Having trouble using date for an interest calculate

Question

I need a little logic help as well as some code help.

I'm writing a program that is suppose to be a simple ATM program. It will do deposits Withdrawals And a balance check.

I also have to have my program record the date of every deposit, and use that date when calculating 5% interest for when the user checks their balance. The part im having problems with is the part where I need to store the deposit amount as well as the date that corresponds to that deposit. I'm supposed to use the GregorianCalender methods. Im just taking a shot in the dark but I think I would use an array for this. But im clueless as to how to implement it. Because I'm new to arrays.

and once I get a date, when checking the balance I need to check how many months its been since the deposit of that amount to calculate interest. Sorry its a pretty elaborate question but I'm a newbie and this is the most complicated thing ive ever written.

 public void printDeposit(){
    Calendar c = new GregorianCalendar();
    BigDecimal depositamt;
    Date date = c.getTime();

    int menuselection;
    System.out.println("Press 1 to deposit, zero to return to the main menu.");
    Scanner sc = new Scanner (System.in);

    menuselection = sc.nextInt();
            if (menuselection==1){
                System.out.println("Please enter the amount: " + date);
                depositamt = sc.nextBigDecimal();
                Transaction tran = new Transaction(depositamt, date);

    }

---

public class Transaction {

BigDecimal amt;
Date date;


public Transaction(BigDecimal amt, Date date) {
    this.amt = amt;
    this.date = date;

}

Answer 1

You should use ArrayList instread of arrays here, but more to the point, you should aim to use a databasehere, even if it is just an in-memory temporary database. Also, the printDeposit method is badly named; that would probably correspond to printing a deposit receipt. You'd want your logic to be something like:

Console console = System.getConsole();
if (console == null) {System.exit(42)}
while(true) (
    String option = console.readLine("What would you like to ask this ATM? %d to Quit, %d to deposit, %d to withdraw:", 0, 1, 2;
    switch (option) {
      case "1": acceptDeposit(); break;
      case "2": acceptWithdrawl(); break;
      case "0": quit(); break;
      default: break;
    }
}

Now, you need to write the acceptDeposit and acceptWithdrawl methods. In database terms, you'd want a balance table and a transaction table. In Java terms, you'd want classes like this:

public class Transaction {
    enum Type {DEPOSIT, WITHDRAWL, INTEREST};
    private Long userId;
    private Type type;
    private BigDecimal amount;
    private Date date;  // Switch to Joda-Time or its Java 8 equivalent when you can.
    // getters, setters, etc.
    public getBalanceChange() {
        if (type != WITHDRAWL) {
            return amount;
        } else {
            return amount.negate();
        }
    }
}

Do you see why you want the user id?

Your application would have a list of transactions, and uneless you keep a map that maps user ids to balances, you'd want a balance method:

public BigDecinmal balance(Long userId) {
    BigDecimal balance = new BigDecimal("0.00");
    for (Transaction t: transactionList) {
        if (userId.equals(t.getUserId())) {
            balance = balance.add(t.getBalanceChange());
        }
    }
}

With a database table, you'd do an UPDATE to the database. Were this app running continually, you'd schedule jobs to compute interest, probably using the Java Quartz library. For a student application, you'd simulate it by having a method:

private void applyInterest(Date from, Date to){...}

In fact, that's an argument against using GregorianCalendarand getting one for the time of each transaction; in practical use, each session will be part of a single day. Instead, have a Date currentDate and a Date interestLastApplied variable, and bump currentDateup periodically.