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Thinking in Java
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Package access

What if you give no access specifier at all, as in all the examples before this chapter? The default access has no keyword, but it is commonly referred to as package access (and sometimes “friendly”). It means that all the other classes in the current package have access to that member, but to all the classes outside of this package, the member appears to be private. Since a compilation unit—a file—can belong only to a single package, all the classes within a single compilation unit are automatically available each other via package access.

Package access allows you to group related classes together in a package so that they can easily interact with each other. When you put classes together in a package, thus granting mutual access to their package-access members, you “own” the code in that package. It makes sense that only code you own should have package access to other code you own. You could say that package access gives a meaning or a reason for grouping classes together in a package. In many languages the way you organize your definitions in files can be arbitrary, but in Java you’re compelled to organize them in a sensible fashion. In addition, you’ll probably want to exclude classes that shouldn’t have access to the classes being defined in the current package.

The class controls which code has access to its members. There’s no magic way to “break in.” Code from another package can’t show up and say, “Hi, I’m a friend of Bob’s!” and expect to see the protected, package-access, and private members of Bob. The only way to grant access to a member is to:

  1. Make the member public. Then everybody, everywhere, can access it.
  2. Give the member package access by leaving off any access specifier, and put the other classes in the same package. Then the other classes in that package can access the member.
  3. As you’ll see in Chapter 6, when inheritance is introduced, an inherited class can access a protected member as well as a public member (but not private members). It can access package-access members only if the two classes are in the same package. But don’t worry about that now.
  4. Provide “accessor/mutator” methods (also known as “get/set” methods) that read and change the value. This is the most civilized approach in terms of OOP, and it is fundamental to JavaBeans, as you’ll see in Chapter 14. public: interface access

    When you use the public keyword, it means that the member declaration that immediately follows public is available to everyone, in particular to the client programmer who uses the library. Suppose you define a package dessert containing the following compilation unit:

    //: c05:dessert:Cookie.java
    // Creates a library.
    package c05.dessert;
    
    public class Cookie {
      public Cookie() {
       System.out.println("Cookie constructor");
      }
      void bite() { System.out.println("bite"); }
    } ///:~


    Remember, the class file produced by Cookie.java must reside in a subdirectory called dessert, in a directory under c05 (indicating Chapter 5 of this book) that must be under one of the CLASSPATH directories. Don’t make the mistake of thinking that Java will always look at the current directory as one of the starting points for searching. If you don’t have a ‘.’ as one of the paths in your CLASSPATH, Java won’t look there.

    Now if you create a program that uses Cookie:

    //: c05:Dinner.java
    // Uses the library.
    import com.bruceeckel.simpletest.*;
    import c05.dessert.*;
    
    public class Dinner {
      static Test monitor = new Test();
      public Dinner() {
       System.out.println("Dinner constructor");
      }
      public static void main(String[] args) {
        Cookie x = new Cookie();
        //! x.bite(); // Can't access
        monitor.expect(new String[] {
          "Cookie constructor"
        });
      }
    } ///:~


    you can create a Cookie object, since its constructor is public and the class is public. (We’ll look more at the concept of a public class later.) However, the bite( ) member is inaccessible inside Dinner.java since bite( ) provides access only within package dessert, so the compiler prevents you from using it.

    The default package

    You might be surprised to discover that the following code compiles, even though it would appear that it breaks the rules:

    //: c05:Cake.java
    // Accesses a class in a separate compilation unit.
    import com.bruceeckel.simpletest.*;
    
    class Cake {
      static Test monitor = new Test();
      public static void main(String[] args) {
        Pie x = new Pie();
        x.f();
        monitor.expect(new String[] {
          "Pie.f()"
        });
      }
    } ///:~


    In a second file in the same directory:

    //: c05:Pie.java
    // The other class.
    
    class Pie {
      void f() { System.out.println("Pie.f()"); }
    } ///:~


    You might initially view these as completely foreign files, and yet Cake is able to create a Pie object and call its f( ) method! (Note that you must have ‘.’ in your CLASSPATH in order for the files to compile.) You’d typically think that Pie and f( ) have package access and therefore not available to Cake. They do have package access—that part is correct. The reason that they are available in Cake.java is because they are in the same directory and have no explicit package name. Java treats files like this as implicitly part of the “default package” for that directory, and thus they provide package access to all the other files in that directory.
    Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire