Java Packages - Complete Tutorial

// java.lang is automatically imported
// No need for: import java.lang.*;

// Import specific class from java.util
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Scanner;

// Import entire package (not recommended for large packages)
import java.util.*;

// Import static members
import static java.lang.Math.PI;
import static java.lang.Math.pow;
import static java.lang.System.out;

public class BuiltInPackagesExample {
    public static void main(String[] args) {
        // Using java.lang (auto-imported)
        String message = "Hello from java.lang!";
        out.println(message); // Using static import
        
        // Using java.util
        Scanner scanner = new Scanner(System.in);
        ArrayList list = new ArrayList<>();
        HashMap map = new HashMap<>();
        
        // Using java.math
        java.math.BigDecimal bigDecimal = new java.math.BigDecimal("123.456");
        
        // Using static imports
        double radius = 5.0;
        double area = PI * pow(radius, 2);
        out.println("Area of circle: " + area);
        
        // Using multiple packages
        java.util.Date utilDate = new java.util.Date();
        java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
        
        out.println("Util Date: " + utilDate);
        out.println("SQL Date: " + sqlDate);
        
        scanner.close();
    }
}

package models;

public class User {
    // Private fields - encapsulated
    private String username;
    private String email;
    private int age;
    
    // Constructor
    public User(String username, String email, int age) {
        this.username = username;
        this.email = email;
        this.age = age;
    }
    
    // Public getters and setters
    public String getUsername() {
        return username;
    }
    
    public void setUsername(String username) {
        this.username = username;
    }
    
    public String getEmail() {
        return email;
    }
    
    public void setEmail(String email) {
        this.email = email;
    }
    
    public int getAge() {
        return age;
    }
    
    public void setAge(int age) {
        if(age >= 0) {
            this.age = age;
        } else {
            throw new IllegalArgumentException("Age cannot be negative");
        }
    }
    
    // Public method
    public void displayInfo() {
        System.out.println("Username: " + username);
        System.out.println("Email: " + email);
        System.out.println("Age: " + age);
    }
    
    // Package-private method
    void updateEmail(String newEmail) {
        this.email = newEmail;
    }
}

package com.example.access;

// Public class - accessible from anywhere
public class AccessControlExample {
    
    // Public field
    public String publicField = "I'm public";
    
    // Protected field
    protected String protectedField = "I'm protected";
    
    // Default (package-private) field
    String defaultField = "I'm package-private";
    
    // Private field
    private String privateField = "I'm private";
    
    // Public method
    public void publicMethod() {
        System.out.println("Public method called");
        privateMethod(); // Can call private method from same class
    }
    
    // Protected method
    protected void protectedMethod() {
        System.out.println("Protected method called");
    }
    
    // Default (package-private) method
    void defaultMethod() {
        System.out.println("Default method called");
    }
    
    // Private method
    private void privateMethod() {
        System.out.println("Private method called");
    }
    
    // Method to demonstrate access
    public void demonstrateAccess() {
        System.out.println("\nAccess from within same class:");
        System.out.println("Public field: " + publicField);
        System.out.println("Protected field: " + protectedField);
        System.out.println("Default field: " + defaultField);
        System.out.println("Private field: " + privateField);
    }
}

// Default (package-private) class - only accessible within same package
class PackagePrivateClass {
    public void sayHello() {
        System.out.println("Hello from package-private class!");
    }
}

package com.example.access;

public class AnotherClassInSamePackage {
    public static void main(String[] args) {
        AccessControlExample example = new AccessControlExample();
        
        System.out.println("Access from same package:");
        
        // Can access public members
        System.out.println("Public field: " + example.publicField);
        example.publicMethod();
        
        // Can access protected members (same package)
        System.out.println("Protected field: " + example.protectedField);
        example.protectedMethod();
        
        // Can access default members (same package)
        System.out.println("Default field: " + example.defaultField);
        example.defaultMethod();
        
        // Cannot access private members
        // System.out.println(example.privateField); // Compilation error
        // example.privateMethod(); // Compilation error
        
        // Can access package-private class
        PackagePrivateClass pkgClass = new PackagePrivateClass();
        pkgClass.sayHello();
    }
}

package com.example.different;

import com.example.access.AccessControlExample;

public class DifferentPackageClass {
    public static void main(String[] args) {
        AccessControlExample example = new AccessControlExample();
        
