# 📚 Java Programming Language – Part 7/10: Packages & Access Modifiers
#Java #Packages #AccessModifiers #Encapsulation

Welcome to Part 7 of our Java series! Today we'll explore how to organize code using packages and control visibility with access modifiers.

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## 🔹 Packages in Java
Packages help organize classes and prevent naming conflicts.

### 1. Creating and Using Packages

// File: com/example/utils/MathHelper.java
package com.example.utils;  // Package declaration

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

// File: MainApp.java
import com.example.utils.MathHelper;

public class MainApp {
    public static void main(String[] args) {
        int sum = MathHelper.add(5, 3);
        System.out.println("Sum: " + sum);
    }
}

### 2. Common Java Packages
| Package | Contents |
|---------|----------|
| java.lang | Core classes (auto-imported) |
| java.util | Collections, date/time |
| java.io | Input/output operations |
| java.net | Networking |

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## 🔹 Access Modifiers
Control visibility of classes, methods, and variables.

### 1. Access Levels Overview
| Modifier | Class | Package | Subclass | World |
|----------|-------|---------|----------|-------|
| public | ✅ | ✅ | ✅ | ✅ |
| protected | ✅ | ✅ | ✅ | ❌ |
| default (no modifier) | ✅ | ✅ | ❌ | ❌ |
| private | ✅ | ❌ | ❌ | ❌ |

### 2. Practical Examples

public class BankAccount {
    private double balance;  // Only accessible within class
    
    public String accountNumber;  // Accessible everywhere
    
    protected String ownerName;  // Accessible in package and subclasses
    
    void displayBalance() {  // Package-private (default)
        System.out.println("Balance: " + balance);
    }
}

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## 🔹 Encapsulation Deep Dive
Proper encapsulation = private fields + public methods.

### 1. Proper Encapsulation Example

public class Student {
    private String id;
    private String name;
    private double gpa;
    
    // Constructor
    public Student(String id, String name) {
        this.id = id;
        this.name = name;
    }
    
    // Getter methods
    public String getId() { return id; }
    public String getName() { return name; }
    public double getGpa() { return gpa; }
    
    // Setter methods with validation
    public void setName(String name) {
        if (name != null && !name.isEmpty()) {
            this.name = name;
        }
    }
    
    public void updateGpa(double newGpa) {
        if (newGpa >= 0 && newGpa <= 4.0) {
            this.gpa = newGpa;
        }
    }
}

### 2. Benefits of Encapsulation
✔️ Better control over data
✔️ Validation in setters
✔️ Hiding implementation details
✔️ Easier to modify internal representation

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## 🔹 Static Import
Import static members directly.

import static java.lang.Math.PI;
import static java.lang.Math.pow;

public class Circle {
    public static double calculateArea(double radius) {
        return PI * pow(radius, 2);
    }
}

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## 🔹 Practical Example: Library Management

package com.library.models;

public class Book {
    private String isbn;
    private String title;
    private String author;
    private boolean isAvailable;
    
    public Book(String isbn, String title, String author) {
        this.isbn = isbn;
        this.title = title;
        this.author = author;
        this.isAvailable = true;
    }
    
    // Getters and setters
    public String getIsbn() { return isbn; }
    public String getTitle() { return title; }
    public boolean isAvailable() { return isAvailable; }
    
    public void setAvailable(boolean available) {
        isAvailable = available;
    }
}

package com.library.system;

import com.library.models.Book;

public class Library {
    public void borrowBook(Book book) {
        if (book.isAvailable()) {
            book.setAvailable(false);
            System.out.println("Book borrowed: " + book.getTitle());
        } else {
            System.out.println("Book not available");
        }
    }
}

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