FocusListener on JTextField not working

Question

I've created an application that uses FocusListener to make sure a text fieid's value is always positive. When the user inputs negative value and then click the "tab" key to move focus away from the text field, the value will be multiplied by -1 so that the resulted value is positive. However, when I ran the application, the text field didn't change. I am not sure what I did wrong, and will appreciate any help.

Here is my code:

import java.awt.event.*;
import javax.swing.*;
import java.awt.*;

public class AlwaysPositive extends JFrame implements FocusListener {
JTextField posField = new JTextField("30",5);

public AlwaysPositive() {
    super("AlwaysPositive");
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    JPanel pane = new JPanel();
    JTextField posField = new JTextField("30",5);
    JButton ok= new JButton("ok");
    posField.addFocusListener(this);
    pane.add(posField);
    pane.add(ok);
    add(pane);
    setVisible(true);
}

public void focusLost(FocusEvent event) {
    try {
        float pos = Float.parseFloat(posField.getText());
        if (pos < 0) 
            pos = pos*-1;
        posField.setText("" + pos);
    } catch (NumberFormatException nfe) {
        posField.setText("0");
    }
}

public void focusGained(FocusEvent event) {
}

public static void main(String[] arguments) {
    AlwaysPositive ap = new AlwaysPositive();
}

}

Answer 1

The main problem is you are shadowing your variables

You declare

 JTextField posField = new JTextField("30",5);

As an instance variable, but in your constructor, you redeclare it again...

public AlwaysPositive() {
    //...
    JTextField posField = new JTextField("30",5);
    posField.addFocusListener(this);
    //...
}

Add attach the focus listener to it, but in the focusLost method, you are referring to the instance variable, which isn't the one that is actually on the screen

Start by changing the declaration within the constructor

public AlwaysPositive() {
    //...
    posField = new JTextField("30",5);
    posField.addFocusListener(this);
    //...
}

However, there are better solutions to use then FocusListener.

For example, you could use an InputVerifier that will allow you to verify the value of the field and make decisions about whether focus should be moved or not.

Take a look at How to Use the Focus Subsystem and Validating Input in particular

You could also use a DocumentFilter to restrict what the user can actually enter, filtering the input as the user types it. Take a look at Text Component Features and Implementing a Document Filter in particular.

You can also take a look at these examples for more ideas

Answer 2

When you create object of same name inside a method, the listener is set to the method object and not to the Class object.