用Account類來模擬一臺ATM機

Account類原始碼：

import java.util.Date;

public class Account {
	private int id = 0;
	private double balance = 0;
	private static double annualInterestRate = 0;
	private Date dateCreated;

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public double getBalance() {
		return balance;
	}

	public void setBalance(double balance) {
		this.balance = balance;
	}

	public double getAnnualInterestRate() {
		return annualInterestRate;
	}

	public void setAnnualInterestRate(double annualInterestRate) {
		this.annualInterestRate = annualInterestRate;
	}

	public Date getDateCreated() {
		return dateCreated;
	}


	public Account() {

	}

	public Account(int id, double balance) {
		this.id = id;
		this.balance = balance;
	}

	public static double getMonthlyInterestRate() {

		return annualInterestRate / 12;

	}

	public void withDraw(double money) {
		balance -= money;
	}

	public void deposit(double money) {
		balance += money;

	}

	/**
	 * @param args
	 */
//	public static void main(String[] args) {
//		// TODO Auto-generated method stub
//		Account account = new Account(1122, 20000);
//		account.dateCreated = new Date();
//		account.setAnnualInterestRate(4.5);
//		account.withDraw(2500);
//		account.deposit(3000);
//		System.out.println("餘額為：" + account.getBalance());
//		System.out.println("月利息為" + getMonthlyInterestRate());
//		System.out.println("開戶日期:" + account.getDateCreated());
//	}

}
 

package object10;

import java.util.*;

public class ATM {

	private static ArrayList<Account> accounts = new ArrayList<Account>();
	private static int numList[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
	private static Scanner scanner;
	private static boolean isLogin = false;


	public static void showMenu() {
		System.out.println("Main menu");
		System.out.println("1.check balance");
		System.out.println("2.withdraw");
		System.out.println("3.deposit");
		System.out.println("4.exit");
		System.out.print("Enter a choice:");

	}

	public static void processMenu(int id, int command) {
		switch (command) {
		case 1:
			System.out.println("" + accounts.get(id).getBalance());
			break;
		case 2:
			System.out.println("Enter an amount to withdraw:");
			double withdrawMoney = scanner.nextDouble();
			accounts.get(id).withDraw(withdrawMoney);
			break;
		case 3:
			System.out.println("Enter an amount to deposit");
			double depositMoney = scanner.nextDouble();
			accounts.get(id).deposit(depositMoney);
			break;
		case 4:
			isLogin = false;
			break;
		default:
			System.out.println("請輸入正確的指令:");
			break;

		}

	}

	public static void main(String[] args) throws Exception {
		// TODO Auto-generated method stub
		for (int i = 0; i <= 9; i++) {
			Account acc = new Account(i, 100);
			accounts.add(acc);
		}
		while (true) {
			System.out.println("請輸入ID:");
			scanner = new Scanner(System.in);
			int id = scanner.nextInt();

			for (int num : numList) {
				if (id == num) {
					isLogin = true;
				}
			}

			if (!isLogin) {
				System.out.println("請輸入正確的id!");
			}
			while (isLogin) {
				showMenu();
				scanner = new Scanner(System.in);
				int command = scanner.nextInt();
				processMenu(id, command);

			}

		}
	}
}
 

出現過的問題：

1.private static ArrayList<Account> accounts= new ArrayList<Account>();
剛開始寫的是
private static ArrayList<Account> accounts;
然後編譯器報錯Exception in thread "main" java.lang.NullPointerException
at object10.ATM.main(ATM.java:53)因為沒有對arrayList進行初始化
2.
case 4:
isLogin = false;
break;
要記得把isLogin設為false,這樣才可以從處理選單跳出到輸入id