Learning Java

Visibility of Variables and Methods

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 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 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.

Figure 6-7. Private, default, protected, and public visibility

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 are used to more restrictive meanings, this may rub you the wrong way. ^[21]

Table 6.1 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 6-1. 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

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 realize 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 in a separate instance of the superclass itself. This can be confusing, because we often forget 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 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 the compiler can prove that the Animal is a Cat. If you found this hard to follow, don’t worry too much. You shouldn’t run into these issues very often.

Interfaces and Visibility

Interfaces behave like classes within packages. An interface can be declared public to make it visible outside of its package. Under the default visibility, an interface is visible only inside of its package. There can be only one public interface declared in a compilation unit.

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^[21] Early on, the Java language allowed for certain combinations of modifiers, one of which was privateprotected. 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.