Java Packages & Access Control Tutorial

1. Defining a Package

A Package acts as a container for group-related classes, interfaces, and sub-packages. It prevents naming conflicts and controls access permissions.

To declare a package, use the package keyword as the very first non-comment statement in your source file.

Syntax & Example

// File: src/com/myapp/utils/MathHelper.java
package com.myapp.utils;

public class MathHelper {
    public static int add(int a, int b) {
        return a + b;
    }
}

Directory Structure: Java maps package names directly to filesystem folders using dots as separators. The file above must reside in the folder path com/myapp/utils/.

2. Understanding CLASSPATH

CLASSPATH is an environment variable (or command-line flag) that tells the Java Virtual Machine (JVM) and Java compiler (javac) where to look for user-defined packages and classes.

How Java Finds Classes:

Current Directory (.): By default, the current working directory is searched.
Environment Variable: You can set a global system variable named CLASSPATH.
Command Line Flag (Recommended): Use the -cp or -classpath flag during compilation and execution.

Usage Example

# Compiling a class 
javac -d . com/myapp/utils/MathHelper.java

# Running a program by specifying the classpath target
java -cp . com.myapp.MainApp

3. Access Protection

Packages serve as visibility boundaries. Java provides four levels of access control to determine whether other classes can access a field, method, or constructor.

Access Modifier	Same Class	Same Package	Subclass (Different Package)	World (Different Package)
`private`	Yes	No	No	No
Default (No modifier)	Yes	Yes	No	No
`protected`	Yes	Yes	Yes	No
`public`	Yes	Yes	Yes	Yes

4. Importing Packages

To use a class located in a different package, you must either refer to it by its Fully Qualified Name or use the import statement.

Types of Imports

Single Class Import: Imports only a specific class.
```
import com.myapp.utils.MathHelper;
```
On-Demand (Wildcard) Import: Imports all public classes in a package.
```
import com.myapp.utils.*;
```
Static Import: Allows direct access to static members (fields/methods) without qualifying them by class name.
```
import static java.lang.Math.PI;
import static java.lang.Math.sqrt;
```

Complete Code Example

// File: src/com/myapp/MainApp.java
package com.myapp;

// Importing explicit class from another package
import com.myapp.utils.MathHelper;

public class MainApp {
    public static void main(String[] args) {
        int result = MathHelper.add(10, 20);
        System.out.println("Result: " + result);
    }
}