java - 如何使用 toString 方法打印第三个对象中的两个对象？

Question

我在作业时遇到问题，因为我对 java 对象的理解有缺陷。我创建了 3 个类。第一个类创建一个对象来保存组成全名的值，第二个类是 MailingAddress 类，它扩展 FullName 类并通过 super() 方法利用继承。第三类是运输标签类，它需要 2 个 MailingAddress 对象...一个对象表示为 ShipTo，另一个表示为 ShipFrom，并具有单一方法 printLabel()。所有对象都会重写 toString() 方法进行输出，下面是我的代码和输出:

public class FullName {

    private String title;
    private String firstName;
    private String middleName;
    private String lastName;

    FullName(){
        title = "";
        firstName = "";
        middleName = "";
        lastName = "";
    }
    FullName(String title, String first, String middle, String last){
        super();
        this.title = title;
        this.firstName = first;
        this.middleName = middle;
        this.lastName = last;
    }
    public String toString(){
        return title + ". " + firstName + " " + middleName + " " + lastName + "\n";
    }
}// end of class

public class MailingAddress extends FullName{

    private String streetAddress, city, province, postal;
    private FullName fn = new FullName();

    MailingAddress(){
        super("","","","");
        streetAddress = "";
        city = "";
        province = "";
        postal = "";
    }

    MailingAddress(String title, String first, String middle, String last, String street, String city, String prov, String postal){
        super(title, first, middle, last);
        this.streetAddress = street;
        this.city = city;
        this.province = prov;
        this.postal = postal;
    }

    public String toString(){
        return fn.toString() + " " + streetAddress + "\n" + city + ", " + province + "\n" + postal;
    }
}// end of class

public class ShippingLabel extends MailingAddress{
    private MailingAddress shipTo;
    private MailingAddress shipFrom;

    private ShippingLabel(MailingAddress shTo, MailingAddress shipFm){
        this.shipTo = shTo;
        this.shipFrom = shipFm;
    }
    private void printLabel(){
        ShippingLabel label = new ShippingLabel(shipTo, shipFrom);
        System.out.println(label.toString());
    }
    public String toString(){
        return "Ship to:" + shipTo + "\n" + "\n" + "Ship From: " + shipFrom;
    }
    public static void main(String[] args){
        MailingAddress shTo = new MailingAddress("Mr","Jonathan","Daniel", "O'Connor","109732","Edmonton", "Alberta", "T5m1K3");
        MailingAddress shipFm = new MailingAddress("Mr","Piercy","Michael","Miller","23 Oreville way","Calgary","Alberta","C3p5o1");
        ShippingLabel sL = new ShippingLabel(shTo,shipFm);
        sL.printLabel();
    } // end of main
}// end of class

输出:

运送至:。
109732艾伯塔省埃德蒙顿T5m1K3

发货地点: .
23 奥雷维尔路艾伯塔省卡尔加里C3p5o1

进程已完成，退出代码为 0

我不明白的是为什么使用 FullName 构造函数的值没有打印在输出中。我希望类 FullName 和 MailingAddress 中的 toString 方法能够协作组成 ShippingLabel。希望通过查看我的代码，您可以了解我的思维过程，并解释它的缺陷所在。

Answer 1

问题出在您的 MailingAddress.toString() 函数中。

您没有使用正确的对象来调用toString()函数。由于 MailingAddress 是从 FullName 继承的，因此您不需要创建 FullName 类型的新成员。您应该使用 super 关键字继承 FullName 的 toString()。

以下是更正后的版本。请参阅the complete code working here :

public String toString()
{
    return super.toString() + " " + streetAddress + "\n" + city + ", " + 
                 province + "\n" + postal;
}

[注意:MailingAddress 和 FullName 之间的关系是 has-a 而不是 is a 即 MailingAddress 有一个 FullName 但 MailingAddress 不是 FullName。上面的解决方案是使用 is-a 关系。请参阅此处 solution using has-a relationship 。 ]