Java `package` Keyword
Usage and Examples

`package` Keyword in Java

In Java, the package keyword is used to define a namespace for classes and interfaces. It plays a fundamental role in organizing your code into modular units, allowing better maintainability, avoiding name conflicts, and making your application structure cleaner.

Why Use Packages?

Imagine working on a large application with hundreds of classes. If all of them were dumped into one folder, it would quickly become overwhelming. Packages help by:

Grouping related classes together
Preventing class name collisions
Controlling access with protected and default visibility
Making codebase modular and easier to navigate

Syntax of the `package` Keyword

The package declaration must be the first line of your Java source file, before any import statements or class definitions.

package com.example.myapp;

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello from my package!");
    }
}

Creating and Using Packages – Step by Step

Step 1: Declare the Package

At the very top of your Java file, write:

package mypackage;

This tells the compiler that this class belongs to the mypackage namespace.

Step 2: Save the File in a Directory Matching the Package Name

Save the file in a directory named mypackage.

For example:


/project-root/
  └── mypackage/
      └── HelloWorld.java

Step 3: Compile the Java File

Navigate to the root directory and compile using:


javac mypackage/HelloWorld.java

Step 4: Run the Class

To run a class in a package, use the fully qualified name:


java mypackage.HelloWorld

Hello from my package!

Accessing a Class from Another Package

You can access a class from another package by importing it using the import keyword.

Example

Step 1: A class in utilities package:

// File: utilities/MessagePrinter.java
package utilities;

public class MessagePrinter {
    public void printMessage() {
        System.out.println("Message from utilities package.");
    }
}

Step 2: Accessing this class from another package:

// File: app/Main.java
package app;

import utilities.MessagePrinter;

public class Main {
    public static void main(String[] args) {
        MessagePrinter printer = new MessagePrinter();
        printer.printMessage();
    }
}

Message from utilities package.

Types of Packages in Java

User-defined packages: Created by developers to organize their application code.
Built-in packages: Provided by Java, like java.util, java.io, java.lang.

Package Naming Conventions

It’s standard to use your organization’s internet domain name in reverse as the prefix, followed by the project/module name. For example:

package com.tutorialkart.datastructures;

Best Practices

Use lowercase letters to avoid conflicts with class names
Choose meaningful names based on functionality
Avoid deeply nested packages unless absolutely necessary

Common Mistakes

Placing the package statement after imports — it must come first
Saving the file outside the matching directory structure
Not compiling from the correct root path