19. Packages & Access Modifiers

[!NOTE] As your application grows from 5 files to 5,000 files, throwing everything into a single folder becomes chaos. Java uses a strict "Package" system mapped directly to your operating system's folders.

Packages (Folders)

A package is simply a directory structure. To avoid naming colllisions globally (where two companies both create a DatabaseConnection class), Java conventionally uses reversed internet domain names as the root package structure.

If your company domain is google.com, your project structure might look like this:

src/
└── com/
    └── google/
        └── auth/
            └── LoginManager.java
        └── database/
            └── QueryExecutor.java

  

At the absolute very top line of LoginManager.java, you MUST declare the package it lives inside:

// Line 1: Package declaration
package com.google.auth;

// Now we import classes from OTHER packages
import java.util.Scanner;
import com.google.database.QueryExecutor;

public class LoginManager { ... }

  

Access Modifiers Matrix

We briefly touched on public and private. But there are actually 4 distinct levels of visibility in Java. Understanding this matrix is critical for interview questions.

The "Default" (Package-Private) Mystery

If you write a variable without ANY keyword (e.g., String token;), it defaults to Package-Private. Any other class living in the exact same folder (package) can see and modify that variable. But if a class in a different folder tries to access it, the compiler throws an error.

When to use what?

• Use private for 99% of your instance variables to enforce Encapsulation.
• Use public for classes, constructors, and methods you want the whole application to use (like a .saveData() method).
• Use protected for variables that you want to hide from developers, but allow inheriting Child classes to manipulate directly for performance logic.