茶歇#201。如何用 Java 建立控制台銀行應用程式

來源：Medium 今天我們將為銀行系統開發一個簡單的 Java 應用程式。它將幫助我們更好地理解 OOP 概念如何在 Java 程式中使用。 首先，我們需要在電腦上安裝Java環境，最好是Java 11。接下來，我們將詳細介紹控制台應用程式的功能。 功能性：

1. 建立一個帳戶;
2. 輸入退出；
3. 顯示最近5筆交易；
4. 現金存款;
5. 顯示目前使用者資訊。

使用的物件導向程式設計概念：

1. 遺產;
2. 多態性；
3. 封裝。

應用程式開發

讓我們在 Eclipse 或 IntelliJ IDEA 中建立一個新的 Java 專案。讓我們定義一個名為SavingsAccount的新介面。

public interface SavingsAccount {

    void deposit(double amount, Date date);
}

我已經實作了一個託管存款方法的介面。每次我向支票帳戶充值時都會呼叫此方法。這裡使用的 OOP 概念是多態性（介面中的方法沒有主體）。透過重寫具有相同名稱和參數的方法，可以在Customer類別中找到此方法的實作。這樣，您就可以在子類別中重寫父介面中的方法。然後我們需要客戶向支票帳戶存錢。但首先，讓我們定義我們的Customer類別。

public class Customer extends Person implements SavingsAccount {

    private String username;

    private String password;

    private double balance;

    private ArrayList<String> transactions = new ArrayList<>(5);

    public Customer(String firstName, String lastName, String address, String phone, String username, String password, double balance, ArrayList<String> transactions, Date date) {
        super(firstName, lastName, address, phone);
        this.username = username;
        this.password = password;
        this.balance = balance;
        addTransaction(String.format("Initial deposit - " + NumberFormat.getCurrencyInstance().format(balance) + " as on " + "%1$tD" + " at " + "%1$tT.", date));
    }

    private void addTransaction(String message) {

        transactions.add(0, message);
        if (transactions.size() > 5) {
            transactions.remove(5);
            transactions.trimToSize();
        }
    }

//Getter Setter

    public ArrayList<String> getTransactions() {
        return transactions;
    }

    @Override
    public void deposit(double amount, Date date) {

        balance += amount;
        addTransaction(String.format(NumberFormat.getCurrencyInstance().format(amount) + " credited to your account. Balance - " + NumberFormat.getCurrencyInstance().format(balance) + " as on " + "%1$tD" + " at " + "%1$tT.", date));
    }

    @Override
    public String toString() {
        return "Customer{" +
                "username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", balance=" + balance +
                ", transactions=" + transactions +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Customer customer = (Customer) o;

        if (Double.compare(customer.getBalance(), getBalance()) != 0) return false;
        if (getUsername() != null ? !getUsername().equals(customer.getUsername()) : customer.getUsername() != null)
            return false;
        if (getPassword() != null ? !getPassword().equals(customer.getPassword()) : customer.getPassword() != null)
            return false;
        return getTransactions() != null ? getTransactions().equals(customer.getTransactions()) : customer.getTransactions() == null;
    }

    @Override
    public int hashCode() {
        int result;
        long temp;
        result = getUsername() != null ? getUsername().hashCode() : 0;
        result = 31 * result + (getPassword() != null ? getPassword().hashCode() : 0);
        temp = Double.doubleToLongBits(getBalance());
        result = 31 * result + (int) (temp ^ (temp >>> 32));
        result = 31 * result + (getTransactions() != null ? getTransactions().hashCode() : 0);
        return result;
    }

