@Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        if (!valueIsDefined) {
            sb.append("<unbound>"); //$NON-NLS-1$
        } else {
            if (value == null) {
                sb.append(BeansUtils.NULL);
            } else {
                Class<?> clazz = value.getClass();
                sb.append(clazz == String.class ? BeansUtils.QUOTE : BeansUtils
                        .idOfClass(clazz));
            }
        }
        sb.append('=');
        sb.append(super.toString());
        return sb.toString();
    }