Question

public class Catalogue() {
  private List<Book> booksAvailable;
  private List<Book> booksRented:

  public Catalogue() {
    booksAvailable.add(new Book("Matrix", 1999, new Genre("SciFi"), 3));
    booksAvailable.add(new Book("Jurassic Park", 1999, new Genre("SciFi"), 3));
    boosAvailable.add(new Book("Terminator", 1999, new Genre("SciFi"), 3));
    booksRented = new LinkedList<Book> ();
  }

  public void rentBook() {
    System.out.println("Rent a book:");
    System.out.println("Enter the title of a book you want to rent: ");
    String name = In.NextLine();
    for (Book book: booksAvailable) {
      if (book.getName.equals(name)) {
        System.out.println("Renting " + name);
        booksAvailable.remove(book);
        booksRented.add(book);
        break;
      } else {
        System.out.println("No such books found");
      }
    }
  }
}

运行此代码时只能租用Matrix手册。当我试图租用另一本像侏罗纪公园的书时，它说没有找到书。当我关闭程序并再次运行并尝试租用第二本书时，它再次说未找到书籍。请帮我解决这个问题。我在这段代码中遇到了什么问题。感谢

Answer 1

当你在迭代时修改列表的其他人指出了它是危险的。

我建议您尝试使用HashMap，尤其是如果name是您正在查看的唯一字段。

public class Catalogue {
    private Map<String, Book> booksAvailable;
    private Map<String, Book> booksRented;

    public Catalogue() {
        booksAvailable = new HashMap<>();
        booksAvailable.put("Matrix", new Book("Matrix", 1999, new Genre("SciFi"), 3));
        booksAvailable.put("Jurassic Park", new Book("Jurassic Park", 1999, new Genre("SciFi"), 3));
        booksAvailable.put("Terminator", new Book("Terminator", 1999, new Genre("SciFi"), 3));
        booksRented = new HashMap<>();
    }

    public void rentBook() {
        System.out.println("Rent a book:");
        System.out.println("Enter the title of a book you want to rent: ");
        Scanner scanner = new Scanner(System.in);
        String name = scanner.nextLine();
        if (booksAvailable.containsKey(name)) {
            Book book = booksAvailable.get(name);
            System.out.println("Renting " + name);
            booksAvailable.remove(name);
            booksRented.put(name, book);
        } else {
            System.out.println("No such books found");
        }
    }
}

Answer 2

当您致电booksAvailable.remove()时，它将从当前迭代中有效删除。

因此，当您访问next()时，可能会导致意外行为。

修改由于您的代码处理，您不能租借第一本书以外的其他书籍。

你在System.out.println("No such books found");语句内部循环中有else。因此，如果您输入的书不是第一本书，则测试失败并打印语句。

要纠正此问题，您可以使用标记来指示是否找到某本书，并在循环外打印该语句;

boolean rented = false; for (Book b : books) { if (found) { rented = true; } } if (!rented) { // print no such book message }

Answer 3

我只是尝试了你的代码（在修改它以进行编译之后）并且它有效。但是，当运行例如Jurasic Park时，它会说找不到该书，然后它将租用它，因为该print语句在for循环中。

我尝试重写它以使用流和选项，我得到了似乎正在运行的代码

public void rentBook() {
    System.out.println("Rent a book:");
    System.out.println("Enter the title of a book you want to rent: ");
    String name = "Jurassic Park";
    Optional<Book> book = booksAvailable.stream().filter(b -> b.name.equals(name)).findFirst();
    if(book.isPresent()) {
        System.out.println("Renting " + name);
        booksAvailable.remove(book.get());
        booksRented.add(book.get());
    }
    else
        System.out.println("No such book found");
}

我只能访问列表中的第一项

3 个答案: