Defining Classes - Beginning Java - page 258

// More members of Outside class...
}
Here the class Inside is nested inside the class Outside . The Inside class is declared as a public mem-
ber of Outside , so it is accessible from outside Outside . Obviously, a nested class should have some spe-
cific association with the enclosing class. Arbitrarily nesting one class inside another would not be sensible.
The enclosing class here is referred to as a top-level class . A top-level class is a class that contains a nested
class but is not itself a nested class.
The nested class here has meaning only in the context of an object of type Outside . This is because the
Inside class is not declared as a static member of the class Outside . Until an object of type Outside has
been created, you can't create any Inside objects. However, when you declare an object of a class contain-
ing a nested class, no objects of the nested class are necessarily created — unless of course the enclosing
class's constructor creates them. For example, suppose you create an object with the following statement:
Outside outer = new Outside();
No objects of the nested class, Inside , are created. If you now want to create an object of the type of the
nested class, you must refer to the nested class type using the name of the enclosing class as a qualifier. For
instance, having declared an object of type Outside , you can create an object of type Inside as follows:
Outside.Inside inner = outer.new Inside(); // Define a nested class object
Here you have created an object of the nested class type that is associated with the object outer that you
created earlier. You are creating an object of type Inside in the context of the object outer . Within non-
static methods that are members of Outside , you can use the class name Inside without any qualification,
as it is automatically qualified by the compiler with the this variable. So you could create a new Inside
object from within a method of the object Outside :
Inside inner = new Inside(); // Define a nested class object
This statement is equivalent to:
this.Inside inner = this.new Inside(); // Define a nested class object
All this implies that a static method cannot create objects of a non-static nested class type. Because the
Inside class is not a static member of the Outside class, such a member could refer to an object that does
not exist — which would be an error if there are no Inside objects extant in the context of an Outside
object. Because Inside is not a static member of the Outside class, if a static method in the Outside class
tried to create an object of type Inside directly, without first invoking an object of type Outside , it would
be trying to create an object outside of that object's legitimate scope — an illegal maneuver.
Further, because the Inside class is not a static member of the Outside class, it cannot in turn contain
any static data members itself. Because Inside is not static, it cannot act as a freestanding class with static
members — this would be a logical contradiction.
You typically use nested classes to define objects that at least have a strong association with objects of
the enclosing class type, and often there is a tight coupling between the two. A further use for nested classes
is for grouping a set of related classes under the umbrella of an enclosing class. You use this approach in
examples later on in the topic.