这是我得到的例外：
Exception in thread "main" java.lang.NullPointerException
at BankAccountDemo.DisplayAccountFees(BankAccountDemo.java:91)
at BankAccountDemo.main(BankAccountDemo.java:30)
每当我尝试打印或查看评估费用的所有帐户时，我也会收到此异常。
这是我的驱动程序。它有5个类，也在下面。其中一个是带有3个后代的抽象类。
DRIVER
import java.io.*;
import java.util.Scanner;
import java.util.ArrayList;
public class BankAccountDemo
{
public static void main(String[] args)
{
while(true) {
Scanner keyboard = new Scanner(System.in);
System.out.println("=========Menu========");
System.out.println("1. Add an account.");
System.out.println("2. To print the names of all accounts together with the fees assessed");
System.out.println("3. To print all accounts together with owner information");
System.out.println("4. To exit program");
System.out.println("Your choice: ");
int input = keyboard.nextInt();
switch(input)
{
case 1:
InputAccount();
break;
case 2:
DisplayAccountFees();
break;
case 3:
ShowAccount();
break;
default:
PrintToFile();
System.exit(0);
}
}
}
private static void InputAccount()
{
System.out.println("Please enter user data as prompted...");
System.out.println("Please enter account type: Press '1' for Checking, '2' for Savings, '3' for Loan");
System.out.println("Account Type: ");
Scanner keyboard = new Scanner(System.in);
int type = keyboard.nextInt();
switch(type)
{
case 1:
{
Checking aChecking = new Checking();
aChecking.AddAccount();
checkAccounts.add(aChecking);
break;
}
case 2:
{
Savings aSavings = new Savings();
aSavings.AddAccount();
savingsAccounts.add(aSavings);
break;
}
case 3:
{
Loan aLoan = new Loan();
aLoan.AddAccount();
loanAccounts.add(aLoan);
break;
}
}
}
private static void DisplayAccountFees()
{
for (int n=0; n < savingsAccounts.size(); n++)
{
Savings aSavings = savingsAccounts.get(n);
Person aPerson = aSavings.getAccountHolder();
System.out.println("Total Fees for: " +aPerson.getPersonName());
System.out.println("with the account number: " +aSavings.getAcctNumber());
System.out.println("are: " + aSavings.accountFee());
}
for (int n=0; n < checkAccounts.size(); n++)
{
Checking aChecking = checkAccounts.get(n);
Person aPerson = aChecking.getAccountHolder();
System.out.println("Total Fees for: " +aPerson.getPersonName());
System.out.println("with the account number: " +aChecking.getAcctNumber());
System.out.println("are: " + aChecking.accountFee());
}
for(int n=0; n < loanAccounts.size(); n++)
{
Loan aLoan = loanAccounts.get(n);
Person aPerson = aLoan.getAccountHolder();
System.out.println("Total Fees for: " +aPerson.getPersonName());
System.out.println("with the account number: " +aLoan.getAcctNumber());
System.out.println("are: " + aLoan.accountFee());
}
}
private static void ShowAccount()
{
for(int n=0; n < savingsAccounts.size(); n++)
{
Savings aSavings = savingsAccounts.get(n);
aSavings.DisplayData();
}
for(int n=0; n < checkAccounts.size(); n++)
{
Checking aChecking = checkAccounts.get(n);
aChecking.DisplayData();
}
for(int n=0; n < loanAccounts.size(); n++)
{
Loan aLoan = loanAccounts.get(n);
aLoan.DisplayData();
}
}
private static void PrintToFile()
{
try
{
FileOutputStream fileOutput = new FileOutputStream("Accounts.dat");
ObjectOutputStream printFile = new ObjectOutputStream(fileOutput);
for (int n=0; n < loanAccounts.size(); n++)
{
Loan aLoan = loanAccounts.get(n);
aLoan.PrintAccountToFile(printFile);
}
for (int n=0; n < checkAccounts.size(); n++)
{
Checking aChecking = checkAccounts.get(n);
aChecking.PrintAccountToFile(printFile);
}
for (int n=0; n < savingsAccounts.size(); n++)
{
Savings aSavings = savingsAccounts.get(n);
aSavings.PrintAccountToFile(printFile);
}
printFile.close();
fileOutput.close();
}
catch(IOException ex)
{
}
}
private static ArrayList<Checking> checkAccounts = new ArrayList<Checking>();
private static ArrayList<Savings> savingsAccounts = new ArrayList<Savings>();
private static ArrayList<Loan> loanAccounts = new ArrayList<Loan>();
}
帐户类
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.Scanner;
public abstract class Account {
private String bankName;
private String bankAddress;
private String acctNumber;
private String acctType;
private double balance;
private Person accountHolder;
public Account()
{
}
public Person setAccountHolder(Person holder)
{
return holder;
}
public void setBankName(String newBankName)
{
bankName = newBankName;
}
public void setAddress(String newBankAddress)
{
bankAddress = newBankAddress;
