public String getValidationErrorMessage() {
    StringJoiner joiner = new StringJoiner(", ");
    for (ValidationFailure vf : getValidationFailures()) {
      joiner.add(vf.getMessage());
    }

    return joiner.toString();
  }
 private void then_ValidationShouldFailWithErrorMessage(String expectedIdValidationMessage) {
   try {
     _product.validate();
     then_AnExceptionShoulBeRaised();
   } catch (ValidationFailure vf) {
     // Then
     assertThat(vf.getMessage(), equalTo(expectedIdValidationMessage));
   }
 }