Java's Module Mechanism (Packages)


I. Packages 

    A. What they are: Packages provide a way of grouping together a
	   set of related classes as one unit

        1. Packages act like a module in that they provide access control
	       mechanisms (more on this in a moment)

        2. Packages help partition the name space so that names can
	       be re-used within different packages

    B. Defining a package

	   a. Use the package keyword to indicate that a class belongs
	      to a certain package

	   b. The package statement must be the first executable statement
	      in a file

	   c. Packages can be nested. If they are nested, then you separate
	       a package name from the one above it by a period.

	       Example: package silhouette;
	                package silhouette.shapes;
			package silhouette.shapes.boxobjects;

    C. Importing Packages

	1. Use the import keyword to access a public class from another
	   package.

	    a. Specifying the name of a class will import that class only
	    b. Specifying an asterisk (*) after a package name will import
	       all public classes from that package

	       example: import java.util.LinkedList -- makes the LinkedList
	                    class accessible
			import java.util.* -- makes all public classes in
			    java.util accessible
	    c. The import statement places the classes returned by the 
	       statement into the namespace of the current file.
	       A namespace is the set of names that are defined in the
	       current scope. A scope is a block of code within which
	       a set of names are valid. A scope can be a file, a class, 
	       a method or a block.
            d. You can also access a class in a different package without
	       importing it. To do so you provide the fully qualified package
	       name for the class. For example:

	       java.util.LinkedList myList = new java.util.LinkedList();

	       You might decide to use the fully qualified name if importing
	       the name would cause a conflict with either a name declared
	       by your class or with a name imported from another package.

        2. import statements should go immediately after a package statement
	   and should precede other executable statements in your file.

	3. Java's libraries are contained in packages. 

	    a. In C++ you use #include's to include system-defined libraries
	       in your program

	    b. In Java you use import statements to include system-defined
	       libraries in your program
	        
	       Example:    C++                    Java
	                   #include <stdio.h>     import java.io.*;

    C. How Java finds classes and CLASSPATH

	    a. The directory containing a package must have the
	       same name as the package

	    b. All classes belonging to the package must be placed in the
	       package's directory

	    c. How Java locates class files

	        i. By looking at directories beneath the current directory
		ii. By examining the set of directories specified by the
		    classpath flag to the java interpreter or compiler
                    javac -classpath .:..:/Users/bvz/silhouette chess.java
		    java -classpath .:..:/Users/bvz/silhouette games.chess
Using this classpath flag, Java will search the current directory, the directory above the current directory, and /Users/bvz/silhouette for the names of package directories. When I execute games.chess, then games must be a subdirectory in one of the current directory, the current directory's parent, or /Users/bvz/silhouette. iii. By examining the set of directories specified by the CLASSPATH environment variable Example: setenv CLASSPATH .:..:/Users/bvz/silhouette The advantage of the CLASSPATH environment variable is that I don't have to specify the -classpath flag every time I compile or execute a java file. d. When executing a java class, the class must be prefixed by its full package name Example: java silhouette.shapes.boxobjects.rectangle D. Member Access Private Package Protected Public Member Member Member Member Visible within same class Yes Yes Yes Yes Visible within same package No Yes Yes Yes by subclass Visible within same package No Yes Yes Yes by non-subclass Visible within different No No Yes Yes package by subclass Visible within different No No No Yes package by non-subclass