Java program for demonstrating packages

Aim

To write a program for demonstrating packages using Java.

 

Theory

A package is a group of related classes. A package is used to restrict access to a class and to create namespaces. If all reasonable class names are already used, then we can create a new package (new namespace) and reuse the class names again.

 

For example, a package mypackage1 can contain a class named MyClass and another package mypackage2 can also contain a class named MyClass.

 

Defining or Creating a Package

A package can be created by including the package statement as first line in a Java source file. Syntax for creating or defining a package is given below:

package mypackage;

 

Multiple source files can include the package statement with the same package name. Packages are maintained as regular directories (folders) in the machine. For example, we have to create a folder named mypackage and store the .class files in that folder.

 

We can also create a hierarchy of packages as shown below:

package pack1.pack2.pack3;

 

Remember that packages are normal folders in the file system. So, pack3 is a sub folder in pack2 and pack2 is a sub folder in pack1.

 

Java Packages and CLASSPATH

It is not mandatory that the driver program (main program) and the package(s) to be at the same location. So how does JVM know the path to the package(s)?

 

There are three options:

1. Placing the package in the current working directory.
2. Specifying the package(s) path using CLASSPATH environment variable.
3. Using the -classpath or -cp options with javac and java commands.

 

Importing or Using Packages

There are multiple ways in which we can import or use a package. They are as follows:

1. Importing all the classes in a package.
2. Importing only necessary classes in a package.
3. Specifying the fully qualified name of the class.

 

Example for Creating and Using Packages

Consider the following hierarchy of folders and files.

 

The code for A.java, B.java, and C.java files is as given below:

A.java

package mypackage;
public class A
{
	public void m1()
	{
		System.out.println("Class A method");
	}
}

 

B.java

package mypackage;
public class B
{
	public void m1()
	{
		System.out.println("Class B method");
	}
}

 

C.java

package mypackage;
public class C
{
	public void m1()
	{
		System.out.println("Class C method");
	}
}

 

Now, we will compile A.java, B.java, and C.java files and place the generated A.class, B.class, and C.class files in the mypackage folder as shown in the figure above.

 

The code for PackageDemo.java file is as given below:

PackageDemo.java

import mypackage.A;
import mypackage.B;
import mypackage.C;
class Driver
{
	public static void main(String args[])
	{
		A obj1 = new A();
		obj1.m1();
		B obj2 = new B();
		obj2.m1();
		C obj3 = new C();
		obj3.m1();
	}
}

 

Now, we can compile and run the above program with the following commands:

javac PackageDemo.java

java Driver

 

Let us change the location of mypackage directory now. So, consider the following hierarchy of folders and files.

 

Now we can use the following commands to compile and run the program:

set CLASSPATH=.;D:\

javac PackageDemo.java

java Driver

 

or we can also use the following commands to compile and run the program.

javac -cp .;D:\ PackageDemo.java

java -cp .;D:\ Driver

 

Pseudo code / Steps

1. Create a package named myshapes.
2. Create java, and Circle.java in the myshapes package.
3. Create java file, where we will create the main() method.
4. In the main() method create objects for the Rectangle, and Circle classes and call the area() and perimeter()

 

Program

Student has to write the program.

 

Input and Output:

Student has to write the input and output of the program.

 

Result

The concept of the packages has been understood and implemented successfully.