将变量传递给扩展类和调用方法

Question

我现在完全不知所措。我正在开发一个程序，该程序将显示两个储蓄账户的Months、Account #和Balance，并在账户利息累积时更新Balance：

• 第一个账户的利息为每月10,002利息，年利率为1.2%。
• 第二个账户的利息为10,003利息，按季度计，年利息为4%。

为了做到这一点，我必须设计四个单独的类。SavingsAccount，SavingsAccountDriver，FlexibleSavingsAccount和CDSavingsAccount。SavingsAccount是FlexibleSavingsAccount和CDSavingsAccount的父类。SavingsAccountDriver是主要的类。

在SavingsAccount中，我有一个在SavingsAccountDriver中调用的方法setAnnualInterestRate()。此方法设置每个帐户的利率。我遇到的问题是将这个值传递给扩展类FlexibleSavingsAccount和CDSavingsAccount，以便通过为每个帐户添加利率来更新余额。如果有人能帮我解决这个问题，我会非常感激的。

SavingsAccountDriver

public class SavingsAccountDriver {
    public static void main (String[] args) {
        SavingsAccount saver1 = new SavingsAccount(10002, 2000); //create new SavingsAccount object
        SavingsAccount saver2 = new SavingsAccount(10003, 3000); //create new SavingsAccount object

        saver1.setAnnualInterestRate(.012); //sets AnnualInterestRate for 'saver1' object
        saver2.setAnnualInterestRate(.04); //sets AnnualInterestRate for 'saver2' object

        System.out.println("\nMonthly balances:\n");
        System.out.println("Month " + " Account# " + " Balance " + "      " + " Month " + " Account# " + " Balance ");
        System.out.println("----- " + " -------- " + " ------- " + "      " + " ----- " + " -------- " + " ------- ");


        System.out.println(saver1.getAccountNumber() + " / " + saver1.getBalance() + " / " + saver1.getInterest());
        System.out.println(saver2.getAccountNumber() + " / " + saver2.getBalance() + " / " + saver2.getInterest());

        /*for(int month = 0; month <= 12; month++) {
            switch(month) { // switch that outputs month, account number, and balance for both accounts (Some non-needed cases used to make output look cleaner)
                case 0:
                    System.out.println(month + "      " + saver1.getAccountNumber() + "     " + saver1.getBalance() + "         " + month + "      " + saver2.getAccountNumber() + "     " + saver2.getBalance());
                    break;
                case 4: 
                    saver1.addInterest();
                    //saver2.addInterest();

                    System.out.println(month + "      " + saver1.getAccountNumber() + "     " + saver1.getBalance() + "         " + month + "      " + saver2.getAccountNumber() + "     " + saver2.getBalance());
                    break;
                case 10:
                    saver1.addInterest();
                    //saver2.addInterest();

                    System.out.println(month + "     " + saver1.getAccountNumber() + "     " + saver1.getBalance() + "        " + month + "     " + saver2.getAccountNumber() + "     " + saver2.getBalance());
                    break;
                case 11:
                    saver1.addInterest();
                    //saver2.addInterest();

                    System.out.println(month + "     " + saver1.getAccountNumber() + "     " + saver1.getBalance() + "        " + month + "     " + saver2.getAccountNumber() + "     " + saver2.getBalance());
                    break;
                case 12:
                    saver1.addInterest();
                    //saver2.addInterest();

                    double totalBalance = saver1.getBalance() + saver2.getBalance();
                    System.out.println(month + "     " + saver1.getAccountNumber() + "     " + saver1.getBalance() + "        " + month + "     " + saver2.getAccountNumber() + "     " + saver2.getBalance());
                    break;
                default:
                    saver1.addInterest();
                    //saver2.addInterest();

                    System.out.println(month + "      " + saver1.getAccountNumber() + "     " + saver1.getBalance() + "        " + month + "      " + saver2.getAccountNumber() + "     " + saver2.getBalance());
                    break;
            }
        }*/
    }
}

SavingsAccount

public class SavingsAccount {
    // variables specific to SavingsAccount class
    public double annualInterestRate;
    private final int ACCOUNT_NUMBER;
    public double balance;

    //constructor with account number and balance parameters
    public SavingsAccount(int account_number, double balance) {
        this.ACCOUNT_NUMBER = account_number;
        this.balance = balance;
    }

    //returns account number
    public int getAccountNumber() {
        return this.ACCOUNT_NUMBER;
    }

    //returns balance
    public double getBalance() {
        return this.balance;
    }

    //sets interest rate
    public void setAnnualInterestRate (double interestRate) {
        this.annualInterestRate = interestRate;
    }
}

FlexibleSavingsAccount

public class FlexibleSavingsAccount extends SavingsAccount{

        public FlexibleSavingsAccount(int account_number, double balance) {
            super(account_number, balance);
        }

        //returns interest
        public double getInterest() {
            return annualInterestRate;  
        }
}

Answer 1

您可能希望添加getInterest作为抽象方法，并将SavingsAccount声明为抽象类。然后，编译器将调用已知的方法，并在子类中强制提供适当的实现。

Answer 2

如果要将getInterest()和saver2声明为SavingsAccount类型，则应该在父类SavingsAccount中声明saver1方法。

按照显示的方式，该方法只对声明为FlexibleSavingsAccount的类可用。

由于您将它们声明为SavingsAccount，所以您只能访问该类的方法。

子类可以访问父类的方法，但不能访问父类的方法。

Answer 3

您已经将实例化的储蓄帐户声明为SavingsAccount类型。它们将无法访问getInterest()方法，因为它位于子类FlexibleSavingsAccount中。

您需要将它们实例化为您希望它们实际成为的实际储蓄帐户：

FlexibleSavingsAccount saver1 = new FlexibleSavingsAccount(10002, 2000); //create new FlexibleSavingsAccount object

现在，saver1将能够访问getInterest()。

额外想法：

更好的方法是将父SavingsAccount编码为一个接口。然后您将在这个接口中声明您的getInterest()方法，同时将方法中的细节留给您的子类。

SavingsAccount：

public interface SavingsAccount {
    public int getAccountNumber();
    public double getBalance();
    public void setAnnualInterestRate (double interestRate);
 }

然后实例化您的帐户：

SavingsAccount saver1 = new FlexibleSavingsAccount(10002, 2000); //create new FlexibleSavingsAccount object

注意，这有一个额外的好处，您可以将实例声明为SavingsAccount接口，这对于将来验证代码总是一个好主意。