我有一个BankAccount类，用于模拟银行帐户信息和一个主要方法，该方法应该从用户输入创建BankAccount对象的数组列表，然后将该信息写入文本文件。
这是我的BankAccount类：
package BankAccount;
public class BankAccount implements Comparable<BankAccount> {
String accountNumber;
String firstName, lastName;
char middleInitial;
double balance;
public static double OVERDRAFT_FEE = 26.00;
public BankAccount(String acno, String fName, char midInit, String lName, double initBal){
acno = accountNumber;
fName = firstName;
midInit = middleInitial;
lName = lastName;
initBal = balance;
}
public String getAccountNumber(){
return accountNumber;
}
public String getFirstName(){
return firstName;
}
public char getMiddleInitial(){
return middleInitial;
}
public String getLastName(){
return lastName;
}
public double getBalance(){
return balance;
}
public void deposit(double amount){
if (amount > 0)
balance = balance + amount;
else
throw new IllegalArgumentException("deposit(double amount): Amount cannot be less than 0.");
}
public void withdraw(double amount){
if (balance >= amount && amount > 0)
balance = balance - amount;
else if(balance < amount && amount > 0)
balance = balance - amount - OVERDRAFT_FEE;
else
throw new IllegalArgumentException("withdraw(double amount): Amount cannot be less than 0.");
}
@Override
public int compareTo(BankAccount another){
if (Integer.parseInt(this.getAccountNumber()) > Integer.parseInt(another.getAccountNumber()))
return 1;
else if(Integer.parseInt(this.getAccountNumber()) < Integer.parseInt(another.getAccountNumber()))
return -1;
else
return 0;
}
}
我的主要方法如下：
public class GeauxBank
{
public static void main(String[] args)
{
ArrayList<BankAccount> accounts = new ArrayList<BankAccount>();
Scanner keyb = new Scanner(System.in);
// write code to read data from acinfo.dbf. Be sure to handle
// FileNotFoundException in a try-catch statement. You don't
// need to do anything in the catch block.
int option;
String acno;
String fName;
char midInit;
String lName;
double initBal=0,amount;
int pos;
do
{
menu();
System.out.println();
System.out.print("Select an option->");
option = keyb.nextInt();
System.out.println();
switch(option)
{
case 1:
System.out.print("Enter a 10-character long account number->");
acno = keyb.next();
System.out.print("Customer's first name ->");
fName = keyb.next();
System.out.print("Customer's middle initial ->");
midInit = keyb.next().charAt(0);
System.out.print("Customer's last name ->");
lName = keyb.next();
System.out.print("Initial Deposit ->");
initBal = keyb.nextDouble();
Collections.sort(accounts);
pos = binarySearch(accounts,acno);
if (pos < 0)
{
try
{
accounts.add(new BankAccount(acno,fName,midInit,lName,initBal));
Collections.sort(accounts);
}
catch(IllegalArgumentException e)
{
System.out.println(e);
}
}
else
System.out.println(acno+" has already being asssigned another customer");
break;
case 2:
System.out.println("Enter the 10-character long account number of the account you wish to close->");
acno = keyb.next();
pos = binarySearch(accounts, acno);
accounts.remove(pos);
break;
case 3:
System.out.println("Enter the 10-character long account number->");
acno = keyb.next();
System.out.println("Enter the amount you wish to deposit->");
amount = keyb.nextDouble();
pos = binarySearch(accounts, acno);
accounts.get(pos).deposit(amount);
break;
case 4:
System.out.println("Enter the 10-character long account number->");
acno = keyb.next();
System.out.println("Enter the amount you wish to withdraw->");
amount = keyb.nextDouble();
accounts.get(binarySearch(accounts, acno)).withdraw(amount);
break;
case 5:
System.out.println("Enter the 10-character long account number->");
acno = keyb.next();
accounts.get(binarySearch(accounts, acno)).getBalance();
break;
case 6:
int i;
int length = accounts.size();
for(i = 0; i < length; i++) {
System.out.print(accounts.get(i).getFirstName()+" ");
System.out.print(accounts.get(i).getMiddleInitial()+" ");
System.out.println(accounts.get(i).getLastName());
}
break;
case 0:
break;
default:
System.out.println("Invalid menu option");
}
}while (option != 0);
try{
PrintWriter writer = new PrintWriter("acinfo.dbf");
int i;
int length = accounts.size();
for(i = 0; i < length; i++){
writer.write(accounts.get(i)+"");
}
writer.close();
}
catch(FileNotFoundException f) {
System.out.println("The file acinfo.dbf does not exist.");
}
}
/**
* displays a text-based menu interface for this application
*/
public static void menu()
{
System.out.println();
System.out.println("G E A U X B A N K L O U I S I A N A L T D.");
System.out.println("=================================================");
System.out.println("[1]...............................open an account");
System.out.println("[2]..............................close an account");
System.out.println("[3].......................................deposit");
System.out.println("[4]....................................withdrawal");
System.out.println("[5]...............................balance inquiry");
System.out.println("[6].............................customers listing");
System.out.println("[0]..........................................exit");
System.out.println("=================================================");
}
/**
* searches an array list of BankAccount objects for a matching
* account number using the Binary Search algorithm.
* @param accounts an array list of BankAccount objects
* @param acno a string
* @return the index of the BankAccount object in the array list or
* with the account number that is the same as acno if such a BankAccount
* object can be found in the accounts array list; otherwise, the negative
* of the position the BankAccount object whose account number would
* have matched acno had it been found.
*/
public static int binarySearch(ArrayList<BankAccount> accounts, String acno)
{
int i;
int low,mid=0,high;
low = 0;
high = accounts.size()-1;
while (low <= high)
{
mid = (low+high)/2;
if (acno.compareTo(accounts.get(mid).getAccountNumber())==0)
return mid;
else if (acno.compareTo(accounts.get(mid).getAccountNumber())< 0)
high = mid-1;
else
low = mid+1;
}
return -1-mid;
}
}
我遇到了binarySearch方法的问题。我可以创建一个银行账户，但是当我尝试创建第二个时，我收到了这条消息：
Exception in thread "main" java.lang.NullPointerException
at java.lang.String.compareTo(String.java:1167)
at geauxbank.GeauxBank.binarySearch(GeauxBank.java:172)
at geauxbank.GeauxBank.main(GeauxBank.java:57)
Java Result: 1
每当我尝试在使用binarySearch方法的switch语句中使用任何其他情况时，它也会给我一个类似的消息。我不知道为什么会这样。此外，当我尝试将信息写入文本文件时，我在文本文件中得到了这个：BankAccount.BankAccount@1b67f74 或者：null null 我真的不知道造成这些问题的原因。任何洞察力和清晰度都将受到赞赏。
答案 0 :(得分：2)
你的作业是错误的。
这将获取参数并使用null字段覆盖它。这些字段保留为null，当您尝试取消引用时，您将获得NPE。
acno = accountNumber;
试
accountNumber = acno;
我建议您制作字段final（除非您需要更改字段），否则您不会遇到此问题。