When you want to gather some fields in an object in order to emphasize a concept, you could either create an external class, or an internal (called either nested (static ones) or inner).
If you want to emphasize the fact that this cooperative class makes strictly no sense (has no use) outside the original object use, you could make it nested/inner.
Thus, when dealing with some hierarchy, you can describe a “nested” interface
, which will be implemented by the wrapping class’s subclasses.
In the JDK, the most significant example would be Map.Entry
inner interface, defined within Map
interface and implemented by various ways by HashMap
, LinkedHashMap
etc…
And of course, Map.Entry
needed to be declared as public
in order to be accessible while iterating the map wherever the code is.