Java 将实例类型更改为已创建的类类型

Question

我是 Java 新手，目前正在学习构造函数。所以我有一个Person 类

class Person {

    private String name;
    private double height;

    Person(String name, double height) {
        this.name = name;
        this.height = height;
    }
}

还有一个Book 类

public class Book {

    private String author;
    private String title;

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

还有一个Test 类

public class TestBook {
    public static void main(String[] args) {
        Book harryp = new Book("Rouling", "Harry Potter");
    }
}

我的任务是更改Book 类，使作者是 Person 类型而不是 String 类型，如下所示：

private String author  -->  private Person author

所以Book 类将如下所示：

public class Book {

    private Person author;
    private String title;

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

所以基本上它应该从Person's构造函数中获取作者的名字？或者它是如何工作的？我应该如何修改Test 类才能使其工作？

Answer 1

您必须先初始化 Person 对象。

Person author = new Person("foo", 100d);

然后你可以改变Book类如下：

public class Book {

    private Person author;
    private String title;

    public Book(Person author, final String title) {
        this.author = author;
        this.title = title;
    }
}

最后初始化Book对象如下：

Person author = new Person("foo", 100d);
Book b = new Book(author, "My Title");

Answer 2

解决方案 1：

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

    public Book(String author, String title) {
        this.author = new Person(author);
        this.title = title;
    }
}

class Person {

    private String name;
    private double height;
    public static final double DEFAULT_HEIGHT = 0.0;

    Person(String name, double height) {
        this.name = name;
        this.height = height;
    }

    Person(String name) {
        this(name, DEFAULT_HEIGHT);
    }
}

您必须在Person 类中再添加一个构造函数，并在将字符串author 传递给Book 时实例化一个新的Person

Book b = new Book("Mike", "Book Title");

或者，将 person 实例传递到 Book 并通过 getName() 访问名称：

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

    public Book(Person author, String title) {
        this.author = author.getName();
        this.title = title;
    }
}

Answer 3

所以基本上它应该从 Person 的构造函数中获取作者的名字？

不一定。您可以通过直接在 Book 构造函数中创建 Person 来保持客户端代码的简单。

public class Book {

    private Person author;
    private String title;

    public Book(String authorName, double height, String title) {
        this.author = new Person(authorName, height);
        this.title = title;
    }
}

你会这样称呼它：

Book book = new Book("author", 170, "book title");

Yon 也可以在构造函数中接受 Person 作为参数：

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

在这种情况下，客户端代码应该传递 Person 而不是 String 和 double。

Person author = new Person("author", 170)
Book book = new Book(author, "book title");

您甚至可以通过重载构造函数来提议两者。
事实上，你应该使用的方式是对类客户端最实用的方式。

Answer 4

author 应该是一个新的Person，其名称取自Book 的构造函数的name 字段。

Answer 5

有两种方法可以解决这个问题：

• 通过将构造函数上的参数类型更改为 Person：

  public class Book {
  
      private Person author;
      private String title;
  
      public Raamat(Person author, String title) {
          this.author = author;
          this.title = title;
      }
  }
  

  • 通过在构造函数上初始化作者：

    public class Book {
    
        private Person author;
        private String title;
    
        public Raamat(String author, String title) {
            this.author = new Person(author);
            this.title = title;
        }
    }