public  void insertString(int offs,
    String str,
    AttributeSet atts) throws BadLocationException 
{
        int length = getLength();
        String newtext = null;
        // is their already a number in the document?
        if( length  > 0 )
        {
            // if so, insert the string
            String text1 = getText(0, offs );
            String text2 = getText(offs, length - offs );
            newtext = text1 + str + text2;
        }
        else
        {
            // document is empty, so take just the new string
            newtext = str;
        }
        try {
            // test wether its pasrable
            Double.parseDouble( newtext );
        }
        catch( NumberFormatException e )
        {
            // it will not be a number if we insert the string
            return;
        }
        super.insertString(offs, str, atts);
}