這裡使用的 OOP 概念是繼承，因為Customer類別會從Person類別接收屬性。也就是說，Person類別的幾乎所有屬性都是繼承的，而父子關係從Person到Customer都是可見的。現在我們需要一個具有兩個類別的所有屬性的建構函數，並且新增一個 superconstructor 關鍵字來指定繼承的屬性。在實作SavingsAccount介面時，我們必須重寫Customer類別中的Deposit方法。為此，我們將在此類中編寫該方法的實作。此外，交易清單被初始化以顯示最後五筆交易。建構函式呼叫addTransaction方法，該方法顯示變更日期和已完成的事務。

private void addTransaction(String message) {

        transactions.add(0, message);
        if (transactions.size() > 5) {
            transactions.remove(5);
            transactions.trimToSize();
        }
    }

現在我們來談談父類別Person。

public class Person {

    private String firstName;

    private String lastName;

    private String address;

    private String phone;

    public Person() {}

    public Person(String firstName, String lastName, String address, String phone) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.address = address;
        this.phone = phone;
    }

//Getters Setters

 @Override
    public String toString() {
        return "Person{" +
                "firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                ", address='" + address + '\'' +
                ", phone='" + phone + '\'' +
                '}';
    }

@Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Person person = (Person) o;

        if (getFirstName() != null ? !getFirstName().equals(person.getFirstName()) : person.getFirstName() != null)
            return false;
        if (getLastName() != null ? !getLastName().equals(person.getLastName()) : person.getLastName() != null)
            return false;
        if (getAddress() != null ? !getAddress().equals(person.getAddress()) : person.getAddress() != null)
            return false;
        return getPhone() != null ? getPhone().equals(person.getPhone()) : person.getPhone() == null;
    }

    @Override
    public int hashCode() {
        int result = getFirstName() != null ? getFirstName().hashCode() : 0;
        result = 31 * result + (getLastName() != null ? getLastName().hashCode() : 0);
        result = 31 * result + (getAddress() != null ? getAddress().hashCode() : 0);
        result = 31 * result + (getPhone() != null ? getPhone().hashCode() : 0);
        return result;
    }

在Person類別中，我們透過對每個屬性套用private存取修飾符來使用封裝的概念。Java 中的封裝可以定義為一種機制，透過該機制將操作這些資料的方法組合成一個單元。本質上，來自 Person 類別的資料僅在該類別中可用，而在其他類別或套件中不可用。最後，我們的主類，稱為Bank。這是我們運行應用程式並與所有類別的功能互動的主類別。

public class Bank {

    private static double amount = 0;
    Map<String, Customer> customerMap;

    Bank() {
        customerMap = new HashMap<String, Customer>();
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        Customer customer;
        Bank bank = new Bank();
        int choice;
        outer:
        while (true) {

            System.out.println("\n-------------------");
            System.out.println("BANK    OF     JAVA");
            System.out.println("-------------------\n");
            System.out.println("1. Registrar cont.");
            System.out.println("2. Login.");
            System.out.println("3. Exit.");
            System.out.print("\nEnter your choice : ");
            choice = sc.nextInt();
            sc.nextLine();
            switch (choice) {
                case 1:
                    System.out.print("Enter First Name : ");
                    String firstName = sc.nextLine();
                    System.out.print("Enter Last Name : ");
                    String lastName = sc.nextLine();
                    System.out.print("Enter Address : ");
                    String address = sc.nextLine();
                    System.out.print("Enter contact number : ");
                    String phone = sc.nextLine();
                    System.out.println("Set Username : ");
                    String username = sc.next();
                    while (bank.customerMap.containsKey(username)) {
                        System.out.println("Username already exists. Set again : ");
                        username = sc.next();
                    }
                    System.out.println("Set a password:");
                    String password = sc.next();
                    sc.nextLine();

                    customer = new Customer(firstName, lastName, address, phone, username, password, new Date());
                    bank.customerMap.put(username, customer);
                    break;

