Your public facing methods/accessors are a sort of contract between a class, and whatever uses it. The contract basically says: "I will behave as you expect me

Your public facing methods/accessors are a sort of contract between a class, and whatever uses it. The contract basically says: "I will behave as you expect me to and as we agreed but only as long as you only interact with me in the predetermined(designed) ways that I allow you to"