為什么我的 ArrayList 中的對象屬性返回 null？

Question

我正在為 Java 中的圖書館（書籍）編寫代碼。 我有一本書 class 具有三個屬性：標題、作者和流派。 Genre 存儲為int ，因為沒有那么多：

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

  public  int genre;
  
  public Book() {
    this.title = title;
    this.author = author;
    this.genre = genre;
  }
  public void setBookTitle(String newBookTitle) {
    this.title = newBookTitle;
  }
  public void setBookAuthor(String newBookAuthor) {
    this.author = newBookAuthor;
  }
  public void setBookGenre(int newBookGenre) {
    this.genre = newBookGenre;
  }

  public String getBookTitle() {
    return  title;
  }
  public String getBookAuthor() {
    return author;
  }
  public int getBookGenre() {
    return genre;
  }
  public String toString() {
    LibraryDatabase libraryDatabase = new LibraryDatabase();
    
    return ("Title: "+this.getBookTitle() +
            "Author: " +this.getBookAuthor()+"Genre: " + this.getBookGenre() //edit this part to refer the integer to a String
            );
  }

  
}

我有一個圖書館數據庫 class 用於存儲 ArrayList 書籍：

    import java.util.*;
    
    public class LibraryDatabase extends Book {
    ArrayList<Book> bookDatabase;
      public LibraryDatabase() {
       
      }
      public void books() {
        Book book1 = new Book();
    
        book1.setBookTitle("Harry Potter");
        book1.setBookAuthor("J.K. Rowling");
        book1.setBookGenre(1);
      }
    
      public ArrayList<Book> getArrayList() {
        return bookDatabase;
      }
      
    }

最后，我有一個客戶 class 基本上只是將以上所有內容組織成一個用戶友好的界面：

import java.io.*;
import java.util.*;

public class Customer {
  
  public Customer() {
  }

  public void run() {
    System.out.print("Welcome to Generic Library Service. Please enter your name.");
    Scanner kbEater = new Scanner(System.in);
    String customerName = kbEater.nextLine();
    System.out.println("Would you like to borrow, return, or donate a book?");
    System.out.println("    1. Borrow");
    System.out.println("    2. Return");
    System.out.println("    3. Donate");
    Scanner kbNeeder = new Scanner(System.in);
    int customerChoice = kbEater.nextInt();
    switch (customerChoice) {
      case 1: borrow();
      break;
      //case 2: returnBook();
      //case 3: donate();
    }
  }
  public void printGenres() {
    
    System.out.println("    1. Fantasy");
    System.out.println("    2. Science Fiction");
    System.out.println("    3. Dystopian");
    System.out.println("    4. Action & Adventure");
    System.out.println("    5. Mystery");
    System.out.println("    6. Horror");
    System.out.println("    7. Thriller & Suspense");
    System.out.println("    8. Historical Fiction");
    System.out.println("    9. Romance");
    System.out.println("    10. Graphic Novel");
    System.out.println("    11. Young Adult");
    System.out.println("    12. Children's");
    System.out.println("    13. Memoir & Biography");
    System.out.println("    14. Food");
    System.out.println("    15. Art & Photography");
    System.out.println("    16. Self-improvement");
    System.out.println("    17. History");
    System.out.println("    18. Travel");
    System.out.println("    19. Humor");
    System.out.println("    20. How-to");
    System.out.println("    21. Science & Tech");
  }
  
  public void borrow() {
    System.out.println("What is the genre of the book you're looking for?");
    printGenres();
    Scanner kbMeter = new Scanner(System.in);
    int genre = kbMeter.nextInt();
    System.out.println("Do you want to see titles, or are you looking for a particular author?");
    System.out.println("    1. Titles now");
    System.out.println("    2. Search by author");
    Scanner kbLeader = new Scanner(System.in);
    int titleOrSearch = kbLeader.nextInt();
    switch(titleOrSearch) {
      case 1:
      titlesNow();
      break;
      case 2:
      //method for searching by author
    }

  }
  public void titlesNow() {
    LibraryDatabase libraryDatabase = new LibraryDatabase("1", "1", 2);
    System.out.println(toString());

  }
  
}

這是 output：

Welcome to Generic Library Service. Please enter your name. **Username**
Here are our books:
Title: nullAuthor: nullGenre: 0

為什么會這樣？ 為什么返回 null 的屬性？ 我想列出 ArrayList 書中所有書籍的標題、作者和類型。 因為目前只有一本書，所以應該打印出那本書（JK羅琳的哈利波特）。

Answer 1

你的問題在這里，在構造函數中：

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

您正在為自己分配title, author, genre ，這些都是null 。 要解決這個問題，只需將一些參數傳遞給構造函數：

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

一切都應該正常工作，而不必使用 setter 方法。

打印錯誤是因為您沒有將book1保存在任何地方。 在 LibraryDatabase 的構造函數中，添加LibraryDatabase bookDatabase = new ArrayList<>(); 在books() function 的末尾，添加bookDatabase.add(book1); .

另外，請注意您的LibraryDatabase在您的toString()方法中沒有做任何事情。

Answer 2

如下更改您的 LibraryDatabase

    import java.util.*;
    
    public class LibraryDatabase extends Book {
    ArrayList<Book> bookDatabase;
      public LibraryDatabase() {
       
      }
      public void books() {
        Book book1 = new Book();
    
        book1.setBookTitle("Harry Potter");
        book1.setBookAuthor("J.K. Rowling");
        book1.setBookGenre(1);
        bookDatabase.add(book1);
      }
    
      public ArrayList<Book> getArrayList() {
        return bookDatabase;
      }
      
    }

將該book1添加到您的 arraylist 中。 還要添加@null_awe 代碼。 你也錯過了。

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