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Java Book

packages (package)

Packages and modules are Java’s mechanisms for organizing code, managing namespaces, and controlling visibility.

Packages group related classes and interfaces, preventing naming conflicts and providing access protection.

Modules (introduced in Java 9) add another layer of encapsulation above packages, allowing explicit dependencies and strong encapsulation.

A package is a namespace that groups related classes and interfaces. The package statement must be the first line in a source file (except comments).

Syntax:

package com.example.myapp;


public class MyClass {
// ...
}

Naming conventions:

  • Use reverse domain name (e.g., com.company.project).
  • All lowercase letters.
  • Underscores are allowed but not common.

Directory structure must match package hierarchy:

Terminal window
src/
└── com/
    └── example/
        └── myapp/
            └── MyClass.java

Compiling:

Terminal window
javac -d . src/com/example/myapp/MyClass.java

The -d . creates directories matching the package structure.

Running:

Terminal window
java com.example.myapp.MyClass

Default package (no package statement) – not recommended for anything but small examples. Classes in default package cannot be imported.

Benefits:

  • Avoid name conflicts.
  • Control access (package‑private).
  • Organize large codebases.