使用 SQLite 在 Java 中使用数据库时出错

Question

Bank class calling the Atm class.The machine class contains the start machine method. Here onwards loop will run forever calling the required methods from another classes not relevant. The create table method should be called only once.Once the Pin is validated the program runs infinitely

import java.util.Scanner;

public class Bank {

    final static int PIN =  5423;//pin to start the ATM operations
    
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter the PIN to start the machine");
        for(int i = 3; i >= 1;--i) {
            int n = scanner.nextInt();
            if(n == PIN) {//if pin was correct , start the ATM
                Atm atm = new Atm();
                atm.startMachine();
                break;
            }
            else if(i > 1)
                System.out.println("Incorrect PIN. You have " + (i-1)  + " tries remaining");
            else {
                System.out.println("Tries exhausted. Contact the main office.");
                break;
            }
        }
        scanner.close();
    }

}

Atm class which has start machine method

class Atm {//class for functioning of ATM

    Scanner scan;
    protected String load;//loading time 
    protected int cashAvailable = 2000000;//initial cash available
    static Database database;
    Atm(){//constructor to initialize fields
        scan = new Scanner(System.in);
        load = "00:00";//the loading time at midnight
        database = new Database();
        database.createTable();//call the database class
    }

    public void startMachine(){//start the machine
        Machine machine = new Machine();
        machine.start();//call the start method
    }
}

And the machine class

public void start() {//method to start the machine
        Withdraw withdraw = new Withdraw();//withdraw object
        Deposit deposit = new Deposit();//deposit object
        while(true) {//infinite loop for functioning of machine
            String currentTime = new SimpleDateFormat("HH:mm").format(new Date());//get the current time
            boolean isLoadingTime = currentTime.equals(load);//if is loading time
            if(isLoadingTime) {//if yes, the call load method
                load();//method to load the cash
            }
            //show the welcome screen at every iteration
            System.out.println("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
            System.out.println();
            System.out.println("Welcome to National Bank");
            SimpleDateFormat formatter = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss");  
            Date date = new Date();
            System.out.print(formatter.format(date) + "  ");
            System.out.println(LocalDateTime.now().getDayOfWeek());
            System.out.println("Press 1 to view your current balance");
            System.out.println("Press 2 to withdraw cash");
            System.out.println("Press 3 to deposit funds");
            System.out.println();
            System.out.println("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
            while(true) {
                int enter_num = scan.nextInt();//get the preferred choice
                if(enter_num == 1) {
                    currentBalance();//method to show current balance
                    break;
                }
                else if(enter_num == 2) {
                    withdraw.debit();//method to withdraw cash
                    break;
                }
                else if(enter_num == 3) {
                    deposit.giveCash();//method to deposit cash
                    break;
                }
                else
                    break;//if not valid option
            }
        }
    }

Answer 1

The obvious errors in your code in update() method are the comma after SET Balance = ? which you must remove and also change the indices of stmt.setInt() to 1 for balance and 2 for accountNo because you are passing 2 paremeters with that order in your UPDATE statement:

String change = "UPDATE accounts SET Balance = ? WHERE AccountNumber = ?";
try {
    PreparedStatement stmt = conn.prepareStatement(change);
    stmt.setInt(1,balance);
    stmt.setInt(2,accountNo);
.............................

Also, the error message:

Abort due to constraint violation (UNIQUE constraint failed: accounts.AccountNumber)

means that you are trying to insert in the table an account number that already exists in the table and this is not allowed because the column AccountNumber is the primary key of the table so it is unique.

Also, inside createTable() what is sc ? Is it a resultset?
If so, what are you trying to do? Are you trying to loop through the rows of the table?
The table is just created and it is empty.

From the code that you posted, I can't see why you get 4 times the message "A new database has been created" , because I don't know how you call the method createTable() .