Question

我是Java的新手，目前我正在学习构造函数。所以我有一个class Person { private String name; private double height; Person(String name, double height) { this.name = name; this.height = height; } }类

Book

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

Test

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

Book

我的任务是更改private String author --> private Person author类，以便作者是Person-type而不是String-type，如下所示：

Book

所以public class Book { private Person author; private String title; public Book(String author, String title) { this.author = author; this.title = title; } }类看起来像这样：

Person's

所以基本上它应该从Test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any()类才能使它工作？

Answer 1

您必须先初始化Person对象。

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

然后您可以更改Book课程，如下所示：

public class Book {

    private Person author;
    private String title;

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

最后按如下方式初始化Book对象：

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

Answer 2

所以基本上应该从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;
    }
}

你会称之为：

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");

你甚至可以通过重载构造函数来提出这两个建议实际上，您应该使用的方式是对类客户端最实用的方式。

Answer 3

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课程中再添加一个构造函数，并在将Person字符串author传递给Book

时立即生成新的

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

person

或者，将getName()实例传递到Book并通过public class Book { private Person author; private String title; public Book(Person author, String title) { this.author = author.getName(); this.title = title; } }访问名称：

create table Ressource (
id_ressource serial primary key, 
nom_r text not null, 
url text not null,  
id_personne int not null, 
foreign key(id_personne) references Personne(primary_key_from_personne));
            ^^^^^^^^^^^^^^^^ 
            (you missed it)

Answer 4

author应该是新的Person，其名称来自name构造函数的Book字段。

Answer 5

有两种方法可以解决这个问题：

通过将构造函数的参数类型更改为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;
    }
}

Java将实例类型更改为已创建的类类型

5 个答案: