gui help... with an actionListener

Question

I'm writing my first GUI program that actually does something and I am having problems with the action listener. The program when complete will take a double input and make conversions from one unit to another based on some radio button selections which i haven't added yet. The problem right now is the Action listener doesn't recognize my text fields.

I have an input text field and an output text fields in separate panels. I created an action listener and I added the input text field to the listener.

ActionListener handler = new HandlerClass(); textField.addActionListener(handler); then I created an in class definition for the handler class but when I write the action preformed method textField and output cannot be resolved by the program. Can anyone see what I'm doing wrong?

public class conversionDisplay extends JFrame{
/**
 * 
 */
private static final long serialVersionUID = 1L;

private JPanel northPanel;
private JPanel southPanel;
private JPanel eastPanel;
private JPanel westPanel;

public conversionDisplay() {
    super("Temperature Conversion");


    northPanel = new JPanel();                          //create northPanel
    northPanel.setLayout(new GridLayout(1,2,5,5));
    northPanel.add(new JPanel());
    JPanel northLabelPanel = new JPanel(new BorderLayout()) ;
    northLabelPanel.add(new JLabel("Input"), BorderLayout.EAST);
    northPanel.add(northLabelPanel);
    JTextField textField =new JTextField(10);
    northPanel.add(textField);
    northPanel.add(new JPanel());


    southPanel = new JPanel();                          //create southPanel
    southPanel.setLayout(new GridLayout(1,2));
    southPanel.add(new JPanel());
    JPanel southLabelPanel = new JPanel(new BorderLayout());
    southLabelPanel.add(new JLabel("Output "), BorderLayout.EAST);
    southPanel.add(southLabelPanel);
    JTextField output;
    southPanel.add(output = new JTextField( 10));
    output.setEditable(false);
    southPanel.add(new JPanel());





    add(northPanel,BorderLayout.NORTH);                 //add north panel
    add(southPanel,BorderLayout.SOUTH);                 //add north panel


    ActionListener handler = new HandlerClass();
    textField.addActionListener(handler);

    setSize(350, 200);
    setLocationRelativeTo(null);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    setVisible(true);
}
private class HandlerClass implements ActionListener{


public void actionPerformed(ActionEvent e) {
    double input = textField.getText();
    if (input != 0)
    {

        output.setText(input);  //Perform conversion
    }

}



}

}

Answer 1

Your textField JTextField is declared inside of a constructor and is thus only visible within that block (again the constructor). You need to make it an instance field of the class.

i.e.,

public class Foo {

  private Bar bar = new Bar();  // this field is visible throughout the object

  public Foo() {
    Baz baz = new Baz(); // this is only visible within this constructor
  } 

So just like the bar variable above is visible while the baz variable that is declared in the constructor is not, you'll want to move your JTextField variable declarations out of the constructor.