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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.

 

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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Current Comments

4 comments so far (post your own) View All Comments Latest 10 Comments:

I tried running java mypackage.HelloWorld and I got the same errors.

Then I went on to create a subdirectory and I still have the same error.

But I'm sure there is a way to get packages to work because I succeeded with the Deitel JHTP5 manuel.

Posted by jean_paris on Friday, 01.30.09 @ 08:35am | #84327

Hi, i have created directory called package1 inside that i have written "shape.java" which i have written methao for area,i Have also created one another directory called package2 in which i have written three source files "circle.java","triangle.java","rectangle.java" importing the package1.And the main function outside the thodse two package directories importing both the packages.tell me how set the class path and run that complete programm.Thanks in advance

Posted by Vicky B on Wednesday, 05.21.08 @ 21:12pm | #60609

Hi..As i followed the above procedures, codings,(except i used different package name as "project1")it compiled successflly, but when i run this, it gives, Exception in thread"main" NoClassDefFoundError: project1/HelloWorld. I created classpath using: sysedit, c:\project1;c:\program files\java\jdk1.6.0_01\bin; Can anyone help me to solve this?
Thanks in Advance
Kalpana.S

Posted by Kalpana on Monday, 11.5.07 @ 11:53am | #35564

Hi..As i followed the above procedures, codings,(except i used different package name as "project1")it compiled successflly, but when i run this, it gives, Exception in thread"main" NoClassDefFoundError: project1/HelloWorld. I created classpath using: sysedit, c:\project1;c:\program files\java\jdk1.6.0_01\bin; Can anyone help me to solve this?
Thanks in Advance
Kalpana.S

Posted by Kalpana on Monday, 11.5.07 @ 11:52am | #35563
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