Java Reference
In-Depth Information
public void merge(PriorityQueue q) {
Cell first = q.head;
// ...
}
We are not accessing the
protected
member of the current object, but
the
protected
member of an object passed as an argument. This is al-
lowed because the class attempting the access is
PriorityQueue
and the
type of the reference
q
is also
PriorityQueue
. If
q
were a subclass of
Pri-
orityQueue
this access would also be valid.
Later the group determines that there is a new requirement: It wants to
be able to merge a
SingleLinkQueue
with a
PriorityQueue
. So it defines an
overloaded version of
merge
that starts like this:
public void merge(SingleLinkQueue q) {
Cell first = q.head;
// ...
}
But this code won't compile.
The problem is that the class attempting to access the
protected
member
is
PriorityQueue
while the type of the reference to the object being
accessed is
SingleLinkQueue
.
SingleLinkQueue
is not the same as, nor a
subclass of,
PriorityQueue
, so the access is not allowed. Although each
PriorityQueue
is a
SingleLinkQueue
, not every
SingleLinkQueue
is a
Prior-
ityQueue
.
The reasoning behind the restriction is this: Each subclass inherits the
contract of the superclass and expands that contract in some way. Sup-
pose that one subclass, as part of its expanded contract, places con-
straints on the values of protected members of the superclass. If a dif-
ferent subclass could access the protected members of objects of the
