java - java中私有(private)成员变量的子类访问

Question

我真的被 Goodrich 和 Tamassia 的“数据结构和算法”中的这个问题难住了。本书介绍了以下两个类:

public class CreditCard {
private String customer;
private String bank;
private String account;
private int limit;
protected double balance;

public CreditCard(String customer, String bank, String account, int limit, double balance) {
    this.customer = customer;
    this.bank = bank;
    this.account = account;
    this.limit = limit;
    this.balance = balance;

}

public CreditCard(String customer, String bank, String account, int limit) {
    this(customer, bank, account, limit, 0.0);
}

public String getCustomer() { return this.customer; }
public String getBank() { return this.bank; }
public String getAccount() { return this.account; }
public int getLimit() { return this.limit; }
public double getBalance() { return this.balance; }

public boolean charge(double price) {
    if(price + this.balance > this.limit)
        return false;

    this.balance += price;
    return true;
}

public void makePayment(double amount) {
   if( amount < 0)
      System.out.println("Cannot process negative payment");
   else
      this.balance -= amount;
}

public void updateCreditLimit() {
    this.limit -= balance;
}

public static void printSummary(CreditCard card) {
    System.out.println("Customer = " + card.customer);
    System.out.println("Bank = " + card.bank);
    System.out.println("Account = " + card.account);
    System.out.println("Balance = " + card.balance);
    System.out.println("Limit = " + card.limit);
}

}

子类

public class PredatoryCreditCard extends CreditCard {
private double apr;

public PredatoryCreditCard(String customer, String bank, String account,
                           int limit, double balance, double apr) {
                               super(customer, bank, account, limit, balance);
                               this.apr = apr;
                           }

public void processMonth() {
    if(this.balance > 0) {
        double monthlyFactor = Math.pow(1 + apr, 1F/12);
        this.balance *= monthlyFactor;
    }
}

public boolean charge(double price) {
    boolean isSuccess = super.charge(price);
    if(!isSuccess)
        this.balance += 5;
    return isSuccess;
}

}

问题:

假设我们更改 CreditCard 类，以便实例变量 balance 具有私有(private)可见性。为什么 PredatoryCreditCard.charge 方法的以下两个实现存在缺陷？

public boolean charge(double price) {
    boolean isSuccess = super.charge(price);
    if(!isSuccess)
       charge(5); //penalty for late fee
     return isSuccess;
 }

第二个:

public boolean charge(double price) {
    boolean isSuccess = super.charge(price);
    if(!isSuccess)
       super.charge(5); //penalty for late fee
     return isSuccess;
 }

据我所知，子类不能直接操作其父类(super class)的私有(private)字段。该字段必须是 protected 或公共(public)的。这允许我们在子类中说 this.balance += 5。我对原则的理解不是问题，我的问题是我被要求回答的问题。从实现的角度来看，我可以在 PredatoryCreditCard.charge 方法中清楚地看到任何缺陷，因为在问题提供的新实现的两种情况下，我们都会更改类的余额字段，因为调用极好的。我找不到新实现中的缺陷，除非我认为我的继承知识存在一些漏洞。

提前谢谢您。

Answer 1

无论哪种情况，如果您的余额足够接近费用(值(value) 5)将超出您的限额，则不会向您收取费用。在第一种情况下:

public boolean charge(double price) {
    boolean isSuccess = super.charge(price);
    if(!isSuccess)
       charge(5); //penalty for late fee
     return isSuccess;
 }

当 isSuccess 失败时会发生什么？我们陷入无限递归调用 this.charge(...)