Packages in Java

 There are folders or directories in our computers for the classification and accessibility of various files, and in Java, we have packages for the same. In Java, Packages are similar to folders, which are mainly used to organize classes and interfaces.

Packages help us to write better and manageable code by preventing naming conflicts. Java provides some built-in packages which we can use but we can also create our own (user-defined) packages.

In this article, we are going to discuss everything about packages in Java along with their syntaxes and examples. Moving forth in this article we are going to learn –

1.     Packages in Java

2.    Advantages of using Packages in Java

3.    Types of Packages in Java

4.    Creating a Package in Java

5.    Example of Java Packages

6.    Package naming Conventions

7.    Compiling a Java Package

8.    Executing Java Package Program

9.    Accessing a Java Package

10. Sub packages in Java

11.  Important Points in Java Package

12. Dividing Classes into Packages

Packages in Java

A package is a collection of similar types of Java entities such as classes, interfaces, subclasses, exceptions, errors, and enums. A package can also contain sub-packages.

Advantages of using Packages in Java

There are several advantages of using Java Packages, some of them, are as follows –

·        Make easy searching or locating of classes and interfaces.

·        Avoid naming conflicts. For example, there can be two classes with the name Student in two packages, university.csdept.Student and college.itdept.Student

·        Implement data encapsulation (or data-hiding).

·        Provide controlled access: The access specifiers protected and default have access control on package level. A member declared as protected is accessible by classes within the same package and its subclasses. A member without any access specifier that is default specifier is accessible only by classes in the same package.

·        Reuse the classes contained in the packages of other programs.

·        Uniquely compare the classes in other packages.

Get to know about Access Specifiers in Java you didn’t know about.

We just put similar classes into the same packages. After that, we simply import the classes from existing packages through the import statement and use them in our program. A package provides access to some classes and others are kept for internal purposes.

Note:

·        Package names are dot-separated, e.g., java.lang.String

·        Packages avoid namespace collision: A package can not contain two classes with the same names, but two different packages can have a class with the same name.

·        The exact name of the class is identified by the structure of its package.

Types of Packages in Java

They can be divided into two categories:

1.     Java API packages or built-in packages and

2.    User-defined packages.

1. Java API packages or built-in packages

Java provides a large number of classes grouped into different packages based on a particular functionality.

Examples:

java.lang: It contains classes for primitive types, strings, math functions, threads, and exceptions.

java.util: It contains classes such as vectors, hash tables, dates, Calendars, etc.

java.io: It has stream classes for Input/Output.

java.awt: Classes for implementing Graphical User Interface – windows, buttons, menus, etc.

java.net: Classes for networking

java. Applet: Classes for creating and implementing applets

2. User-defined packages

As the name suggests, these packages are defined by the user. We create a directory whose name should be the same as the name of the package. Then we create a class inside the directory.

Creating a Package in Java

To create a package, we choose a package name and to include the classes, interfaces, enumerations, etc, inside the package, we write the package with its name at the top of every source file.

There can be only one package statement in each type of file. If we do not write class, interfaces, inside any package, then they will be placed in the current default package.

Example of Java Package

We can create a Java class inside a package using a package keyword.

package com.abc.packagedemo; //package

class Example

{

public static void main(String args[])

{

System.out.println("Welcome to abc’s Java Tutorial");

}

}

Output:

Welcome to abc Java Tutorial

How do Packages in Java Work?

The names of packages and the directory structure are closely related to each other.

For example, if a package name is university.engineering.csedept, then there are three directories- university, engineering, and csedept such that csedept is present in engineering and engineering is present in university.

The package university can be considered as a top-level package while engineering is a subpackage of university and cse dept is a sub-package of engineering.

Package Naming Conventions

Packages names follow the reverse order of domain names, that is, org.abc.tutorials. For example, in a university, the recommended convention is university.engineering.mech or university.tech.it or university.arts.history etc.

In the following package:

java.util.Vector

·        java is a top-level package

·        util is a sub package

·        and Vector is a class which is present in the subpackage util.

Compiling a Java Package

If you are using an IDE (Integrated Development Environment), then for compiling the package, you need to follow the syntax given below:

javac -d directory javaFileName

For example,

javac -d . Example.java

-d specifies the destination where to locate the generated class file. You can use any directory name like /home (in case of Linux), C:/folderName (in case of windows), etc. If you want the package to be present in the same directory, you can use the dot ( . )

Executing Java Package Program

You need to use a fully qualified name e.g. com.abc.MyClass etc to run the class.

To Compile:

javac -d . MyClass.java

Here -d represents the destination. The . represents the current folder.

To run:

java com.abc.MyClass

Accessing Packages or Classes from Another Package

If we want to access all the classes and interfaces of an existing package then we use the import statement. We can do it in three different ways:

·        import package.*;

·        import package.classname;

·        fully qualified name.