        System.out.println("Access from different package:");
        
        // Can only access public members
        System.out.println("Public field: " + example.publicField);
        example.publicMethod();
        
        // Cannot access protected, default, or private members
        // System.out.println(example.protectedField); // Compilation error
        // System.out.println(example.defaultField);   // Compilation error
        // System.out.println(example.privateField);   // Compilation error
        
        // Cannot access package-private class from different package
        // PackagePrivateClass pkgClass = new PackagePrivateClass(); // Compilation error
    }
}

// Subclass in different package
class SubClass extends AccessControlExample {
    public void testAccess() {
        System.out.println("Access from subclass in different package:");
        
        // Can access public members
        System.out.println("Public field: " + publicField);
        publicMethod();
        
        // Can access protected members (through inheritance)
        System.out.println("Protected field: " + protectedField);
        protectedMethod();
        
        // Cannot access default or private members
        // System.out.println(defaultField); // Compilation error
        // System.out.println(privateField); // Compilation error
    }
}

// File: com/example/mainapp/controllers/UserController.java
package com.example.mainapp.controllers;

import com.example.mainapp.models.User;
import com.example.utils.StringHelper;

public class UserController {
    
    public User createUser(String username, String email) {
        // Validate username using StringHelper from utils sub-package
        if(StringHelper.isValidUsername(username)) {
            return new User(username, email);
        } else {
            throw new IllegalArgumentException("Invalid username");
        }
    }
    
    public void displayUserInfo(User user) {
        System.out.println("User: " + user.getUsername());
        System.out.println("Email: " + user.getEmail());
    }
}

// File: com/example/utils/StringHelper.java
package com.example.utils;

public class StringHelper {
    
    public static boolean isValidUsername(String username) {
        if(username == null || username.trim().isEmpty()) {
            return false;
        }
        // Username must be 3-20 characters, alphanumeric
        return username.matches("^[a-zA-Z0-9]{3,20}$");
    }
    
    public static String capitalize(String str) {
        if(str == null || str.isEmpty()) {
            return str;
        }
        return str.substring(0, 1).toUpperCase() + str.substring(1).toLowerCase();
    }
}

// File: com/example/mainapp/Main.java
package com.example.mainapp;

import com.example.mainapp.controllers.UserController;
import com.example.mainapp.models.User;
import com.example.utils.StringHelper;

public class Main {
    public static void main(String[] args) {
        System.out.println("=== Working with Sub-packages ===");
        
        // Create user controller
        UserController controller = new UserController();
        
        try {
            // Create valid user
            User user1 = controller.createUser("john123", "john@example.com");
            controller.displayUserInfo(user1);
            
            // Try to create invalid user
            User user2 = controller.createUser("ab", "invalid@example.com");
        } catch (IllegalArgumentException e) {
            System.out.println("Error: " + e.getMessage());
        }
        
        // Use StringHelper from utils package
        String name = "jane doe";
        String capitalized = StringHelper.capitalize(name);
        System.out.println("\nOriginal: " + name);
        System.out.println("Capitalized: " + capitalized);
        
        // Check username validity
        String[] usernames = {"alice", "bob123", "charlie_doe", "a", "verylongusername123456789"};
        System.out.println("\n=== Username Validation ===");
        for(String username : usernames) {
            boolean valid = StringHelper.isValidUsername(username);
            System.out.println(username + ": " + (valid ? "Valid" : "Invalid"));
        }
    }
}

/**
 * Provides utility classes for common operations in the application.
 * 
 * 
This package contains helper classes that provide reusable functionality
 * for string manipulation, mathematical calculations, date formatting,
 * and other common tasks.
 * 
 * 
Package Specification
 * 

 *   
{@link com.example.utils.StringHelper} - String manipulation utilities
 *   
{@link com.example.utils.MathHelper} - Mathematical operations
 *   
{@link com.example.utils.DateHelper} - Date and time utilities
 *   
{@link com.example.utils.ValidationHelper} - Input validation
 * 
 * 
 * @since 1.0
 * @author Your Name
 * @version 1.0
 */
package com.example.utils;