Java 通过将其声明为SavingsAccount，您（编译器）所知道的是它至少是一个SavingsAccount，并且SavingsAccount没有getInterest（）方法。它不能做任何其他的假设，我想我不明白。通过扩展类，您应该可以访问父类的成员。你有

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通过将其声明为

SavingsAccount

，您（编译器）所知道的是它至少是一个

SavingsAccount

，并且

SavingsAccount

没有

getInterest（）

方法。它不能做任何其他的假设，我想我不明白。通过扩展类，您应该可以访问父类的成员。你有错误吗？是的。如果查看我发布的最后一小段代码，getInterest（）方法应该返回对象的AnnualInterest。在本例中，我有两个SavingsAccount类型的对象（“saver1”和“saver2”）。当我运行代码时，我得到以下错误：找不到符号（参考主类saver1.getInterest（）和saver2.getInterest（）中的部分。您将无法从

SavingsAccount

对象中运行

getInterest（）

。

SavingsAccount

没有

getInterest（）

已定义。如果要使用该方法，必须将对象初始化为

FlexibleSavingsAccount

或

CDSavingsAccount

这是为什么？我是否必须声明另外两个FlexibleSavingsAccount类型的对象？我想我可以使用SavingsAccount对象，因为其他类都是从该类扩展而来的。通过将其声明为

SavingsAccount

，您（编译器）只知道它至少是

SavingsAccount

，并且

SavingsAccount

没有

getInterest（）

method。它不能做任何其他假设。不确定你所说的未来验证代码是什么意思？在这里创建一个接口会释放可以包含在基类中的共享行为，并迫使作者编写代码两次，如果需要更改，则需要两次维护，如果需要输入错误，则需要两次？是的ract基类而不是interface可以处理这个问题。我可能会同时做一个接口和一个抽象基类。不确定你所说的未来验证代码是什么意思？在这里创建一个接口会释放基类中可能包含的共享行为，并迫使作者编写两次代码，如果需要，还需要两次维护更改并输入2次错误？是的，一个抽象基类而不是接口将处理此问题。我可能会同时执行接口和抽象基类。

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;
            }
        }*/
    }
}

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;
    }
}

public class FlexibleSavingsAccount extends SavingsAccount{

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

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

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

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

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