                case 2:
                    System.out.println("Enter username : ");
                    username = sc.next();
                    sc.nextLine();
                    System.out.println("Enter password : ");
                    password = sc.next();
                    sc.nextLine();
                    if (bank.customerMap.containsKey(username)) {
                        customer = bank.customerMap.get(username);
                        if (customer.getPassword().equals(password)) {
                            while (true) {
                                System.out.println("\n-------------------");
                                System.out.println("W  E  L  C  O  M  E");
                                System.out.println("-------------------\n");
                                System.out.println("1. Deposit.");
                                System.out.println("2. Transfer.");
                                System.out.println("3. Last 5 transactions.");
                                System.out.println("4. User information.");
                                System.out.println("5. Log out.");
                                System.out.print("\nEnter your choice : ");
                                choice = sc.nextInt();
                                sc.nextLine();
                                switch (choice) {
                                    case 1:
                                        System.out.print("Enter amount : ");
                                        while (!sc.hasNextDouble()) {
                                            System.out.println("Invalid amount. Enter again :");
                                            sc.nextLine();
                                        }
                                        amount = sc.nextDouble();
                                        sc.nextLine();
                                        customer.deposit(amount, new Date());
                                        break;

                                    case 2:
                                        System.out.print("Enter beneficiary username : ");
                                        username = sc.next();
                                        sc.nextLine();
                                        System.out.println("Enter amount : ");
                                        while (!sc.hasNextDouble()) {
                                            System.out.println("Invalid amount. Enter again :");
                                            sc.nextLine();
                                        }
                                        amount = sc.nextDouble();
                                        sc.nextLine();
                                        if (amount > 300) {
                                            System.out.println("Transfer limit exceeded. Contact bank manager.");
                                            break;
                                        }
                                        if (bank.customerMap.containsKey(username)) {
                                            Customer payee = bank.customerMap.get(username); //Todo: check
                                            payee.deposit(amount, new Date());
                                            customer.withdraw(amount, new Date());
                                        } else {
                                            System.out.println("Username doesn't exist.");
                                        }
                                        break;

                                    case 3:
                                        for (String transactions : customer.getTransactions()) {
                                            System.out.println(transactions);
                                        }
                                        break;

                                    case 4:
                                        System.out.println("Titularul de cont cu numele: " + customer.getFirstName());
                                        System.out.println("Titularul de cont cu prenumele : " + customer.getLastName());
                                        System.out.println("Titularul de cont cu numele de utilizator : " + customer.getUsername());
                                        System.out.println("Titularul de cont cu addresa : " + customer.getAddress());
                                        System.out.println("Titularul de cont cu numarul de telefon : " + customer.getPhone());
                                        break;
                                    case 5:
                                        continue outer;
                                    default:
                                        System.out.println("Wrong choice !");
                                }
                            }
                        } else {
                            System.out.println("Wrong username/password.");
                        }
                    } else {
                        System.out.println("Wrong username/password.");
                    }
                    break;

                case 3:
                    System.out.println("\nThank you for choosing Bank Of Java.");
                    System.exit(1);
                    break;
                default:
                    System.out.println("Wrong choice !");
            }}}}

使用java.util庫，我們呼叫Scanner來讀取鍵盤。透過其定義（稱為Bank ）轉換Customer對象，我們建立了一個Bank()類型的新物件。一段時間後，我們將顯示開始功能表。使用nextLine時，會讀取從鍵盤新增的數字。下面我們有一個新的建構函式來保存我們的地圖，也就是客戶端資料。Map.put用於保存或更新客戶資料。客戶=新客戶（名字，姓氏，地址，電話，用戶名，密碼，新日期（））；

bank.customerMap.put(username, customer);

如果有連接，我們會得到一個帶有選項的新選單。相同的方法透過使用while和switch來呼叫應用程式功能。第 1 階段：為您的活期帳戶儲值。步驟2：顯示最近5筆交易。第三階段：我們將客戶端資料從卡片輸出到控制台。第四階段：關閉選單。可以在此處找到該程式的原始程式碼。我希望這個範例能幫助您更熟悉 Java 中 OOP 概念的使用。