PACKAGES

A PACKAGE in Java is a collection of classes, sub-packages(package inside a package), and interfaces. It helps to organize classes into a folder structure and make it easy to locate and use them. More importantly, it helps to improve code reusability.

In java we have several built-in packages, for example when we need user input, we import a package as follows:

import java.util.Scanner; 

Here:

• java is a top level package
• util is a sub package of java
• Scanner is a class which is present in the sub package util.

Advantages of using a package in Java

Reusability: While developing a project in java, we often feel that there are few things that we are writing again and again in our code. Using packages, you can create such things in form of classes inside a package and whenever you need to perform that same task, just import that package and use the class.

Better Organization: Again, in large java projects where we have several hundreds of classes, it is always required to group the similar types of classes in a meaningful package name so that you can organize your project better and when you need something you can quickly locate it and use it, which improves the efficiency.

Name Conflicts: We can define two classes with the same name in different packages so to avoid name collision, we can use packages

Types of packages in Java

1)User defined package: The package we create is called user-defined package.

2)Built-in package: The packages that are already defined,like java.io.*, java.lang.* etc are known as built-in packages.

Syntax to create a package:

package <nameOfPackage>; 

 

Command:package calculate;

 
How to Create a package?

• Choose the name of the package
• Include the package command as the first line of code in your Java Source File.
• The Source file contains the classes, interfaces, etc you want to include in the package
• Compile to create the Java packages.

Step 1) Type the following package program in notepad. 

package p1;

class Test

{

public void m1()

{

System.out.println("It is method m1 of the class Test");

}

public static void main(String args[])

{

Test t = new Test(); 

t.m1();

}

}

Step 2) In next step, save this file as Test.java

Step 3) In this step, we compile the file. 

I:\>javac Test.java

Note: Here Test.class will be created but no package is created.

Note:

We can create a package in two ways
Method 1 : Manually creating a directory for package.
Method 2 : Creating a directory for package using a command.

Method1

Manually creating a directory for package.

Step 4) create a folder named by package p1 

C:\Users\Student\Desktop\java prg\p1>dir

 Volume in drive C has no label.

 Volume Serial Number is B460-E456

Directory of C:\Users\Student\Desktop\java prg\p1>

24-05-2022  13:15    <DIR>          .

24-05-2022  13:15    <DIR>          ..

24-05-2022  13:16               517 Test.class

24-05-2022  12:28               202 Test.java

               2 File(s)            719 bytes

               2 Dir(s)  246,153,756,672 bytes free

 

Step 5) Compile the same file using the following code

C:\Users\Student\Desktop\java prg\p1>javac -d . Test.java

Here –d creates a directory and the "." operator represents the current working directory.

Step 6)Compile the same file using the following code

C:\Users\Student\Desktop\java prg\p1>javac -d .. Test.java

Here “..” indicates the parent directory. In our case file will be saved in parent directory which is C Drive

Step 7) To execute the code mention the fully qualified name of the class i.e. the package name followed by the sub-package name followed by the class name 

C:\Users\Student\Desktop\java prg\p1>java p1.Test

It is method m1 of the class Test

Example:

Procedure to create a package
//save the below program as Test.java 

package tstpkg;

public class Test

{

public static void main(String args[])

{

System.out.println("Testing a Package functionality");

}

}

Compile and create a package “tstpkg” in current directory as follows 

javac -d . Test.java

Run/execute Java package program
java tstpkg.Test

Output:
Testing a Package functionality

Example:

package com.rkblog;
public class Packrk
{
    public static void main(String[] args)
    {
    System.out.println(" It will dispaly package name");
    }
}     

Output:

ubuntu@ubuntu:~/Desktop/pk$ ls
Packrk.java
ubuntu@ubuntu:~/Desktop/pk$ javac -d . Packrk.java
ubuntu@ubuntu:~/Desktop/pk$ ls
com  Packrk.java
ubuntu@ubuntu:~/Desktop/pk$ cd com/
ubuntu@ubuntu:~/Desktop/pk/com$ ls
rkblog
ubuntu@ubuntu:~/Desktop/pk/com$ cd rkblog/
ubuntu@ubuntu:~/Desktop/pk/com/rkblog$ ls
Packrk.class
ubuntu@ubuntu:~/Desktop/pk/com/rkblog$ pwd
/home/ubuntu/Desktop/pk/com/rkblog
ubuntu@ubuntu:~/Desktop/pk/com/rkblog$ cd ..
ubuntu@ubuntu:~/Desktop/pk/com$ cd ..
ubuntu@ubuntu:~/Desktop/pk$ ls
com  Packrk.java
ubuntu@ubuntu:~/Desktop/pk$ javac -d . Packrk.java
ubuntu@ubuntu:~/Desktop/pk$ java com.rkblog.Packrk
 It will dispaly package name

Method 2 : Creating a directory for package using a command.

Step 4) To create a package using Method2, use the following command during compilation. javac –d . Test.java

Note: Here –d creates a directory and the "." operator represents the current working directory.

