) + " degrees
Centigrade.");
public void convertF2K(String input)
{ double temp = 0.0;
temp = Double.parseDouble(input);
resultsField.setText(temp + " degrees Fahrenheit is "+((5/9)*(temp-32)+273) + " degrees
Kelvin.");
public void convertC2F(String input)
double temp = Double.parseDouble(input);
resultsField.setText(temp + " degrees Centigrade is " +((9/5)*temp+32) + " degrees
Fahrenheit.");
public void convertC2K(String input)
double temp = Double.parseDouble(input);
resultsField.setText(temp + " degrees Centigrade is " +(temp+273) + " degrees Kelvin.");
public void convertK2F(String input)
double temp = Double.parseDouble(input);
resultsField.setText(temp + " degrees Kelvin is " +((9/5)*(temp-273)+32) + " degrees
Fahrenheit.");
public void convertK2C(String input)
double temp = Double.parseDouble(input);
resultsField.setText(temp + " degrees Kelvin is " + (temp-273) + " degrees Centigrade.");
public static void main(String args[])
Temperature app = new Temperature();
app.addWindowListener(new WindowAdapter()
public void windowClosing(WindowEvent e)
     System.exit(0);
} // end main
} // end class
C:\jdk1.2.1\bin>java Temperature
Exception occurred during event dispatching:
java.lang.NumberFormatException: Please enter a temperature:
at java.lang.FloatingDecimal.readJavaFormatString(Compiled Code)
at java.lang.Double.parseDouble(Double.java:188)
at Temperature.convertC2F(Temperature.java:129)
at Temperature$RadioButtonHandler.itemStateChanged(Temperature.java:95)
at javax.swing.AbstractButton.fireItemStateChanged(Compiled Code)
at javax.swing.AbstractButton$ForwardItemEvents.itemStateChanged(Abstrac
tButton.java:1112)
at javax.swing.DefaultButtonModel.fireItemStateChanged(Compiled Code)
at javax.swing.JToggleButton$ToggleButtonModel.setSelected(JToggleButton
.java:222)
at javax.swing.JToggleButton$ToggleButtonModel.setPressed(JToggleButton.
java:239)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonL
istener.java:217)
at java.awt.Component.processMouseEvent(Component.java:3126)
at java.awt.Component.processEvent(Compiled Code)
at java.awt.Container.processEvent(Compiled Code)
at java.awt.Component.dispatchEventImpl(Compiled Code)
at java.awt.Container.dispatchEventImpl(Compiled Code)
at java.awt.Component.dispatchEvent(Compiled Code)
at java.awt.LightweightDispatcher.retargetMouseEvent(Compiled Code)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:1732)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:1645)
at java.awt.Container.dispatchEventImpl(Compiled Code)
at java.awt.Window.dispatchEventImpl(Window.java:714)
at java.awt.Component.dispatchEvent(Compiled Code)
at java.awt.EventQueue.dispatchEvent(Compiled Code)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:68)

Answer 1

Of course you can also just stick your code in a try... catch block and handle the exceptions as they arise. This would probably be sub-optimal if you were getting a lot of them though.
Of course you could combine both and create a helper method to parse your double values.
public double getDoubleValueFromString(String input) {
  double myDouble = 0;
  try {
    if(!isEmptyOrNull(input)) {
      myDouble =  Double.parseDouble(input);
  catch (NumberFormatException ex) {
    // Handle this exception how ever you want.
  finally {
    // This is to annoy a certain somebody in the forums
    return myDouble;