}
public void setAcctNumber(String newAcctNumber)
{
acctNumber = newAcctNumber;
}
public void setBalance(double newBalance)
{
balance = newBalance;
}
public void setAcctType(String newAcctType)
{
acctType = newAcctType;
}
public Person getAccountHolder()
{
return accountHolder;
}
public String getBankName()
{
return bankName;
}
public String getAddress()
{
return bankAddress;
}
public String getAcctNumber()
{
return acctNumber;
}
public String getAcctType()
{
return acctType;
}
public double getBalance()
{
return balance;
}
public abstract double accountFee();
public abstract void DisplayData();
public abstract void AddAccount();
public abstract void PrintAccountToFile(ObjectOutputStream printFile);
public void readInputAccount()
{
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter the name of your bank: ");
bankName = keyboard.nextLine();
System.out.println("Enter the address of your bank: ");
bankAddress = keyboard.nextLine();
System.out.println("Enter your account number: ");
acctNumber = keyboard.nextLine();
System.out.println("Enter your current balance: ");
balance = keyboard.nextDouble();
}
public void PrintBankInfo(ObjectOutputStream printFile)
{
try
{
printFile.writeUTF("------------------------------------------");
printFile.writeUTF(" Bank information: ");
printFile.writeUTF("------------------------------------------");
printFile.writeUTF("Bank name: " + getBankName());
printFile.writeUTF("Bank Address: " + getAddress());
}
catch(IOException ex)
{
}
}
public void DisplayBankInfo()
{
System.out.println("------------------------------------------");
System.out.println(" Bank information: ");
System.out.println("------------------------------------------");
System.out.println("Bank name: " + getBankName());
System.out.println("Bank Address: " + getAddress());
}
}
检查课程
import java.util.Scanner;
import java.io.*;
public class Checking extends Account {
private double monthFee;
private int checksAllowed;
private int checksUsed;
public Checking()
{
}
public double accountFee()
{
int tooManyChecks = checksUsed - checksAllowed;
double fee = monthFee + (3.00 * tooManyChecks);
return fee;
}
public double getMonthFee()
{
return monthFee;
}
public int getChecksAllowed()
{
return checksAllowed;
}
public int getChecksUsed()
{
return checksUsed;
}
public void setMonthFee(double newMonthFee)
{
monthFee = newMonthFee;
}
public void setChecksAllowed(int newChecksAllowed)
{
checksAllowed = newChecksAllowed;
}
public void setChecksUsed(int newChecksUsed)
{
checksUsed = newChecksUsed;
}
public void AddAccount()
{
Person aPerson = new Person();
aPerson.personInput();
setAccountHolder(aPerson);
readInputAccount();
Scanner keyboard = new Scanner(System.in);
System.out.println("Please enter the monthly fee: ");
monthFee = keyboard.nextDouble();
System.out.println("Please enter the number of free checks allowed: ");
checksAllowed = keyboard.nextInt();
System.out.println("Please enter the number of checks used: ");
checksUsed = keyboard.nextInt();
}
public void DisplayData()
{
Person aPerson = getAccountHolder();
aPerson.DisplayPersonInfo();
DisplayBankInfo();
System.out.println("------------------------------------------");
System.out.println(" Account information: ");
System.out.println("Account Type : Checking");
System.out.println("Bank account number :" + getAcctNumber());
System.out.println("Account balance :" + getBalance());
System.out.println("Monthly fee :" + getMonthFee());
System.out.println("Number of checks used :" + getChecksUsed());
System.out.println("Number of free checks :" + getChecksAllowed());
}
public void PrintAccountToFile(ObjectOutputStream printFile)
{
try
{
Person aPerson = getAccountHolder();
aPerson.PrintInfo(printFile);
PrintBankInfo(printFile);
printFile.writeUTF("------------------------------------------");
printFile.writeUTF(" Account information: ");
printFile.writeUTF("Account Type : Checking");
printFile.writeUTF("Bank account number :" + getAcctNumber());
printFile.writeUTF("Account balance :" + getBalance());
printFile.writeUTF("Monthly fee :" + getMonthFee());
printFile.writeUTF("Number of checks used :" + getChecksUsed());
printFile.writeUTF("Number of free checks :" + getChecksAllowed());
printFile.writeUTF("Fees accessed : " + accountFee());
}
catch(IOException ex)
{
}
}
}
贷款类
import java.util.Scanner;
import java.io.*;