Step 5) When you execute the above command, it creates a package(directory) p1 in the current working directory, that is the location where we use above command. P1 directory contains the file named Test.class.

Example 1: 

Filename : Rk, Kk

 

Step 1: create a directory pk and then create a file Rk.java as shown 

ubuntu@ubuntu:~/Desktop/rkjava/$ mkdir pk

ubuntu@ubuntu:~/Desktop/rkjava/$ cd pk

ubuntu@ubuntu:~/Desktop/rkjava/pk$gedit Rk.java

ubuntu@ubuntu:~/Desktop/rkjava/pk$ ls

 lsRk.class  Rk.java
 

//Program :file name: Rk.java

package pk;
public class Rk
{
    public int add(int a,int b)
    {
        return a+b;
    }
    public int sub(int a,int b)
    {
        return a-b;
    }
    public int mul(int a,int b)
    {
        return a*b;
    }
    public int div(int a,int b)
    {
        return a/b;
    }
}

Step 2: create a file with Kk.java

import Pk.Rk;  // import <Package name>.<Class name>
class Kk
{
    public static void main(String []args)
    {
        Rk c=new Rk();
        int a=Integer.parseInt(args[0]);
        int b=Integer.parseInt(args[1]);
        int z=c.mul(a,b);
        int v=c.sub(a,b);
        System.out.println("Addition is:"+c.add(a,b));
        System.out.println("Division is:"+c.div(a,b));
        System.out.println("Multiplication is:"+z);
        System.out.println("Subtraction is:"+v);    
    }
}

Step 3: use the following command to run the package program 

ubuntu@ubuntu:~/Desktop$ mkdir pk
ubuntu@ubuntu:~/Desktop$ cd pk
ubuntu@ubuntu:~/Desktop/pk$ gedit Rk.java
ubuntu@ubuntu:~/Desktop/pk$ javac -d . Rk.java
ubuntu@ubuntu:~/Desktop/pk$ ls
pk  Rk.class  Rk.java
ubuntu@ubuntu:~/Desktop/pk$ cd ..
ubuntu@ubuntu:~/Desktop$ gedit Kk.java
ubuntu@ubuntu:~/Desktop$ javac -d . Kk.java
ubuntu@ubuntu:~/Desktop$ ls K*
Kk.class  Kk.java
ubuntu@ubuntu:~/Desktop$ java Kk 4 2
Addition is:6
Division is:2
Multiplication is:8
Subtraction is:2
                                            (OR)

Command to compile and to create a package Rk

ubuntu@ubuntu:~/Desktop/rkjava$ javac -d . Rk.java

Command to compile 

ubuntu@ubuntu:~/Desktop/rkjava$ javac  Kk.java

Command to execute 

ubuntu@ubuntu:~/Desktop/rkjava$ java  Kk 44 22

Output:

ubuntu@ubuntu:~/Desktop/rkjava$ javac -d . Rk.java
ubuntu@ubuntu:~/Desktop/rkjava$ javac  Kk.javaubuntu@ubuntu:~/Desktop$ java Kk 4 2
Addition is:6
Division is:2
Multiplication is:8
Subtraction is:2

Example 2: 

Filename : Mensuration.java

package Calc;

public class Mensuration

{

public double volume(double l,double b,double h)

{

return l*b*h;

}

public double tsa(double l,double b,double h)

{

return 2*(l*b+b*h+l*h);

}

public double lsa(double l,double b,double h)

{

return 2*h*(l+b);

}

public double diag(double l,double b,double h)

{

return Math.pow(l*l+b*b+h*h,0.5);

}

public double peri(double l,double b,double h)

{

return 4*(l+b+h);

}

}

 

File Name : UseCalc.java 

import Calc.Mensuration;

import java.util.Scanner;

class UseCalc

{

public static void main(String x[])

{

double l,b,h;

Scanner s=new Scanner(System.in); 

Mensuration m=new Mensuration();

System.out.println("Enter Length, Breadth & Height:");

l=s.nextDouble();

b=s.nextDouble(); 

h=s.nextDouble();

System.out.println("Volume Of Cuboid: "+m.volume(l,b,h)); 

System.out.println("Total Surface Area :"+m.tsa(l,b,h)); 

System.out.println("Lateral Surface Area: "+m.lsa(l,b,h));

System.out.println("Length of diagonal of the cuboid: "+Math.round(m.diag(l,b,h))); 

System.out.println("Perimeter of Cuboid: "+m.peri(l,b,h));

}

}

Command to compile and to create a package Calc 

I:\>javac -d . Mensuration.java

Command to compile 

I:\>javac UseCalc.java 

Command to execute 

I:\>java UseCalc 

Output:
Enter Length, Breadth & Height: 

12
13
14
Volume Of Cuboid: 2184.0 

Total Surface Area :1012.0 

Lateral Surface Area: 700.0
Length of diagonal of the cuboid: 23 

Perimeter of Cuboid: 156.0