@Override
    protected boolean validationCriteria(JComponent c) {        
        if ( (c instanceof JTextComponent) ) {
            String input = ((JTextComponent)c).getText();
            if ( input.equals("") )
                return false;
            
            for(int i = 0; i < input.length(); i++) {
                if ( !Character.isDigit(input.charAt(i)) ) {
                    return false;
                }    
            }
        }
        return true;
    }