One of the most important aspects of object-oriented design is data hiding, or encapsulation. By treating an object in some respects as a “black box” and ignoring the details of its implementation, we can write more resilient, simpler code with components that can be easily reused.
By default, the variables and methods of a class are accessible to members of the class itself and to other classes in the same package. To borrow from C++ terminology, classes in the same package are friendly. We’ll call this the default level of visibility. As you’ll see as we go on, the default visibility lies in the middle of the range of restrictiveness that can be specified.
The modifiers public
and private
, on the other
hand, define the extremes. As we mentioned earlier, methods and
variables declared as private
are
accessible only within their class. At the other end of the spectrum,
members declared as public
are
accessible from any class in any package, provided the class itself can
be seen. (The class that contains the methods must also be public
to be seen outside of its package, as
we discussed previously.) The public
members of a class should define its most general functionality—what the
black box is supposed to do.
Figure 6-7 illustrates the four
simplest levels of visibility, continuing the example from the previous
section. Public members in TextArea
are accessible from anywhere. Private members are not visible from
outside the class. The default visibility allows access by other classes
in the package.
The protected
modifier
allows special access permissions for subclasses. Contrary to how it
might sound, protected
is slightly
less restrictive than the default level of accessibility. In addition to
the default access afforded classes in the same package, protected
members are visible to subclasses of
the class, even if they are defined in a different package. If you are a
C++ programmer used to more restrictive meanings, this may rub you the
wrong way.[18]
Table 6-1 summarizes the levels of visibility available in Java; it runs generally from most to least restrictive. Methods and variables are always visible within a declaring class itself, so the table doesn’t address that scope.
Subclasses add two important (but unrelated) complications
to the topic of visibility. First, when you override methods in a
subclass, the overriding method must be at least as visible as the
overridden method. While it is possible to take a private
method and
override it with a public
method in a
subclass, the reverse is not possible; you can’t override a public
method with a private
method. This restriction makes sense
if you recall that subtypes have to be usable as instances of their
supertype (e.g., a Mammal
is a
subclass of Animal
and, therefore,
must be usable as an Animal
). If we
could override a method with a less visible method, we would have a
problem: our Mammal
might not be able
to do all the things an Animal
can.
However, we can reduce the visibility of a variable. In this case, the
variable acts like any other shadowed variable; the two variables are
distinct and can have separate visibilities in different classes.
The next complication is a bit harder to follow: the protected
variables of
a class are visible to its subclasses, but only through objects of the
subclass’s type or its subtypes. In other words, a subclass can see a
protected
variable of its superclass
as an inherited variable, but it can’t access that same variable via a
reference to the superclass itself. This statement could be confusing
because it might not be obvious that visibility modifiers don’t restrict
access between instances of the same class in the same way that they
restrict access between instances of different classes. Two instances of
the same class can access all of each other’s members,
including private ones, as long as they refer to
each other as the correct type. Said another way: two instances of
Cat
can access all of each other’s
variables and methods (including private ones), but a Cat
can’t access a protected member in an
instance of Animal
unless the
compiler can prove that the Animal
is
a Cat
. That is, Cat
s have the special privileges of being an
Animal
only with respect to other
Cat
s, not just any Animal
. If you find this hard to follow, don’t
worry too much. If you run into this as a problem in the real world, you
are probably trying to do something trickier than you should.
Interfaces behave like classes within packages. An
interface can be declared public
to make it
visible outside its package. Under the default visibility, an interface
is visible only inside its package. Like classes, only one public
interface can be declared in a
compilation unit (file).
[18] Early on, the Java language allowed for certain combinations
of modifiers, one of which was private
protected
. The meaning of private
protected
was to limit visibility strictly to subclasses
(and remove package access). This was later deemed confusing and
overly complex. It is no longer supported.
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