Input Validator : FocusLost event handling

Question

I am new to working with FocusListener and I am trying to use it validate the input of a textfield to be an integer.
This seems like a seemingly simple thing to do, but I am getting errors.
Here is a portion of the code where I make the textfield and assign it to a focus listener.
InputValidator i = new InputValidator();
resolutionlabel = new JLabel("Resolution: ");
p.add(resolutionlabel);         
resolutiontextbox = new JTextField("3200");        
resolutionlabel.setLabelFor(resolutiontextbox);
resolutiontextbox.addFocusListener(i);
p.add(resolutiontextbox);Here is the important parts of the focus listener code:
public class InputValidator extends FocusAdapter
    public InputValidator()
    public void focusLost(FocusEvent event)
     validate((TextField)(event.getSource()));
    private void validate(TextField field)
}The error I am getting is on the line where I call the validate function and is listed as :
java.lang.ClassCastException: javax.swing.JTextField

Answer 1

this might be an alternative way - it won't accept non-digit characters
import javax.swing.*;
import java.awt.*;
import javax.swing.text.*;
class Testing extends JFrame
  public Testing()
    setLocation(400,300);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    JPanel jp = new JPanel();
    JTextField tf = new JTextField(new NumberLimiter(),"3200",10);
    jp.add(tf);
    getContentPane().add(jp);
    pack();
  class NumberLimiter extends PlainDocument
    public void insertString(int offs, String str, AttributeSet a) throws BadLocationException
      char[] characters = str.toCharArray();
      for(int x = 0, y = characters.length; x < y; x++)
        if(Character.isDigit(characters[x]) == false)
          java.awt.Toolkit.getDefaultToolkit().beep();
          return;
      super.insertString(offs, str, a);
  public static void main(String[] args){new Testing().setVisible(true);}
}