public class Loan extends Account {
private double monthPayment;
private int daysOverDue;
public Loan()
{
}
public void setMonthPayment(double newMonthPayment)
{
monthPayment = newMonthPayment;
}
public void setDaysOverDue(int newDaysOverDue)
{
daysOverDue = newDaysOverDue;
}
public double getMonthPayment()
{
return monthPayment;
}
public int getDaysOverDue()
{
return daysOverDue;
}
public double accountFee()
{
double fee = 0.001 * monthPayment * daysOverDue;
return fee;
}
public void AddAccount(){
Scanner keyboard = new Scanner(System.in);
Person aPerson = new Person();
aPerson.personInput();
setAccountHolder(aPerson);
readInputAccount();
System.out.println("Please enter the monthly payment amount: ");
monthPayment = keyboard.nextDouble();
System.out.println("Please enter the number of days past the grace period: ");
daysOverDue = keyboard.nextInt();
}
public void DisplayData()
{
Person aPerson = getAccountHolder();
aPerson.DisplayPersonInfo();
}
public void PrintAccountToFile(ObjectOutputStream printFile)
{
try
{
Person aPerson = getAccountHolder();
aPerson.PrintInfo(printFile);
printFile.writeUTF("------------------------------------------");
printFile.writeUTF(" Account information: ");
printFile.writeUTF("Account Type: Loan");
printFile.writeUTF("Bank account number: " + getAcctNumber());
printFile.writeUTF("Account balance: " + getBalance());
printFile.writeUTF("Monthly payment amount: " + getMonthPayment());
printFile.writeUTF("Number of days past the grace period: " + getDaysOverDue());
printFile.writeUTF("Fees assessed: " + accountFee());
}
catch(IOException ex)
{
}
}
}
SAVINGS CLASS
import java.io.*;
import java.util.Scanner;
public class Savings extends Account {
private double minBal;
private double intRate;
private double avgBal;
public Savings()
{
}
public double accountFee()
{
double fee = 0.00;
if (avgBal < minBal)
{
fee = 50.00;
}
return fee;
}
public double getMinBal()
{
return minBal;
}
public double getIntRate()
{
return minBal;
}
public double getAvgBal()
{
return avgBal;
}
public void setMinBal(double newMinBal)
{
minBal = newMinBal;
}
public void setIntRate(double newIntRate)
{
minBal = newIntRate;
}
public double getChecks()
{
return intRate;
}
public void setChecks(double newIntRate)
{
intRate = newIntRate;
}
public void setAvgBal(double newAvgBal)
{
avgBal = newAvgBal;
}
public void AddAccount()
{
Scanner keyboard = new Scanner(System.in);
Person aPerson = new Person();
aPerson.personInput();
setAccountHolder(aPerson);
readInputAccount();
System.out.println("Please enter the minimum balance: ");
minBal = keyboard.nextDouble();
System.out.println("Please enter the average balance: ");
avgBal = keyboard.nextDouble();
System.out.println("Please enter interest rate");
intRate = keyboard.nextDouble();
}
public void DisplayData()
{
Person aPerson = getAccountHolder();
aPerson.DisplayPersonInfo();
DisplayBankInfo();
System.out.println("------------------------------------------");
System.out.println(" Account information: ");
System.out.println("Account Type: Savings Account");
System.out.println("Bank account number: " + getAcctNumber());
System.out.println("Account balance: " + getBalance());
System.out.println("Minimum balance: " + getMinBal());
System.out.println("Average balance: " + getAvgBal());
System.out.println("Interest rate: " + getIntRate());
}
public void PrintAccountToFile(ObjectOutputStream printFile)
{
try
{
Person aPerson = getAccountHolder();
aPerson.PrintInfo(printFile);
PrintBankInfo(printFile);
printFile.writeUTF("------------------------------------------");
printFile.writeUTF(" Account information: ");
printFile.writeUTF("Account Type: Savings Account");
printFile.writeUTF("Bank account number: " + getAcctNumber());
printFile.writeUTF("Account balance: " + getBalance());
printFile.writeUTF("Minimum balance: " + getMinBal());
printFile.writeUTF("Average balance: " + getAvgBal());
printFile.writeUTF("Interest rate: " + getIntRate());
printFile.writeUTF("Fees assessed: " + accountFee());
}
catch(IOException ex)
{
}
}
}
答案 0 :(得分：6)
问题是您的setAccountHolder方法实际上没有设置任何内容：
而不是：
public Person setAccountHolder(Person holder)
{
return holder;
}
它应该是：
public void setAccountHolder(Person holder) {
this.accountHolder = holder;
}
答案 1 :(得分：5)
1。）您在哪里初始化 savingsAccounts 属性？
2。）当你这样做时
DisplayAccountFees()
你开始做
Savings aSavings = savingsAccounts.get(n);
你确定get（n）总是返回数据吗？