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

public class NumericField
{
  public NumericField ()
  {
    JFrame frame = new JFrame("Event Test");
    Container content = frame.getContentPane();
    content.setLayout(new FlowLayout());
    frame.setSize(300,300);

    JTextField tf = new JTextField(new NumericDocument(), "", 10);
    content.add(tf);

    frame.setVisible(true);
  }
  
  private static class NumericDocument extends PlainDocument
  {
    public void insertString(int offs, String str, AttributeSet a) 
      throws BadLocationException
    {
      
      if (str == null)
        return;

      StringBuffer digits = new StringBuffer();
      for (int i = 0; i < str.length(); i++)
        {
          if (Character.isDigit(str.charAt(i)))
            digits.append(str.charAt(i));
        }
      super.insertString(offs, digits.toString(), a);
    }
  }

  public static void main(String[] args)
  {
    new NumericField();
  }
}