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Create Your Own Package


The package to which the source file belongs is specified with the keyword package at the top left of the source file, before the code that defines the real classes in the package. 
Suppose we have a source file called "HelloWorld.java" and we want to put this file in a package "mypackage" then the following code is written as shown in the given example:

package mypackage;
class HelloWorld {
  public static void main (String args[]) {
    System.out.println("Hello World!");

Before running this program make sure to do the following things:

  1. Create a directory "mypackage" .
  2. Save the  source file as "HelloWorld.java" in the created directory.
  3. Set the class path as  set CLASSPATH = .;C:\;        
  4. Go to the "mypackage" directory and compile the program as
    C:\mypackage>javac HelloWorld.java
  5. Run the program.

If you try to run this program, you will get the following exceptions (or error):

C:\mypackage>java HelloWorld
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong name: mypackage/HelloWorld)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$000(Unknown Source)

This is, because the class "HelloWorld" belongs to the package "mypackage". So If we want to run it, we have to tell the JVM about its fully-qualified class name as (mypackage.HelloWorld) instead of its plain class name (HelloWorld). Fully-qualified class name is the name of the java class that includes its package name. 

Now run the program as shown:

C:\mypackage>java mypackage.HelloWorld
Hello World!

The ways to Compile the Package:

Compile in the same directory:  If you have a hierarchy of packages to compilation then you can compile the package without going to the subdirectories and specifying the complete directory path with the class . Suppose, you have a hierarchy of packages as "india.mycompany.mainpackage.mypackage" including the class "HelloWorld" then type the following command shown as:

C:\javac C:\india\mycompany\mainpackage\mypackage\HelloWord.java


This command will reach to the last subdirectory and compile the class "HelloWorld".

Compile into the Different Directory: On the other hand, if you want to compile the same package available in the hierarchy manner to another directory (location) then syntax is shown as:

C:\javac -d <target_directory> <complete_directorypath>

Suppose, you want to save the compiled package to the location "D:\myfolder" then type the following command shown as:

C:\javac -d D:\myfolder C:\ india\mycompany\mainpackage\mypackage\HelloWord.java


This command puts the folder "india" along with its subfolders and the class file "HelloWorld.class" to the new location as D:\myfolder.



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