Java写图书馆借书还书的小系统

感谢您的提问。以下是Java写图书馆借书还书的小系统的示例代码：

import java.util.*;

class Book {
  private int id;
  private String title;
  private String author;
  private boolean available;

  public Book(int id, String title, String author) {
    this.id = id;
    this.title = title;
    this.author = author;
    this.available = true;
  }

  public int getId() {
    return id;
  }

  public String getTitle() {
    return title;
  }

  public String getAuthor() {
    return author;
  }

  public boolean isAvailable() {
    return available;
  }

  public void setAvailable(boolean available) {
    this.available = available;
  }

  public String toString() {
    return "Book[" + id + "] - " + title + " by " + author + " (available=" + available + ")";
  }
}

class Library {
  private List<Book> books = new ArrayList<Book>();

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

  public Book removeBook(int id) {
    for (Book book : books) {
      if (book.getId() == id && book.isAvailable()) {
        book.setAvailable(false);
        return book;
      }
    }
    return null;
  }

  public void returnBook(int id) {
    for (Book book : books) {
      if (book.getId() == id && !book.isAvailable()) {
        book.setAvailable(true);
        break;
      }
    }
  }

  public List<Book> getBooks() {
    return books;
  }

  public String toString() {
    StringBuilder sb = new StringBuilder();
    for (Book book : books) {
      sb.append(book.toString()).append("\n");
    }
    return sb.toString();
  }
}

public class LibraryApplication {
  public static void main(String[] args) {
    Library library = new Library();

    Book book1 = new Book(1, "Java Programming", "James Gosling");
    Book book2 = new Book(2, "Effective Java", "Joshua Bloch");
    Book book3 = new Book(3, "Thinking in Java", "Bruce Eckel");

    library.addBook(book1);
    library.addBook(book2);
    library.addBook(book3);

    System.out.println("Library catalog:");
    System.out.println(library);

    Book borrowedBook = library.removeBook(2);
    if (borrowedBook != null) {
      System.out.println("Borrowed book: " + borrowedBook.getTitle());
      System.out.println("Library catalog after borrowing:");
      System.out.println(library);
    } else {
      System.out.println("Book is not available for borrowing.");
    }

    library.returnBook(2);
    System.out.println("Library catalog after returning:");
    System.out.println(library);
  }
}

请注意，这只是一个简单的示例代码，仅用于说明如何实现图书馆借书还书的系统。在实际应用中，您可能需要考虑更多的方面，例如检查图书的借阅期限，生成借阅报告等等。