Class access

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  1. There can be only one public class per compilation unit (file). The idea is that each compilation unit has a single public interface represented by that public class. It can have as many supporting “friendly” classes as you want. If you have more than one public class inside a compilation unit, the compiler will give you an error message.
  2. The name of the public class must exactly match the name of the file containing the compilation unit, including capitalization. So for Widget, the name of the file must be Widget.java, not widget.java or WIDGET.java. Again, you’ll get a compile-time error if they don’t agree.
  3. It is possible, though not typical, to have a compilation unit with no public class at all. In this case, you can name the file whatever you like.
Note that a class cannot be private (that would make it accessible to no one but the class), or protected.[26] So you have only two choices for class access: “friendly” or public. If you don’t want anyone else to have access to that class, you can make all the constructors private, thereby preventing anyone but you, inside a static member of the class, from creating an object of that class. [27] Here’s an example:

//: Lunch.java
// Demonstrates class access specifiers.
// Make a class effectively private
// with private constructors:
 
class Soup {
  private Soup() {}
  // (1) Allow creation via static method:
  public static Soup makeSoup() {
    return new Soup();
  }
  // (2) Create a static object and
  // return a reference upon request.
  // (The "Singleton" pattern):
  private static Soup ps1 = new Soup();
  public static Soup access() {
    return ps1;
  }
  public void f() {}
}
 
class Sandwich { // Uses Lunch
  void f() { new Lunch(); }
}
 
// Only one public class allowed per file:
public class Lunch {
  void test() {
    // Can't do this! Private constructor:
    //! Soup priv1 = new Soup();
    Soup priv2 = Soup.makeSoup();
    Sandwich f1 = new Sandwich();
    Soup.access().f();
  }
} ///:~ 

Up to now, most of the methods have been returning either void or a primitive type so the definition:

  public static Soup access() {
    return ps1;
  }

might look a little confusing at first. The word before the method name ( access) tells what the method returns. So far this has most often been void, which means it returns nothing. But you can also return a handle to an object, which is what happens here. This method returns a handle to an object of class Soup.

The class Soup shows how to prevent direct creation of a class by making all the constructors private. Remember that if you don’t explicitly create at least one constructor, the default constructor (a constructor with no arguments) will be created for you. By writing the default constructor, it won’t be created automatically. By making it private, no one can create an object of that class. But now how does anyone use this class? The above example shows two options. First, a static method is created that creates a new Soup and returns a handle to it. This could be useful if you want to do some extra operations on the Soup before returning it, or if you want to keep count of how many Soup objects to create (perhaps to restrict their population).


[26] Actually, a Java 1.1 inner class can be private or protected, but that’s a special case. These will be introduced in Chapter 7.

[27] You can also do it by inheriting (Chapter 6) from that class.



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