Java: Packages - Defining

Multiple classes of larger programs are usually grouped together into a package. Packages correspond to directories in the file system, and may be nested just as directories are nested.

Reasons for using packages

To allow classes to be combined in a single ".jar" file.
To control the namespace.
To limit the scope of names.

Package declarations

Each file must have a package declaration which precedes all non-comment code. The package name must be the same as the enclosing directory. For example, here are two files in the packagetest directory.

package packagetest;
class ClassA {
    public static void main(String[] args) {
        ClassB.greet();
    }
}

and

package packagetest;
class ClassB {
    static void greet() {
        System.out.println("Hi");
    }
}

Note that these source files must be named ClassA.java and ClassB.java (case matters) and they must be in a directory named packagetest.

Compiling and running packages from a command line

To compile the above example, you must be outside the packagetest directory. To compile the classes:

   javac packagetest/ClassB.java
   javac packagetest/ClassA.java

To run the main program in ClassA.

   java packagetest.ClassA

   java packagetest/ClassA

In windows the "/" can be replaced by the "\" in the javac command, but not in the java command. Generally use a forward slash ("/") because it is used more commonly than the backslash in other places as well.