Question

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Are static inner classes a good idea or poor design?

Java supports static inner classes, what the designer of language must have thought when they added it, what real world issues it solves?

Answer 1

Static inner classes are useful when you want the inner class to be “logically scoped” within the outer class (i.e. because it is closely related to the outer class and does not make sense on its own), but don’t need the “code scope” features of regular inner classes i.e. there are no private fields or methods of the outer class that the inner class needs to access, and instances of the inner class don’t logically belong to a specific outer class instance.

For example, AbstractMap has a static inner class SimpleEntry that represents a map entry. Since a map entry does not need access to the map that contains it, it can be static.