Java Reference
In-Depth Information
ce to an object of
Outer
must be supplied when the
Inner
constructor is
invoked via
super
. For example:
class Unrelated extends Outer.Inner {
Unrelated(Outer ref) {
ref.super();
}
}
When the construction of an
Unrelated
object reaches the point where
the superclass constructor is invoked, there must be an object of class
Outer
to which the superclass object can be bound. Since
Unrelated
is not
itself an inner class of
Outer
, there is no implicit enclosing object. Sim-
ilarly, because
Unrelated
is not a subclass of
Outer
, the current object of
Unrelated
is not a valid enclosing object. We must provide an explicit ref-
erence to an
Outer
object for the superclass object to bind to. We chose
to supply that reference using an argument to the
Unrelated
construct-
or, which uses it as an explicit binding reference in the invocation of the
superclass constructor.
Note that you cannot use the inner class creation syntax to externally
provide an
Outer
object, as in
Outer ref = new Outer();
Unrelated u = ref.new Unrelated(); // INVALID
because this syntax supplies an enclosing object for the
Unrelated
class
and
Unrelated
is
not
an inner class.
An inner class can extend another, unrelated, inner class provided an
appropriate enclosing instance is supplied to the superclass, as just de-
scribed. The resulting inner class then has two enclosing instancesone
for the extended class and one for the superclass. Such designs are con-
voluted, however, and are best avoided.
