Java Package Tutorial

(Thai version here)
  by Patrick Bouklee
 
 

Introduction

Many times when we get a chance to work on a small project, one thing we intend to do is to put all java files into one single directory. It is quick, easy and harmless. However if our small project gets bigger, and the number of files is increasing, putting all these files into the same directory would be a nightmare for us. In java we can avoid this sort of problem by using Packages.

Packages are nothing more than the way we organize files into different directories according to their functionality, usability as well as category they should belong to. An obvious example of packaging is the JDK package from SUN (java.xxx.yyy) as shown below:

Figure 1. Basic structure of JDK package

Basically, files in one directory (or package) would have different functionality from those of another directory. For example, files in java.io package do something related to I/O, but files in java.net package give us the way to deal with the Network. In GUI applications, it's quite common for us to see a directory with a name "ui" (user interface), meaning that this directory keeps files related to the presentation part of the application. On the other hand, we would see a directory called "engine", which stores all files related to the core functionality of the application instead.

Packaging also help us to avoid class name collision when we use the same class name as that of others. For example, if we have a class name called "Vector", its name would crash with the Vector class from JDK. However, this never happens because JDK use java.util as a package name for the Vector class (java.util.Vector). So our Vector class can be named as "Vector" or we can put it into another package like com.mycompany.Vector without fighting with anyone. The benefits of using package reflect the ease of maintenance, organization, and increase collaboration among developers. Understanding the concept of package will also help us manage and use files stored in jar files in more efficient ways.

How to create a package

Suppose we have a file called HelloWorld.java, and we want to put this file in a package world. First thing we have to do is to specify the keyword package with the name of the package we want to use (world in our case) on top of our source file, before the code that defines the real classes in the package, as shown in our HelloWorld class below: 

// only comment can be here
package world;

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

One thing you must do after creating a package for the class is to create nested subdirectories to represent package hierachy of the class. In our case, we have the world package, which requires only one directory. So, we create a directory world and put our HelloWorld.java into it.

Figure 2. HelloWorld in world package (C:\world\HelloWorld.java)

That's it!!! Right now we have HelloWorld class inside world package. Next, we have to introduce the world package into our CLASSPATH.

Setting up the CLASSPATH

From figure 2 we put the package world under C:. So we just set our CLASSPATH as: 

set CLASSPATH=.;C:\;
We set the CLASSPATH to point to 2 places, . (dot) and C:\ directory.
Note: If you used to play around with DOS or UNIX, you may be familiar with . (dot) and .. (dot dot). We use . as an alias for the current directory and .. for the parent directory. In our CLASSPATH we include this . for convenient reason. Java will find our class file not only from C: directory but from the current directory as well. Also, we use ; (semicolon) to separate the directory location in case we keep class files in many places.

When compiling HelloWorld class, we just go to the world directory and type the command: 

C:\world\javac HelloWorld.java
If you try to run this HelloWorld using java HelloWorld, you will get the following error: 
C:\world>java HelloWorld
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong name: 
world/HelloWorld)
        at java.lang.ClassLoader.defineClass0(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:442)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:101)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:248)
        at java.net.URLClassLoader.access$1(URLClassLoader.java:216)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:191)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:290)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:286)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
The reason is right now the HelloWorld class belongs to the package world. If we want to run it, we have to tell JVM about its fully-qualified class name (world.HelloWorld) instead of its plain class name (HelloWorld). 
C:\world>java world.HelloWorld
C:\world>Hello World
Note: fully-qualified class name is the name of the java class that includes its package name

To make this example more understandable, let's put the HelloWorld class along with its package (world) be under C:\myclasses directory instead. The new location of our HelloWorld should be as shown in Figure 3:

Figure 3. HelloWorld class (in world package) under myclasses directory

We just changed the location of the package from C:\world\HelloWorld.java to C:\myclasses\world\HelloWorld.java. Our CLASSPATH then needs to be changed to point to the new location of the package world accordingly. 

set CLASSPATH=.;C:\myclasses;
Thus, Java will look for java classes from the current directory and C:\myclasses directory instead.

Someone may ask "Do we have to run the HelloWorld at the directory that we store its class file everytime?". The answer is NO. We can run the HelloWorld from anywhere as long as we still include the package world in the CLASSPATH. For example, 

C:\>set CLASSPATH=.;C:\;

C:\>set CLASSPATH // see what we have in CLSSPATH
CLASSPATH=.;C:\;

C:\>cd world

C:\world>java world.HelloWorld
Hello World

C:\world>cd ..

C:\>java world.HelloWorld
Hello World

Subpackage (package inside another package)

Assume we have another file called HelloMoon.java. We want to store it in a subpackage "moon", which stays inside package world. The HelloMoon class should look something like this: 
package world.moon;

public class HelloMoon {
  private String holeName = "rabbit hole";
  
  public getHoleName() {
    return hole;
  }
  
  public setHole(String holeName) {
    this.holeName = holeName;
  }
}

If we store the package world under C: as before, the HelloMoon.java would be c:\world\moon\HelloMoon.java as shown in Figure 4 below:

Figure 4. HelloMoon in world.moon package

Although we add a subpackage under package world, we still don't have to change anything in our CLASSPATH. However, when we want to reference to the HelloMoon class, we have to use world.moon.HelloMoon as its fully-qualified class name.

How to use package

There are 2 ways in order to use the public classes stored in package.
1. Declare the fully-qualified class name. For example, 
...
world.HelloWorld helloWorld = new world.HelloWorld();
world.moon.HelloMoon helloMoon = new world.moon.HelloMoon();
String holeName = helloMoon.getHoleName();
...
2) Use an "import" keyword: 
import world.*;  // we can call any public classes inside the world package 
import world.moon.*;  // we can call any public classes inside the world.moon package
import java.util.*;  // import all public classes from java.util package
import java.util.Hashtable;  // import only Hashtable class (not all classes in java.util package)
Thus, the code that we use to call the HelloWorld and HelloMoon class should be 
...
  HelloWorld helloWorld = new HelloWorld(); // don't have to explicitly specify world.HelloWorld anymore
  HelloMoon helloMoon = new HelloMoon(); // don't have to explicitly specify world.moon.HelloMoon anymore
...
Note that we can call public classes stored in the package level we do the import only. We can't use any classes that belong to the subpackage of the package we import. For example, if we import package world, we can use only the HelloWorld class, but not the HelloMoon class.

Using classes stored in jar file

Jar files are the place where we put a lot of files to be together. We compress these files and make them as a single bundle. Jar files may also include directories, subdirectories to represent class and package hierachy. Normally, we can see what is inside a jar file by using the command jar -tvf fileName.jar as shown in Figure 5:

Figure 5. JSDK package viewed by jar -tvf jsdk.jar command

From figure 5 we see that there is a class called javax.servlet.http.Cookie. We can call this class by 

import javax.servlet.http.Cookie; // import only Cookie class or  
import javax.servlet.http.*; // import the whole javax.servlet.http package

But we have to include this package in the CLASSPATH as well. 

set CLASSPATH=.;D:\JSDK2.0\lib\jsdk.jar;
Note that if the package is stored inside a jar file, we have to include the jar file with its extension (.jar) in the CLASSPATH. However, if the package is a plain directory, we just put the name of directory into the CLASSPATH.
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