Packages

1. Introduction

A package in Java is a mechanism to group related classes, interfaces, and sub-packages.
Packages help in:

• Organizing code
• Avoiding name conflicts
• Providing access protection
• Building modular applications

Java packages are similar to folders in a file system, allowing clean structure and easy maintenance.

2. Types of Packages in Java

Java provides two types of packages:

1. Built-in Packages (predefined)
2. User-Defined Packages (created by developers)

2.1 Built-in Packages

Examples:

• java.lang → Core classes (String, Math, Integer)
• java.util → Utility classes (ArrayList, HashMap, Date)
• java.io → Input/output classes
• java.time → Date and time API
• java.net → Networking
• java.sql → Database connectivity

These packages are part of the JDK.

2.2 User-Defined Packages

Developers create packages to organize large projects.

Example:

package com.example.project;

3. Creating a Package

Step 1: Declare package at the top of the Java file

package mypackage;

public class Student {
    public void display() {
        System.out.println("Inside Student class");
    }
}

Step 2: Compile with package directory creation

javac -d . Student.java

-d . instructs the compiler to create directories based on package name.

Step 3: Use the package

import mypackage.Student;

public class Main {
    public static void main(String[] args) {
        Student s = new Student();
        s.display();
    }
}

4. Using import Statement

To use a class from another package, import it:

import mypackage.Student;

OR import all:

import mypackage.*;

Importing improves readability but does not increase runtime memory.

5. Sub-packages

Packages inside other packages are known as sub-packages.

Example:

com.company.product

Directory structure:

com/
   company/
       product/
           Product.java

Organizing large projects becomes easier with sub-packages.

6. Access Modifiers and Packages

Packages play an important role in access control.

Package-level (default) access is restricted to classes inside the same package.

7. Static Import

Java allows importing static members:

import static java.lang.Math.PI;
import static java.lang.Math.sqrt;

System.out.println(PI);
System.out.println(sqrt(25));

8. Real-World Use of Packages

Large applications follow layered architecture using packages:

com.company.app
      ├── controller
      ├── service
      ├── model
      ├── repository
      └── config

Each layer contains logically grouped classes, improving:

• Maintainability
• Readability
• Scalability

9. Package Naming Conventions

Follow industry style:

• Domain in reverse order → com.google.api
• Use lowercase
• No special characters
• Short and meaningful

Examples:

• com.amazon.payment
• org.springframework.core
• com.myapp.utils

10. Complete Example Demonstrating Package Use

File: mypack/Message.java

package mypack;

public class Message {
    public void print() {
        System.out.println("Hello from mypack");
    }
}

File: Main.java

import mypack.Message;

public class Main {
    public static void main(String[] args) {
        Message m = new Message();
        m.print();
    }
}

Compile:

javac -d . Message.java Main.java

Run:

java Main

Output:

Hello from mypack

11. Summary

• Packages group related classes and avoid naming conflicts.
• Two types: built-in and user-defined.
• Use package to declare a package and import to use classes.
• Packages improve code organization and readability.
• Sub-packages help structure large applications.
• Access modifiers behave differently across packages.

This completes Packages in Java.

Next topic: classes-and-objects-practical.mdx if you want the next file in sequence.

Written By: Shiva Srivastava