The Declaring Classes page obliquely refers to Inheritance and Interfaces, but just as the previous page on this topic said very little on why you would use an Interface, and when you would use one instead of or as well as a superclass, this page also assumes you would know when and why you would use either or both constructs.
To be fair there is a promise to talk about them later: "The lesson on interfaces and inheritance will explain how and why you would use the extends and implements keywords", but there are two things wrong with the promise. First there is no hyperlink, so the reader is left to scurry around looking for lessons "on interfaces and inheritance". Second, we have already lessons on interfaces and inheritance, which say very little about how and why when you would use one rather than the other.
My general observation of these tutorials is that the language mechanics are better described than the more abstract ideas. This applies to the Defining Methods page, which sets out the rules quite clearly.