Java debugging interview questions

Question 1

total() adds up a cart's line items. Why does it throw on every call?

public static long total(long[] lineItemsCents) {
    long sum = 0;
    for (int i = 0; i <= lineItemsCents.length; i++) {
        sum += lineItemsCents[i];
    }
    return sum;
}

Accepted Answer

Line 3 (Off-by-one): Valid indexes run from 0 to length - 1. With `<=` the loop reads index `length` on its last pass and throws ArrayIndexOutOfBoundsException.

Question 2

greetingFor() builds the email greeting for the password-reset flow. What happens for an email that isn't registered?

public String greetingFor(String email) {
    Optional<User> user = userRepository.findByEmail(email);
    return "Hi " + user.get().getFirstName() + ",";
}

Accepted Answer

Line 3 (Null / undefined handling): findByEmail returns Optional.empty() for unknown emails, and get() on an empty Optional throws NoSuchElementException. The reset endpoint returns a 500, which also tells attackers which emails exist.

Question 3

emailExists() is called on every signup. After some traffic, all requests hang waiting for a database connection. Why?

public boolean emailExists(String email) throws SQLException {
    Connection conn = dataSource.getConnection();
    PreparedStatement ps = conn.prepareStatement("SELECT 1 FROM users WHERE email = ?");
    ps.setString(1, email);
    ResultSet rs = ps.executeQuery();
    return rs.next();
}

Accepted Answer

Line 2 (Resource leak): The connection, statement and result set are never closed, so every call keeps a pooled connection checked out. Once the pool is exhausted, new requests block until they time out.

Question 4

isAdmin() gates the admin dashboard. Why are real admins being locked out?

public boolean isAdmin(HttpServletRequest request) {
    String role = request.getHeader("X-Role");
    return role == "admin";
}

Accepted Answer

Line 3 (Wrong API usage): `==` compares object references, not contents. The header value is a new String built at runtime, so it is never the same object as the "admin" literal and the check is always false.

Question 5

toCsv() exports 200k rows. It takes minutes and churns the garbage collector. Why?

public static String toCsv(List<String[]> rows) {
    String csv = "";
    for (String[] row : rows) {
        csv += String.join(",", row) + "
";
    }
    return csv;
}

Accepted Answer

Line 4 (Performance trap): Strings are immutable, so each += copies everything built so far into a new string. Over n rows that is O(n²) copying and a huge amount of garbage.

Java debugging interview questions

Java gotchas interviewers love

5 Java spot-the-bug questions

Summing an array

Calling Optional.get()

JDBC connection never closed

Comparing strings with ==

Building a CSV with +=

Attempts distribution