Understanding Packages and Interfaces in Java
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by Debjani Mallick
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Setting up the classpath

When we write a java program file having extension .java, it is kept under a particular directory. After compilation, the class file is created in the same directory. To execute the program we have to specify the class filename only. If a package is included then the name of the package must be a directory and after compiling the java file, the class files are stored under that package directory. To run a class of a particular package the following commands must be used.

Listing 3

java packagename.classname

However, this command must be given from the directory in which the package directory is present. For our Firstappplication example, if we are placing the Example package under C directory then we can set the classpath as:

Listing 4

set classpath = .;C:\;

The above statement sets the classpath to point at two places: .(dot) and C directory. Dot is used as an alias to represent the current directory. Semicolon is used to separate the directory location in case class files are placed at many locations.

As mentioned earlier, if we have placed our java file inside Example directory (which is placed inside C directory), for compiling we need to go to the Example directory and write the command as shown below.

Listing 5

C:\Example\javac Firstapplication.java

If we try to run the same by using java Firstapplication, then we get an error because the Firstapplication program is now a part of the package Example. For running the application we need to specify the fully qualified class name. Let us take another example; we have a package called Hello containing the file HelloWorld.java within Example package. Whenever we want to reference the HelloWorld class, we have to use Example.Hello.HelloWorld as its fully classified class name. For using the classes stored in a package, either we can declare the fully classified class name as shown:

Listing 6

//for first package 
Example.Firstapplication firstApplication = newExample.Firstapplication();
//for the subpackage 
Example.Hello.HelloWorld helloWorld = new Example.Hello.HelloWorld(); 

or we can use the import statement:

Listing 7

import.Example.*; //import all the public classes inside Example package
import.Example.Hello.*; //import all the public classes inside Hello package
import.Example.Firstapplication; //import only Firstapplication inside Example

By using the first import statement we can use the public classes present inside Example package but not the classes of Hello package.


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