What causes "Number format exception / empty string" error in the following code?

Question

I am trying to write a GUI temperature converter. It has one JTextField and two JButtons. TextField accepts the temperature which the user wants to convert and the user presses the appropriate button. Whenever I click on anyone of the buttons, I get a "Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: empty String" error. Please Help!

public class tempcon extends JFrame {
  private JPanel panel;
  private JLabel messageLabel;
  public JTextField tempC; 
  private JButton calcButton, calcButton1;
  private final int WINDOW_WIDTH = 300;
  private final int WINDOW_HEIGHT = 140;

public tempcon() {
  setTitle("Temperature convertion");
  setSize(WINDOW_WIDTH, WINDOW_HEIGHT);
  setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  buildPanel();
  add(panel);
  setVisible(true);
}

public double getTempC(){
 return Double.parseDouble(tempC.getText());
}

private void buildPanel() {
    tempC = new JTextField(10);
    messageLabel = new JLabel("Enter tempurture");
    calcButton = new JButton("Convert to Fahrenheit");
    calcButton1 = new JButton("Convert to Celcius");
    calcButton.addActionListener(new CalcButtonListener());
    calcButton1.addActionListener(new CalcButtonListener1());
    panel = new JPanel();
    panel.add(messageLabel);
    panel.add(tempC);
    panel.add(calcButton);
    panel.add(calcButton1);

}

public static void main(String[] args){
 new tempcon().buildPanel();   
}
}

class CalcButtonListener1 implements ActionListener {
 public void actionPerformed(ActionEvent e) {
        double input;
        double temp;
        input = new tempcon().getTempC();
        temp = input * 1.8 + 32;
        JOptionPane.showMessageDialog(null, "That is " + temp + " 
  degrees Celsius.");
    }

}

 class CalcButtonListener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        double input;
        double temp;
        input = new tempcon().getTempC();
        temp = (input - 32)*1.8;
        JOptionPane.showMessageDialog(null, "That is " + temp + " 
 degrees Fehrenheit.");
}

public static void main(String[] args) {
tempcon myTempWindowInstance = new tempcon();
}
}

Arnaud · Accepted Answer · 2017-11-06 15:19:35Z

2

The problem is that you are recreating a new frame in your action listeners : new tempcon().getTempC() .

The textfields in these new frames are obviously empty and you get your error.

Consider referring to the same instance of tempcon everywhere, that is simply replace

new tempcon().getTempC();

with

getTempC();

, which will call the getTempC() method of the outer tempcon instance .

answered Nov 6, 2017 at 15:19

Arnaud

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