求助

Question

求助我写了下面的程序但有错：请帮我看一下package exercise2;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 (balance >= amt) {
            balance -= amt;
            return true;
        } else {
            return false;
        }
    }
}
package exercise2;public class SavingsAccount extends Account {    private double interestRate;    public SavingsAccount(double balance, double interestRate) {
        super(balance);
    }
}
package exercise2;public class CheckingAccount extends Account {    private double overdraftProtection;    public CheckingAccount(double balance) {
        super(balance);    }    public CheckingAccount(double balance, double overdraftProtection) {
        super(balance);
        this.overdraftProtection = overdraftProtection;
    }    @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 exercise2;public class Customer {    private Account[] accounts;
    private String firstname, Lastname;
    private static int numberOfAccounts;    public Customer(String firstname, String Lastname) {
        this.firstname = firstname;
        this.Lastname = Lastname;
    }    public Customer() {
        accounts = new Account[3];
    }    public void addAccount(Account account) {
        accounts[numberOfAccounts] = account;
        numberOfAccounts++;
    }    public Account getAccounts(int in_index) {
        return accounts[in_index];
    }    public static int getNumberOfAccounts() {
        return numberOfAccounts;
    }    public String getLastname() {
        return Lastname;
    }    public String getFirstname() {
        return firstname;
    }
}package exercise2;
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 int_index) {
        return customers[int_index];
    }}
package exercise2;import java.text.NumberFormat;public class TextBanking {    public static void main(String[] args) {
        NumberFormat currency_format = NumberFormat.getCurrencyInstance();
        Bank bank = new Bank();
        Customer customer;        // Create several customers and their accounts
        bank.addCustomer("Jane", "Simms");
        customer = bank.getCustomers(0);     //该客户两个卡
        customer.addAccount(new SavingsAccount(500.00, 0.05));
        customer.addAccount(new CheckingAccount(200.00, 400.00));        bank.addCustomer("Owen", "Bryant");
        customer = bank.getCustomers(1);    //该客户一个卡
        customer.addAccount(new CheckingAccount(200.00));        bank.addCustomer("Tim", "Soley");
        customer = bank.getCustomers(2);
        customer.addAccount(new SavingsAccount(1500.00, 0.05));
        customer.addAccount(new CheckingAccount(200.00));   //该客户两个卡        bank.addCustomer("Maria", "Soley");
        customer = bank.getCustomers(3);
        // Maria and Tim have a shared checking account
        customer.addAccount(bank.getCustomers(2).getAccounts(1));//该客户分享上一客户的
        //第二个卡
        customer.addAccount(new SavingsAccount(150.00, 0.05));//该客户自己又单独有一个
        //一共两个        // Generate a report
        System.out.println("\t\t\tCUSTOMERS REPORT");
        System.out.println("\t\t\t================");        for (int cust_idx = 0; cust_idx < bank.getNumberOfCustomers(); cust_idx++) {
            {
                customer = bank.getCustomers(cust_idx);                System.out.println();
                System.out.println("Customer: "
                        + customer.getLastname() + ", "
                        + customer.getFirstname());                for (int acct_idx = 0; acct_idx < customer.getNumberOfAccounts(); acct_idx++) {
                    {
                        Account account = customer.getAccounts(acct_idx);
                        String account_type = "Savings Account";
                        if (account instanceof SavingsAccount) {
                            System.out.println(account_type + ":current balance is" + currency_format.format(account.getBalance()));                        } else {
                            System.out.println("Checking Account" + ":current balance is" + currency_format.format(account.getBalance()));                        }                        System.out.println();
                    }
                }
                // Determine the account type
                /*** Step 1:
                 **** Use the instanceof operator to test what type of account
                 **** we have and set account_type to an appropriate value, such
                 **** as "Savings Account" or "Checking Account".
                 ***/
                // Print the current balance of the account
                /*** Step 2:
                 **** Print out the type of account and the balance.
                 **** Feel free to use the currency_format formatter
                 **** to generate a "currency string" for the balance.
                 ***/
            }
        }
    }
}错误是这样的
Exception in thread "main" java.lang.NullPointerException
        at exercise2.Customer.addAccount(Customer.java:19)
        at exercise2.TextBanking.main(TextBanking.java:15)
Java Result: 1但我不知道是怎么错啦该怎么改请帮我看一下谢谢

Answer 1

//Customer 类下accounts没有实例化
 private Account[] accounts; 

Answer 2

/**Bank类中*/
public void addCustomer(String firstname,String Lastname)
  {
  Customer customer=new Customer(firstname, Lastname);  customers[numberOfCustomers]=customer;
  numberOfCustomers++;  }这个里面红色部分调用的是
public Customer(String firstname, String Lastname) {
  this.firstname = firstname;
  this.Lastname = Lastname;
  }这个构造函数，里面并没有创建Account 数组，即所创建的Customer并没有将Account 数组初始化即是null，那么对该Customer对象调用addAccount方法就会抛空指针异常了，因为方法里面引用的Account 数组是个空数组

Answer 3

我不是已经发通过 public Customer() {
        accounts = new Account[3];
    }对其进行初始化了吗我还是不理解啊

Answer 4

customer.addAccount(new SavingsAccount(500.00, 0.05));这句不已经把它出过去了吗