Packages (Cont.)

Importing a Package
There are many packages to choose from. In the previous example, we used the ArrayList class from the java.util package. This package also contains date and time facilities, random-number generator and other utility classes. To import a whole package, end the sentence with an asterisk sign (*).

The example will import ALL the classes in the java.util package: 
import java.util.*;

User-Defined Packages
To create your own package, you need to understand that Java uses a file system directory to store them.

Just like folders on your computer: To create a package, use the package keyword: 
└── root
  └── mypack
    └── MyPackageClass.java

MyPackageClass.java (a user-defined package example) 
package mypack;
class MyPackageClass {
  public static void main( String[ ] args ) {
    System.out.println( "This is my package!" );
  }
}
undcemcs02.und.edu (a use scenario) 
undcemcs02> # Create the file. 
undcemcs02> emacs MyPackageClass.java

undcemcs02> # List the file contents.
undcemcs02> cat MyPackageClass.java
package mypack;
class MyPackageClass {
  public static void main( String[ ] args ) {
    System.out.println( "This is my package!" );
  }
}
undcemcs02> # Compile the file.
undcemcs02> javac MyPackageClass.java

undcemcs02> # Compile the packages.
undcemcs02> javac -d . MyPackageClass.java

undcemcs02> # This forces the compiler to create the "mypack" package.

undcemcs02> # The -d keyword specifies the destination for where to
undcemcs02> # save the class file.
undcemcs02> # You can use any directory name, like c:/user (windows), or
undcemcs02> # if you want to keep the package within the same directory,
undcemcs02> # you can use the dot sign ".", like in the example above.
undcemcs02> # Note: The package name should be written in lower case to
undcemcs02> # avoid conflict with class names.

undcemcs02> ls 
mypack/     MyPackageClass.class      MyPackageClass.java

undcemcs02> ls mypack/
MyPackageClass.class

undcemcs02> # When we compiled the package in the example above, a new
undcemcs02> # folder was created, called "mypack".
undcemcs02> # To run the MyPackageClass.java file, write the following:
undcemcs02> java mypack/MyPackageClass
This is my package!
shell> java mypack/MyPackageClass

      Output: