Three-attempt ATM PIN verification

import java.util.*;

public class Main {
    private static final Scanner IN = new Scanner(System.in).useLocale(Locale.ROOT);

java.util.* imports utility type names such as Scanner, Locale and collections. Main is the public class matching Main.java. The private static final IN field holds one shared Scanner for System.in; final prevents replacing that reference. The scanner uses Locale.ROOT.

    static String text() {
        if (!IN.hasNextLine()) throw new IllegalArgumentException("Incomplete input.");
        String value = IN.nextLine().trim();
        if (value.isEmpty() || value.length() > 200) throw new IllegalArgumentException("Text must have 1–200 characters.");
        return value;
    }

text checks whether a line exists, reads it and removes leading/trailing whitespace. Reject empty text and lengths above 200 by throwing IllegalArgumentException. Otherwise return the string. One field is read per line, so these helpers do not mix nextInt with leftover newlines.

    static int integer(int low, int high) {
        int value = Integer.parseInt(text());
        if (value < low || value > high) throw new IllegalArgumentException("Integer out of range.");
        return value;
    }

Integer.parseInt converts the returned string to int or throws NumberFormatException, a subtype of IllegalArgumentException. Reject values outside the caller’s inclusive low/high limits, then return the accepted integer.

    static double number(double low, double high) {
        double value = Double.parseDouble(text());
        if (!Double.isFinite(value) || value < low || value > high) throw new IllegalArgumentException("Number out of range.");
        return value;
    }

Double.parseDouble reads the string as a double. Double.isFinite rejects NaN and infinities as well as the explicit low/high range check. Return only a finite accepted value; exceptions transfer control to a matching catch block.

    static void money(String label, double value) { System.out.printf(Locale.ROOT, "%s: %.2f%n", label, value); }

money is a formatting helper: %s prints the label, %.2f prints two decimal places, and %n ends the line. Locale.ROOT fixes locale-sensitive formatting. The helper can also display non-currency quantities; it does not change the stored value.

static class ATMPinChecker {
        private final String username, pinStored;
        ATMPinChecker(String username, String pinStored) {
            if (!pinStored.matches("[0-9]{4}")) throw new IllegalArgumentException("Stored PIN must be four digits.");
            this.username=username; this.pinStored=pinStored;
        }

Keeping the PIN as text preserves leading zeros. The constructor requires exactly four digits for the stored PIN, then assigns final identity fields.

        boolean verifyPIN(String enteredPIN) { return pinStored.equals(enteredPIN); }
        void authenticate() {
            System.out.println("User: " + username);

equals compares the stored and entered character sequences; == would compare String references instead. Print the demonstration username before asking for attempts.

            for (int attempt=1; attempt<=3; attempt++) {
                String entered = text();
                if (verifyPIN(entered)) { System.out.println("PIN ENTERED CORRECT"); return; }
                System.out.println("Incorrect PIN, attempt " + attempt + ".");
            }

Read one nonempty PIN line per attempt. A match prints success and returns from authenticate immediately. A mismatch prints its attempt number and continues while attempts remain.

            System.out.println("ACCOUNT LOCKED");
        }
    }

If no earlier return occurred, print ACCOUNT LOCKED. This is the exercise’s in-memory control flow, not an actual external account lock.

    // Demo username/stored four-digit PIN, then up to three entered PIN lines.
    public static void main(String[] args) {
        try { new ATMPinChecker(text(),text()).authenticate(); }
        catch (IllegalArgumentException e) { System.out.println("Invalid input: " + e.getMessage()); }
    }
}

main reads username and stored PIN, constructs an object and calls authenticate. The catch reports missing/invalid input lines. The supplied program does not use a banking service.