Interfaces should 'typically' be adjectives. Its not mandatory/compulsory. Most important point while framing interfaces is: You don't want to provide implimentation details for various reasons. All you wanted to do is provide a contract or more specifically a set of criteria to be followed. Thats it. Interface only deals with criteria. If you think in these terms, you can easily conclude why the interfaces like Map etc. weren't named like adjectives. Map interface provides criteria for map data structure. Any thing that claims to be a map data structure should atleast fulfill the criteria described by Map interface.