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Exploring Java

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Objects in Java

5.7 Variable and Method Visibility

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 stronger, simpler code with components that can be easily reused.

Basic Access Modifiers

By default, the variables and methods of a class are accessible to members of the class itself and 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 always accessible, from any class in any package. Of course, the class that contains the methods must also be public, as we just discussed. The public members of a class should define its most general functionality--what the black box is supposed to do. Figure 5.8 illustrates the three simplest levels of visibility.

Figure 5.8 continues with 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 and so used to more restrictive meanings for both the default and protected levels of access, this may rub you the wrong way.

What was private protected?

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 somewhat inconsistent and overly complex and is no longer supported.[5]

[5] The meaning of the protected modifier changed in the Beta2 release of Java, and the private protected combination appeared at the same time. They patched some potential security holes, but confused many people.

Table 5.1 summarizes the levels of visibility available in Java; it runs generally from most restrictive to least. Methods and variables are always visible within a class, so the table doesn't address those:

Table 5.1: Visibility Modifiers
Modifier Visibility
private None
none (default) Classes in the package
protected Classes in package and subclasses inside or outside the package
public All classes

Subclasses and Visibility

There are two important (but unrelated) notes we need to add to the discussion of visibility with regards to class members in subclasses. First, when you override methods of a class in a subclass, it's not possible to reduce their visibility. While it is possible to take a private method of a class and override it to be public in a subclass, the reverse is not possible. This makes sense when you think about the fact that subtypes have to be usable as instances of their supertype (e.g., a Mammal is a type of Animal). If we could reduce the visibility of an overridden method, this would be a problem. However, we can reduce the visibility of a variable because it simply results in a shadowed variable. As with all shadowed variables, the two variables are distinct and can have separate visibilities in their different class forms.

The second point is that protected variables of a class are visible to its subclasses, but unlike C++, only in objects of the subclass's type or its subtypes. In other words, a subclass can see a protected variable from its superclass as an inherited variable, but it can't access the variable in a separate instance of the superclass itself. This can be confusing because often we forget that visibility modifiers don't resrtict between multiple instances of the same class in the same way that they do instances of different classes. Two instances of the same type of object can normally access all of each other's members, including private ones. 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 it can prove that the Animal is a Cat.

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