我正在为一个面向对象的编程类介绍一个简单的程序，我有3个类：Account，ATM和main / Runner类。
当我尝试使用循环初始化ATM数组时，我在Account类的构造函数中遇到空指针异常错误。我是Java的新手，所以我真的不知道会发生什么。
以下是代码：
import java.util.Date;
public class Account {
private int account_id;
private double account_balance;
private static double annual_interest_rate;
private Date date_Created;
Account(){
account_id = 0;
account_balance=0;
annual_interest_rate = 0;
date_Created = new Date();
}
Account(int account_id_in,
double account_balance_in, double annual_interest_rate_in) {
account_id = account_id_in;
account_balance = account_balance_in;
annual_interest_rate = annual_interest_rate_in;
date_Created = new Date();
}
int get_account_id(){
return account_id;
}
double get_account_balance(){
return account_balance;
}
double get_annual_interest_rate(){
return annual_interest_rate;
}
Date get_date_created(){
return date_Created;
}
void set_account_id(int account_id_in){
account_id = account_id_in;
}
void set_account_balance(double account_balance_in){
account_balance = account_balance_in;
}
void set_annual_interest_rate(double annual_interest_rate_in){
annual_interest_rate = annual_interest_rate_in;
}
double get_monthly_interest_rate(){
return annual_interest_rate/12;
}
double get_monthly_interest(){
return (account_balance * get_monthly_interest_rate())/100;
}
void perform_deposit(double deposit_in){
account_balance += deposit_in;
}
void perform_withdraw(double withdraw_amount){
account_balance -= withdraw_amount;
}
}
public class ATM {
private Account[] acct = new Account [10];
public ATM(){
//acct = new Account[10];
for(int i = 0; i<10 ; i++){
acct[i].set_account_id(i+1);
acct[i].set_account_balance(100); // here iam getting an error.
}
//you must set their ids to values as specified in the assignment
} //these are the teacher's comments and instructions.
public void displayMenu(){
System.out.println("ATM Menu:");
System.out.println("\tenter 1 for viewing the current balance");
System.out.println("\tenter 2 for withdrawing money");
System.out.println("\tenter 3 for for depositing money");
System.out.println("\tenter 4 for exiting the main menu");
}
public boolean checkID(int id){
for(int i = 0; i<10 ; i++){
if(acct[i].get_account_id() == id){
return true;
}
}
return false;
}
public double checkBalance(int idToSearch){
int indexOfAccountToReturn = 0;
for(int i = 0; i<10; i++){
if(acct[i].get_account_id() == idToSearch){
indexOfAccountToReturn = i;
break;
}
}
return acct[indexOfAccountToReturn].get_account_balance();
}
public void withdrawFunds(int id, double amount){
for(int i=0; i<10; i++){
if(acct[i].get_account_id() == id){
acct[i].perform_withdraw(amount);
break;
}
}
}
public void depositFunds(int id, double amount){
for(int i=0; i<10; i++){
if(acct[i].get_account_id() == id){
acct[i].perform_deposit(amount);
break;
}
}
}
}
import java.util.Scanner;
import javax.swing.JOptionPane;
public class Banking_Finance_Main {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
ATM atm = new ATM();
Scanner input = new Scanner(System.in);
do{
System.out.println("Account ID?");
int id = input.nextInt();
while(!(atm.checkID(id))){
String entry =
JOptionPane.showInputDialog("Incorrect Input");
id = Integer.parseInt(entry);
}
//prevent user to proceed without the correct id; use checkID method and store appropriately
do{
atm.displayMenu();
System.out.println("Your choice?");
int choice = input.nextInt();
while((choice >4) || (choice <1)){
String entry = JOptionPane.showInputDialog("Incorrect Imput");
choice = Integer.parseInt(entry);
}
//prevent user to proceed without the correct choice 1-4;
if(choice==1){
System.out.println("Enter the Account id to check the balance: ");
int idToSearch = input.nextInt();
System.out.println("The balance in this ACccount is: $" + atm.checkBalance(idToSearch));
}else if(choice==2){
System.out.println("Enter the Account id to perform withdrawal: ");
int idToSearch = input.nextInt();
System.out.println("Enter the amount to be withdrawn: ");
double amountToWithdraw = input.nextDouble();
atm.withdrawFunds(idToSearch, amountToWithdraw);
}else if(choice==3){
System.out.println("Enter the Account id to perform deposit: ");
int idToSearch = input.nextInt();
System.out.println("Enter the amount to be deposited: ");
double amountToDeposit = input.nextDouble();
atm.depositFunds(idToSearch, amountToDeposit);
}else{
break;
}
}while(true);
}while(true);
}
}
如果我不恰当地使用本网站，我很抱歉，这是我的第一个问题，请耐心等待。 我设法通过以下修复来传递错误：
public ATM(){
for(int i = 0; i<10 ; i++){
acct[i] = new Account();
acct[i].set_account_id(i+1);
acct[i].set_account_balance(100);
}
//you must set their id's to values as specified in the assignment
}
但是我不确定这是否正确。是吗？
答案 0 :(得分：4)
这里的问题是这一行：
private Account[] acct = new Account [10];
仅初始化10个帐户实例的空间，它实际上并不构建10个帐户实例并将它们放入数组中。
在ATM构造函数的for循环中，您需要先调用acct [i] = new Account（）;
答案 1 :(得分：0)
除了创建数组之外，还需要用对象填充它。在ATM构造函数中更新代码：
for (int i = 0; i < 10; i++) {
acct[i] = new Account(); // add this line to crate account
// and place it in array
acct[i].set_account_id(i + 1); // also you are getting error here
acct[i].set_account_balance(100); // not here because you ware trying to
// invoke method on `null`
}