Packages In Java

Packages

Packages are an encapsulation mechanism that group related classes and interfaces into a single unit.

What is a package?

A package groups related types (classes/interfaces) for modularity, naming, and access control.

Examples

• java.sql — classes and interfaces related to database operations.
• java.io — classes and interfaces for file I/O operations.

Main advantages

• Resolve name conflicts (unique identification).
• Improve modularity of the application.
• Improve maintainability.
• Provide security for components (access control).

Naming convention

Use the internet domain name in reverse followed by module/submodule/class. Example:

com.icicibank.loan.housing.Account

• com.icicibank — reversed domain
• loan — module
• housing — submodule
• Account — class

Code example

package com.code.java; public class Test { public static void main(String[] args) { System.out.println("hello"); } }

Compiling & class file placement

javac Test.java
The generated .class file is placed in the current working directory.

javac -d

javac -d . Test.java

-d specifies the destination root where the .class files are placed (creates folder structure for the package if needed).

• If destination doesn't exist you can specify another path: javac -d F: Test.java
• If specified directory is not present and not creatable: compile error (directory not found).

Execution

Run using fully qualified name:

java com.code.java.Test

Rules & Conclusion

• At most one package statement is allowed per source file.
• If used, the package declaration must be the first non-comment statement in the file.
• Valid Java source file structure (when package is present):

1. package statement
2. import statements
3. class / interface / enum declaration

Access modifiers (summary)

Recommended: make data members private, expose methods as public when needed.

Context	public	private	protected	default
Within same class	Allowed	Allowed	Allowed	Allowed
Child class (same package)	Allowed	Not allowed	Allowed	Allowed
Non-child class (same package)	Allowed	Not allowed	Allowed	Allowed
Child class (outside package)	Allowed	Not allowed	Allowed (use child reference)	Not allowed
Non-child class (outside package)	Allowed	Not allowed	Not allowed	Not allowed

Access order (least → most permissive): private < default < protected < public