Effective Java, Third Edition

To recap, there are four different kinds of nested classes, and each has its place. If a nested class needs to be visible outside of a single method or is too long to fit comfortably inside a method, use a member class. If each instance of a member class needs a reference to its enclosing instance, make it nonstatic; otherwise, make it static. Assuming the class belongs inside a method, if you need to create instances from only one location and there is a preexisting type that characterizes the class, make it an anonymous class; otherwise, make it a local class.

In summary, interfaces should be used only to define types. They should not be used merely to export constants.

Default methods are, however, extremely useful for providing standard method implementations when an interface is created, to ease the task of implementing the interface (Item 20).