Non-static inner classes contain a reference to an instance of the outer class. Hence, serializing a non-static inner class will result in an
attempt at serializing the outer class as well. If the outer class is not serializable, serialization will fail, resulting in a runtime error.
Making the inner class static (i.e., "nested") avoids this problem, as no reference to an instance of the outer class is required.
Serializing the inner class can be done independently of the outer class. Hence, inner classes implementing Serializable should be
static if the outer class does not implement Serializable.
Be aware of the semantic differences between an inner class and a nested one:
- an inner class can only be instantiated within the context of an instance of the outer class.
- a nested (
static) class can be instantiated independently of the outer class.
