Question

在构造函数之间以某种方式设置值，然后在Main中调用所述值，它们变为null。代码只是让用户输入书籍作者，标题和流派，然后显示这些值。

package book;
import java.util.Scanner;

public class Book {
    Scanner input = new Scanner(System.in);
    public String author;
    public String title;
    public String genre;

    public Book(String author, String title, String genre) {
        String bookAuthor = author;
        String bookTitle = title;
        String bookGenre = genre;
        System.out.println(bookAuthor + bookTitle + bookGenre);
        System.out.println(author + title + genre);
    }

    public Book() {
        String bookAuthor;
        String bookTitle;
        String bookGenre;
    }

    public void setTitle() {
        System.out.print("What is the title of the book? ");
        title = input.nextLine();
    }

    public void setAuthor() {
        System.out.print("Who wrote this book? ");
        author = input.nextLine();
    }

    public void setGenre() {
        System.out.print("What genre category does this book fall under? ");
        genre = input.nextLine();
    }

    public void getAuthor() {
        System.out.printf("\nAuthor: %s", author);
    }

    public void getTitle() {
        System.out.printf("\nTitle: %s", title);
    }

    public void getGenre() {
        System.out.printf("\nGenre: %s", genre);
    }

    public static void main(String[] args) { //starting point of any Java program
        Book book = new Book(); //create new instance of Book() so that main can modify it
        book.setTitle();
        book.setAuthor();
        book.setGenre();
        Book myBook = new Book(book.author, book.title, book.genre);
        System.out.println(myBook.author + myBook.title + myBook.genre);
        myBook.getAuthor();
        myBook.getTitle();
        myBook.getGenre();
    }
}

Answer 1

你的问题在这里：

public Book(String author, String title, String genre){
    String bookAuthor = author;
    String bookTitle = title;
    String bookGenre = genre;
    System.out.println(bookAuthor + bookTitle + bookGenre);
    System.out.println(author + title + genre);
    }

您正在书籍构造函数中声明本地/堆栈变量并将输入参数分配给它们。然后它们在构造函数退出时消失，因为它们不再在范围内。我认为你的意思是这样做，并将构造函数参数分配给对象数据成员：

public Book(String authorIn, String titleIn, String genreIn){
    author = authorIn;
    title = titleIn;
    genre = genreIn;
    }

此外，那些无效的getter方法没有任何意义。

Answer 2

正如OldProgrammer所说，你不需要隐藏变量，但更重要的是，要让用户交互超出Book类。它应该只封装Book信息，所有用户交互都应该去其他地方。此外，setter和getter应该是真正的setter和getter，即setter方法设置字段和getter方法返回。

例如getter / setter对，请考虑字符串字段foo：

private String foo;

// getter
public String getFoo() {
   return foo;
}

// setter 
public void setFoo(String foo) {
   this.foo = foo;
}

例如，Book的简单版本可能如下所示：

public class SimpleBook {
   private String author;
   private String title;

   public SimpleBook(String author, String title) {
      this.author = author;
      this.title = title;
   }

   // getter method returns the field's value
   public String getAuthor() {
      return author;
   }

   // setter method sets the field's value
   public void setAuthor(String author) {
      this.author = author;
   }
   public String getTitle() {
      return title;
   }
   public void setTitle(String title) {
      this.title = title;
   }

   // so the book can be displayed cleanly
   @Override
   public String toString() {
      return "Book [Author: " + author + ", title: " + title + "]";
   }

}

然后用户交互可以进入测试类：

import java.util.Scanner;

public class TestSimpleBook {
   public static void main(String[] args) {
      Scanner scanner = new Scanner(System.in);

      System.out.print("Enter an Author: ");
      String author = scanner.nextLine();

      System.out.print("Enter an Title: ");
      String title = scanner.nextLine();

      SimpleBook book = new SimpleBook(author, title);

      System.out.println("Your Book: " + book);

      scanner.close();
   }
}

不知何故失去这些数据值Java

2 个答案: