Gotcha roundup III — APIs

API traps you will encounter in code and on the exam

Streams are single-use and lazy

Stream<String> stream = List.of("a", "b", "c").stream();
stream.forEach(System.out::println);  // works
stream.forEach(System.out::println);  // IllegalStateException — already consumed

A Stream pipeline does no work until a terminal operation (forEach, collect, count, findFirst, …) is called. Intermediate operations (filter, map, sorted) are lazy. Calling a terminal operation twice on the same stream throws IllegalStateException. Re-use the source collection, not the stream.

Optional.get() misuse

Optional<String> opt = findUser(id);
String name = opt.get();   // NoSuchElementException if empty!

// Safer alternatives:
String name = opt.orElse("anonymous");
opt.ifPresent(u -> System.out.println(u));

Optional.get() throws NoSuchElementException if the Optional is empty. Use orElse, orElseGet, orElseThrow, or ifPresent — they express intent more clearly and cannot silently throw.

try-with-resources close order

Resources are closed in reverse declaration order — last declared, first closed:

try (var a = openA(); var b = openB()) {
    // use a and b
}   // b.close() called first, then a.close()

If both close() and the body throw, the close exception is suppressed and the body exception propagates. Retrieve suppressed exceptions with Throwable.getSuppressed().

java.time objects are immutable

LocalDate date = LocalDate.of(2024, 1, 1);
date.plusDays(30);        // silent no-op — returns a new LocalDate, discarded
date = date.plusDays(30); // correct — reassign the variable

Every method on LocalDate, LocalTime, LocalDateTime, ZonedDateTime, and Duration returns a new object; the original is unchanged. Forgetting to use the return value is a silent bug and a favourite OCP trap.

ResultSet is 1-indexed

JDBC ResultSet.getXxx(int columnIndex) counts from 1, not 0:

String name = rs.getString(1);  // first column — not 0
int    age  = rs.getInt(2);

Using index 0 throws SQLException. The named overload rs.getString("name") is less fragile and is preferred in production.

ConcurrentModificationException

List<String> list = new ArrayList<>(List.of("a", "b", "c"));
for (String s : list) {
    if (s.equals("b")) list.remove(s);  // ConcurrentModificationException
}

Modifying a collection while iterating with a for-each loop throws ConcurrentModificationException. Safe alternatives:

• iterator.remove() — the only safe remove during iteration.
• list.removeIf(s -> s.equals("b")) — the idiomatic one-liner.
• Collect matches into a separate list, then call list.removeAll(...).