求助

Question

我写了下面的关于银行账号的程序但运行有错求帮助：
package exercisel.banking;
public class Account {
   protected  double balance;    public Account(double init_balance) {
        this.balance = init_balance;
    }    protected  double getBalance() {
        return balance;
    }    public boolean deposit(double amt)
    {
        balance+=amt;
        return  true;    }
    public boolean withdraw(double amt)
    {
        if(amt<=balance)
        {
          balance-=amt;
          return true;
        }
     else
         return false;
    }}package exercisel.banking;
public class Bank {
    private Customer[] customers;
    private static int numberOfCustomers;    public Bank() {
        customers = new Customer[5];
    }    public void addCustomer(String firstName, String LastName) {
        Customer customer = new Customer(firstName, LastName);
        customers[numberOfCustomers] = customer;
        numberOfCustomers++;    }    public static int getNumberOfCustomers() {
        return numberOfCustomers;
    }    public Customer getCustomers(int customer_index) {
        return customers[customer_index];
    }
}
package exercisel.banking;public class CheckingAccount extends Account {    private double overdraftProtection;    public CheckingAccount(double balance) {
        super(balance);    }    public CheckingAccount(double balance, double protect) {
        super(balance);
        this.overdraftProtection = protect;
    }    @Override
    public boolean withdraw(double amt) {
        if (balance >=amt) {
            balance-=amt;
            return true;
        } else 
            {
               if(amt-balance<=overdraftProtection)
               {
                   balance-=amt;
                   return  true;
               }
            else
                return
                   false;
            }
                
            
        
    }}package exercisel.banking;
public class Customer {
    private Account account;
    private String firstname,Lastname;    public Customer(String firstname, String Lastname) {
        this.firstname = firstname;
        this.Lastname = Lastname;
    }    public Account getAccount() {
        return account;
    }
    public void setAccount(Account account)
    {
       this.account=account;    }    public String getLastname() {
        return Lastname;
    }    public String getFirstname() {
        return firstname;
    }    
}package exercisel.banking;public class SavingsAccount extends Account{     private double interestRate;    public SavingsAccount(double balance, double interestRate) {
        super(balance);    }
}package exercisel.banking;public class TestBanking {    public static void main(String[] args) {
        Bank bank = new Bank();
        Customer customer;
        Account account;        //
        // Create bank customers and their accounts
        //        System.out.println("Creating the customer Jane Smith.");
        bank.addCustomer("Jane", "Simms");
        //code
        customer = bank.getCustomers(0);
        System.out.println("Creating her Savings Account with a 500.00 balance and 3% interest.");
        //code
        SavingsAccount savingsaccount = new SavingsAccount(500.00, 0.03);
        System.out.println("Creating the customer Owen Bryant.");
        //code
        bank.addCustomer("Owen", "Bryant");
        customer = bank.getCustomers(1);
        System.out.println("Creating his Checking Account with a 500.00 balance and no overdraft protection.");
        //code
        CheckingAccount checkingaccount1 = new CheckingAccount(500.00);
        System.out.println("Creating the customer Tim Soley.");
        bank.addCustomer("Tim", "Soley");
        customer = bank.getCustomers(2);
        System.out.println("Creating his Checking Account with a 500.00 balance and 500.00 in overdraft protection.");
        //code
        CheckingAccount checkingaccount2 = new CheckingAccount(500.00, 500.00);
        System.out.println("Creating the customer Maria Soley.");
        //code
        bank.addCustomer("Maria", "Soley");
        customer = bank.getCustomers(3);
        System.out.println("Maria shares her Checking Account with her husband Tim.");
        customer.setAccount(bank.getCustomers(2).getAccount());        System.out.println();        //
        // Demonstrate behavior of various account types
        //        // Test a standard Savings Account
        System.out.println("Retrieving the customer Jane Smith with her savings account.");
        customer = bank.getCustomers(0);
        account = customer.getAccount();
        // Perform some account transactions
        System.out.println("Withdraw 150.00: " + account.withdraw(150.00));
        System.out.println("Deposit 22.50: " + account.deposit(22.50));
        System.out.println("Withdraw 47.62: " + account.withdraw(47.62));
        System.out.println("Withdraw 400.00: " + account.withdraw(400.00));
        // Print out the final account balance
        System.out.println("Customer [" + customer.getLastname()
                + ", " + customer.getFirstname()
                + "] has a balance of " + account.getBalance());        System.out.println();        // Test a Checking Account w/o overdraft protection
        System.out.println("Retrieving the customer Owen Bryant with his checking account with no overdraft protection.");
        customer = bank.getCustomers(1);
        account = customer.getAccount();
        // Perform some account transactions
        System.out.println("Withdraw 150.00: " + account.withdraw(150.00));
        System.out.println("Deposit 22.50: " + account.deposit(22.50));
        System.out.println("Withdraw 47.62: " + account.withdraw(47.62));
        System.out.println("Withdraw 400.00: " + account.withdraw(400.00));
        // Print out the final account balance
        System.out.println("Customer [" + customer.getLastname()
                + ", " + customer.getFirstname()
                + "] has a balance of " + account.getBalance());        System.out.println();        // Test a Checking Account with overdraft protection
        System.out.println("Retrieving the customer Tim Soley with his checking account that has overdraft protection.");
        customer = bank.getCustomers(2);
        account = customer.getAccount();
        // Perform some account transactions
        System.out.println("Withdraw 150.00: " + account.withdraw(150.00));
        System.out.println("Deposit 22.50: " + account.deposit(22.50));
        System.out.println("Withdraw 47.62: " + account.withdraw(47.62));
        System.out.println("Withdraw 400.00: " + account.withdraw(400.00));
        // Print out the final account balance
        System.out.println("Customer [" + customer.getLastname()
                + ", " + customer.getFirstname()
                + "] has a balance of " + account.getBalance());        System.out.println();        // Test a Checking Account with overdraft protection
        System.out.println("Retrieving the customer Maria Soley with her joint checking account with husband Tim.");
        customer = bank.getCustomers(3);
        account = customer.getAccount();
        // Perform some account transactions
        System.out.println("Deposit 150.00: " + account.deposit(150.00));
        System.out.println("Withdraw 750.00: " + account.withdraw(750.00));
        // Print out the final account balance
        System.out.println("Customer [" + customer.getLastname()
                + ", " + customer.getFirstname()
                + "] has a balance of " + account.getBalance());    }
}
请帮我写明原因谢谢

Answer 1

是因为你的TestBanking这个类中，account = customer.getAccount();这句代码里得到的account就是空的

Answer 2

在你的bank类中的
          public Customer getCustomers(int customer_index) {
  Account a=new Account(100);
  customers[customer_index].setAccount(a);
  return customers[customer_index];
  
  }
因为你的custom中account是没赋值的

Answer 3

   但我不已经通过这个SavingsAccount savingsaccount = new SavingsAccount(500.00, 0.03);为account赋值了吗

Answer 4

在你的代码中没看到这个。就算是有这个，也不对呀。Account是Customer对象的一个属性。你在对Customer对象初始化的时候，并没有对其初始化。但你也可以在使用时对其初始化。但你也没有这样的操作（这个就是5楼给你的建议了）。所以Accout将返回其默认是null