i need some explanation..

[Ernest Lee]

Classes can be nested to any depth. Top-level static classes can be nested only within other static top-level classes or interfaces. (what this means??)

If the inner class is sub-classed outside of the outer class (only possible with top-level nested classes) (why??)

[Michael Ernest]

Translation: static inner classes cannot be nested to any depth, and are an exception to the rule.
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Michael Ernest, co-author of: The Complete Java 2 Certification Study Guide