It's not always easy for Jacks and cats to get along. Most Jacks and cats are fine but not all Jacks can be trusted around cats because they are hunters by nature. If the Jack is new to the family then I would introduce the Jack and cat very slowly and only letting them see each other for a little bit each day, supervised. If the Jack isn't new to the family then make sure the Jack and the cat are never left alone especially if you detect any discomfort between them.