Introduction to Packages in Java

Packages organize related classes, interfaces, and sub-packages in Java. They're like folders on your computer, but for code.

What are Packages?

A package is a namespace that groups related classes and interfaces. If you have 50 classes, you don't dump them all in one place—you organize them into logical groups.

Why Use Packages?

1. Organization: Group related classes together
2. Name Conflicts: Avoid naming conflicts between classes
3. Access Control: Control visibility of classes and members
4. Reusability: Make classes easier to find and use
5. Maintainability: Easier to maintain and understand large codebases

Creating and Using Packages

Package Declaration

To create a package, use the package keyword at the top of your Java file:

package com.example.myapp;

public class MyClass {
    // Class implementation
}

Package Naming Conventions

Java packages follow reverse domain name conventions:

// Good package names
com.company.project
org.organization.library
edu.university.department

// Avoid
myPackage
utils
helpers

Directory Structure

Package structure must match the directory structure:

src/
├── com/
│   ├── example/
│   │   ├── myapp/
│   │   │   ├── MyClass.java
│   │   │   └── AnotherClass.java
│   │   └── utils/
│   │       └── Helper.java

Importing Classes

Single Class Import

import java.util.ArrayList;

public class MyClass {
    ArrayList<String> list = new ArrayList<>();
}

Import All Classes from Package

import java.util.*;

public class MyClass {
    ArrayList<String> list = new ArrayList<>();
    HashMap<String, Integer> map = new HashMap<>();
}

Fully Qualified Names

You can also use fully qualified names without imports:

public class MyClass {
    java.util.ArrayList<String> list = new java.util.ArrayList<>();
}

Package Access

Default (Package-Private) Access

Classes and members with no access modifier are accessible within the same package:

package com.example.myapp;

class PackagePrivateClass {
    int packagePrivateField;
    void packagePrivateMethod() {}
}

Public Access

Public classes are accessible from anywhere:

package com.example.myapp;

public class PublicClass {
    public void publicMethod() {}
}

Common Package Patterns

Layered Architecture

com.example.myapp
├── controller/
├── service/
├── repository/
├── model/
└── util/

Feature-Based Organization

com.example.myapp
├── user/
│   ├── UserController.java
│   ├── UserService.java
│   └── UserRepository.java
├── product/
│   ├── ProductController.java
│   ├── ProductService.java
│   └── ProductRepository.java

Built-in Java Packages

Java provides many built-in packages:

• java.lang: Fundamental classes (String, Object, etc.)
• java.util: Utility classes (Collections, Date, etc.)
• java.io: Input/Output operations
• java.net: Networking operations
• java.sql: Database operations

Package Compilation

Compiling with Packages

# Compile from source root
javac com/example/myapp/MyClass.java

# Run with fully qualified name
java com.example.myapp.MyClass

Using Classpath

# Compile with classpath
javac -cp . com/example/myapp/MyClass.java

# Run with classpath
java -cp . com.example.myapp.MyClass

Packages organize Java code, prevent naming conflicts, and provide access control. Use reverse domain names (com.company.project), keep them lowercase, and match your directory structure. They're essential for any project bigger than a handful of classes.