java如何从具有特定属性的ArrayList打印一个或多个对象？

Question

我有一个读书班：

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

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

    public String getBookTitle() {
        return  title;
    }
    
    public String getBookAuthor() {
        return author;
    }
    
    public String getBookGenre() {
        if (genre == 1) {
          return ("Fantasy");
        }
        else if (genre == 2) {
          return ("Science Fiction");
        }
        else if (genre == 3) {
          return ("Dystopian");
        }
        return "genre";
        
    }

    @Override
    public String toString() {
        return ("Title: " + this.getBookTitle() + " \nAuthor: " + this.getBookAuthor() + " \nGenre: " + this.getBookGenre() + "\n");
    }
    
}

我有一个LibraryDatabase类，它有两个Book对象的ArrayList：

import java.util.*;
public class LibraryDatabase extends Book {
  List<Book> bookDatabase = new ArrayList<>();

  public LibraryDatabase() {
      super();
  }

  public List<Book> books() {
    Book book1 = new Book("Harry Potter", "J.K. Rowling", 1);
    Book book2 = new Book("Neuromancer", "William Gibson", 2);

    bookDatabase.add(book1);
    bookDatabase.add(book2);
    return bookDatabase;
  }
}

我想允许用户选择他们想要的类型，并让控制台打印出将该类型作为属性的所有对象。 我在一个单独的类中使用此方法：

public void showTitles() {
    LibraryDatabase libraryDatabase = new LibraryDatabase();
    Scanner kbMeter = new Scanner(System.in);
    String genre = kbMeter.nextLine();
    if (genre.equals(libraryDatabase.getBookGenre())) {
      //???
    }

  }

我不知道在if语句中放入什么才能打印出所有具有该类型的对象。谢谢你的帮助

Answer 1

你的设计有一些根本性的问题。你的LibraryDatabase不应该扩展Book

您可以在LibraryDatabase类中公开findBookByGenre(String genre)方法，也可以筛选books()返回的列表

        libraryDatabase.books().forEach(book -> {
            if ("aaa".equals(book.getBookGenre())) {
                System.out.println(book.getBookTitle());
            }
        });

Answer 2

通过数据库中每个Book的简单for循环应该可以工作：

for (Book book : bookDatabase) {
    if (book.getBookGenre().equals(genre) {
        System.out.println(book);
    }
}

Answer 3

我建议至少像这样重构books()方法以防止引用转义

public Collection<Book> getBooks() {
    Book book1 = new Book("Harry Potter", "J.K. Rowling", 1);
    Book book2 = new Book("Neuromancer", "William Gibson", 2);

    bookDatabase.add(book1);
    bookDatabase.add(book2);
    return Collections.unmodifiableCollection(bookDatabase);
} 

和重构showTitles方法，使其使用java 8功能和更通用的功能：

public void showTitles(Consumer<Book> consumer) {
    LibraryDatabase libraryDatabase = new LibraryDatabase();
    Scanner kbMeter = new Scanner(System.in);
    String genre = kbMeter.nextLine();
    libraryDatabase.getBooks()
                .stream()
                .filter(b -> b.getBookGenre().equals(genre))
                .forEach(consumer);

}

并像这样使用此API：

LibraryDatabase db = new LibraryDatabase();
db.showTitles(b -> System.out.println(String.format("%s, %s, %s", b.getBookAuthor(), b.getBookTitle(), b.getBookGenre())));