什么是面向对象编程（OOP）？Java中是如何体现OOP的三大特性（封装、继承、多态）的？

public class BankAccount {
    private double balance;

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }

    public double getBalance() {
        return balance;
    }
}

public class Animal {
    public void makeSound() {
        System.out.println("The animal makes a sound");
    }
}

public class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("The dog barks");
    }
}

public class Shape {
    public void draw() {
        System.out.println("Drawing a shape");
    }
}

public class Circle extends Shape {
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

public class Rectangle extends Shape {
    @Override
    public void draw() {
        System.out.println("Drawing a rectangle");
    }
}

public class Main {
    public static void main(String[] args) {
        Shape shape1 = new Circle();
        Shape shape2 = new Rectangle();

        shape1.draw(); // 输出: Drawing a circle
        shape2.draw(); // 输出: Drawing a rectangle
    }
}

public class Book {
    private String title;
    private String author;

    public Book(String title, String author) {
        this.title = title;
        this.author = author;
    }

    public String getTitle() {
        return title;
    }

    public String getAuthor() {
        return author;
    }
}

public class Novel extends Book {
    private String genre;

    public Novel(String title, String author, String genre) {
        super(title, author);
        this.genre = genre;
    }

    public String getGenre() {
        return genre;
    }
}

public class Library {
    public void displayBook(Book book) {
        System.out.println("Title: " + book.getTitle() + ", Author: " + book.getAuthor());
        if (book instanceof Novel) {
            Novel novel = (Novel) book;
            System.out.println("Genre: " + novel.getGenre());
        }
    }

    public static void main(String[] args) {
        Library library = new Library();
        Book book1 = new Book("The Great Gatsby", "F. Scott Fitzgerald");
        Novel book2 = new Novel("1984", "George Orwell", "Dystopian");

        library.displayBook(book1);
        library.displayBook(book2);
    }
}

public abstract class Animal {
    public abstract void makeSound();
}

public class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("The dog barks");
    }
}

public class Cat extends Animal {
    @Override
    public void makeSound() {
        System.out.println("The cat meows");
    }
}

public class AnimalFactory {
    public static Animal createAnimal(String type) {
        if ("dog".equalsIgnoreCase(type)) {
            return new Dog();
        } else if ("cat".equalsIgnoreCase(type)) {
            return new Cat();
        } else {
            throw new IllegalArgumentException("Unknown animal type: " + type);
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Animal dog = AnimalFactory.createAnimal("dog");
        Animal cat = AnimalFactory.createAnimal("cat");

        dog.makeSound(); // 输出: The dog barks
        cat.makeSound(); // 输出: The cat meows
    }
}