Java对象: Method doesn't print as it should

Question

我有两个类(class):公寓和建筑。公寓等级:

public class Apartment {
    private String familyName;
    private int rent;
    private boolean isRented;
}

Building 类有一个 Apartment 数组、一个 numOfApartments 变量(指示该建筑有多少套公寓)以及一个 FINAL 变量(指示一栋建筑中的公寓最大数量)。

private Apartment[] apartments;
private int numOfApartments;
private final int MAX_APARTMENTS=20;

public Building() {
    this.apartments=new Apartment[MAX_APARTMENTS];
    this.numOfApartments=0;
}

现在我想做一个方法，将公寓的租金提高一定的金额作为参数:

public void raiseRent(int amount) {
    for (int i=0; i<numOfApartments; i++) {
        apartments[i].setRent(apartments[i].getRent() + amount);
    }
}

addAppartment 方法:

public boolean addApartment(Apartment a){ //adds an appartment to the building if
                                           //there's enough space in the array.
        if (this.numOfApartments<this.MAX_APARTMENTS)
        {
            this.apartments[this.numOfApartments]=new Apartment(a);
            this.numOfApartments++;
            return true;
        }
        return false;
    }

公寓类字符串ToString方法:

public String toString()
{
    if (this.isRented) //if the apartment is rented, prints:
    {
        return "Apartment rent: " + this.rent + ". rented to " + this.familyName + " family.";
    }
    else
        return "Apartment rent: " + this.rent  + ". not rented yet.";
}

主要是:

String familyName; int rent;
System.out.println("enter family name and rent by this order");
familyName=scan.next();
rent=scan.nextInt();

Apartment a=new Apartment(familyName,rent);  //build new apartment object.
Building b=new Building();
System.out.println(b.addApartment(a));
b.raiseRent(200); //initiate the raiseRent method.

这里的一切都有效，但是当我写这个时:

 System.out.println(a);

租金显示为旧租金，而不是更新后的租金。当我写下这段话时:

System.out.println(b.apartments[0]);

它有效。

为什么当我写 System.out.println(a); 时不起作用？

谢谢您的回答。

Answer 1

在你的

public boolean addApartment(Apartment a) 方法:

this.apartments[this.numOfApartments]=new Apartment(a);

创建一个新公寓，该公寓似乎是由现有公寓 build 而成。这是副本。

提高租金后，

公寓 a=new Apartment(familyName,rent);//构建新的公寓对象。

你传递给

b.addApartment(a)

不会更新租金。这是因为 Java 是按值传递的。它是您的 Building Apartment[] 集合中包含的 new Apartment 中的一个"new"int，即使您从现有公寓中设置它(this.rent = other.rent)