3. 简单的图书管理系统：

import java.util.ArrayList;
import java.util.Scanner;

public class Library {
    private ArrayList<Book> books;

    public Library() {
        books = new ArrayList<>();
    }

    public void addBook(Book book) {
        books.add(book);
    }

    public void removeBook(Book book) {
        books.remove(book);
    }

    public void displayBooks() {
        for (Book book : books) {
            System.out.println(book);
        }
    }

    public static void main(String[] args) {
        Library library = new Library();
        
        // 添加一些示例图书
        library.addBook(new Book("The Great Gatsby", "F. Scott Fitzgerald", 1925));
        library.addBook(new Book("To Kill a Mockingbird", "Harper Lee", 1960));
        library.addBook(new Book("Pride and Prejudice", "Jane Austen", 1813));

        Scanner scanner = new Scanner(System.in);

        while (true) {
            System.out.println("\n请选择一个操作：");
            System.out.println("1. 显示所有图书");
            System.out.println("2. 添加一本新图书");
            System.out.println("3. 删除一本图书");
            System.out.println("4. 退出");

            int choice = scanner.nextInt();

            switch (choice) {
                case 1:
                    library.displayBooks();
                    break;
                case 2:
                    System.out.print("请输入书名：");
                    String title = scanner.next();
                    System.out.print("请输入作者名：");
                    String author = scanner.next();
                    System.out.print("请输入出版年份：");
                    int year = scanner.nextInt();
                    library.addBook(new Book(title, author, year));
                    System.out.println("添加成功！");
                    break;
                case 3:
                    System.out.print("请输入要删除的书名：");
                    String bookTitle = scanner.next();
                    boolean bookRemoved = false;
                    for (Book book : library.books) {
                        if (book.getTitle().equals(bookTitle)) {
                            library.removeBook(book);
                            bookRemoved = true;
                            System.out.println("删除成功！");
                            break;
                        }
                    }
                    if (!bookRemoved) {
                        System.out.println("未找到该书，删除失败！");
                    }
                    break;
                case 4:
                    System.out.println("程序已退出。");
                    return;
                default:
                    System.out.println("输入无效，请重新选择。");
            }
        }
    }
}

class Book {
    private String title;
    private String author;
    private int year;

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

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    @Override
    public String toString() {
        return "书名：" + title + "\t作者：" + author + "\t出版年份：" + year;
    }
}

   4. 简单的网络爬虫示例代码：

import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;

import java.io.IOException;

public class WebCrawler {
    private static final int MAX_DEPTH = 2; // 爬取的最大深度
    private static int depth = 0; // 当前深度

    public static void main(String[] args) {
        String url = "https://www.example.com"; // 起始 URL
        crawl(url);
    }

    public static void crawl(String url) {
        if (depth > MAX_DEPTH) {
            return;
        }
        try {
            Document document = Jsoup.connect(url).get(); // 获取页面文档对象
            Elements links = document.select("a[href]"); // 获取所有链接元素
            for (Element link : links) {
                String nextUrl = link.absUrl("href");
                System.out.println(nextUrl); // 输出当前页的链接

                // 递归爬取下一层页面
                depth++;
                crawl(nextUrl);
                depth--;
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}