A nested class is a class defined within another class. A nested class should exist
only to serve its enclosing class. If a nested class would be useful in some other
context, then it should be a top-level class. There are four kinds of nested classes:
static member classes, nonstatic member classes, anonymous classes, and local
classes. All but the first kind are known as inner classes. This item tells you when
to use which kind of nested class and why.
A static member class is the simplest kind of nested class. It is best thought of
as an ordinary class that happens to be declared inside another class and has
access to all of the enclosing class’s members, even those declared private. A
static member class is a static member of its enclosing class and obeys the same
accessibility rules as other static members. If it is declared private, it is accessible
only within the enclosing class, and so forth.
If you declare a member class that does not require access to an enclosing
instance, always put the static modifier in its declaration, making it a static
rather than a nonstatic member class. If you omit this modifier, each instance will
have an extraneous reference to its enclosing instance. Storing this reference costs
time and space, and can result in the enclosing instance being retained when it
would otherwise be eligible for garbage collection. And should you ever
need to allocate an instance without an enclosing instance, you’ll be unable to do
so, as nonstatic member class instances are required to have an enclosing instance.