1. By using * after the import statement, we can access all the classes of the package but not the sub-packages.

Syntax:

For importing all the classes:

import packageName.*;

Code to illustrate the above concept:

package com.abc.packagedemo; //package

class MyClass

{

public void printName(String name)

{

System.out.println(name);

}

}

package com.abc.packagedemo1;

import com.abc.packagedemo.*; //importing all the classes

public class MyClass1

{

public static void main(String args[])

{

// Initializing the String variable with a value

String name = "abc's Java Tutorial";

// Creating an instance of class MyClass from another package.

MyClass obj = new MyClass();

obj.printName(name);

}

}

Output:

abc’s Java Tutorial

2. By using a particular class name after the import statement, we can access that particular class package but not the sub-packages.

Syntax:

For importing a particular class:

import packageName.className;

Code to illustrate the above concept:

package com.abc.packagedemo; //package

class MyClass

{

public void printName(String name)

{

System.out.println(name);

}

}

package com.tabc.packagedemo1;

import com.abc.packagedemo.MyClass; //importing a particular class MyClass

public class MyClass1

{

public static void main(String args[])

{

// Initializing the String variable with a value

String name = "abc's Java Tutorial";

// Creating an instance of class MyClass from another package.

MyClass obj = new MyClass();

obj.printName(name);

}

}

Output:

abc’s Java Tutorial

3. Using a Fully qualified name means we can access the declared class of different packages without using the import statement. But you need to use a fully qualified name every time when you are accessing the class or interface which is present in a different package.

This type of technique is generally used when two packages have the same class name example class Date is present in both the packages java.util and java.sql.

Code to illustrate the above concept:

package com.abc.packagedemo; //package

class MyClass

{

public void printName(String name)

{

System.out.println(name);

}

}

package com.abc.packagedemo1;

public class MyClass1

{

public static void main(String args[])

{

// Initializing the String variable with a value

String name = "abc's Java Tutorial";

// Using fully qualified name to access the class of different package

com.abc.packagedemo.MyClass obj = new com.abc.packagedemo.MyClass();

obj.printName(name);

}

}

Output:

abc’s Java Tutorial

Note: If you import a package, you can not import the sub-packages. If you import a package, all the classes and interface of that package will be imported but the classes and interfaces of the sub-packages will not be accessible. Hence, you will have to import the subpackage as well.

Note: The sequence of the program must be package name then import statement.After both the class begins.

Subpackage in Java

The package present inside a package is called the subpackage. It is created to further categorize packages.

For example, If we create a package inside the abc package then that will be called subpackage.

Let’s say we have created another package inside abc and the name of subpackage is tutorials. So if we create a class in this subpackage it should have a package declaration in the following manner:

package abc.tutorials.classname;

Here  abc is a package while tutorials is a subpackage of  abc.

The standard of defining the package is domain.company.package
example com. abc.tutorials.javatutorial or org.iit.dao.

Code to explain Subpackage

package com. abc.tutorials.javatutorial;

class Demo

{

public static void main(String args[])

{

System.out.println("Inside a sub-package");

}

}

To compile: javac -d. Demo.java

To run: java com. abc.tutorials.javatutorial

Output:

Inside a sub-package

Using Static Import

‘Static Import’ is a feature introduced in Java programming language for Java versions 5 and above that allows fields and members of a class which are declared as public and static to be directly used inside the Java code without specifying the name of the class in which these public static fields or methods are defined.

Code to illustrate the use of static import statement

// Note static keyword after import.

package com. abc.packagedemo;

import static java.lang.System.*;

class StaticImportDemo

{

public static void main(String args[])

{

// We don't need to use 'System.out'

// as we imported the package using static.

out.println(" abc");

}

}

Output:

 abc

Important points on Packages in Java

·        Every class belongs to a package. If you do not mention any package, the classes in the file move into a special unnamed package which is the same for all the files which do not belong to a specified package.

·        Multiple classes and interfaces in a file can be part of the same package.

·        If the package name is specified, then the directory name must match the package name.

·        We can access the classes declared as public in another package using:
import package-name.class-name

Dividing Your Classes Into Packages

You might be thinking that how to categorize your classes into packages. There is no standard method to do this but thee are two methods which you can follow:

1. Divide by Layer

This is the first method in which we divide the classes according to the layers of the application. For example, if your application contains a network layer, then you would create a package named network.

All classes which are related to the network of the application are located in the network package.

2. Divide by Application Functionality

You can also divide your classes on the basis of what part of the application functionality they belong to. Thus, if your application has a functionality area that calculates interest you would create a Java package named interest.

All classes related directly or indirectly to the interest calculations would go into that package. If the number of classes in the root package becomes very large, they can also be moved to the sub-packages.

Summary

Packages are essential for better management and accessing of code. Coming to the end of our tutorial, we learned about the packages in Java in detail along with its two types.

We also covered the working, the techniques to access the packages in other packages in Java, with the help of syntax and coding examples. This will be surely helpful for your further study of the Java language.

Thank you for reading our article. If you have any queries related to Packages in Java, do let us know by dropping a comment below.