Optional

Optional is a container that explicitly represents value or absence, replacing null-return patterns and eliminating accidental NullPointerException through type-safe APIs.

of(null) throws NPE as a bug detector; ofNullable(null) returns empty as a safe wrapper: use of() when null is a programming error, ofNullable() when null is legitimate absence.

orElse always evaluates the default (eager); orElseGet evaluates only when empty (lazy): use orElseGet for expensive defaults like database calls.

Optional is for return types only; not fields (not Serializable), not parameters (awkward API), not collections (use empty collection), and never .get() without checking.

Use map for transformation, flatMap for nested Optionals, filter for conditions, orElse for defaults, orElseThrow for required values, and ifPresent for actions: never isPresent()+get().

Never Optional<Collection> (return empty collection), never Optional<Integer> (use OptionalInt), and avoid Optional in hot loops (heap allocation overhead).

Java 9 added Optional.stream() for stream integration, or() for lazy fallback chains, and ifPresentOrElse() for handling both present and empty cases.

Use stream filter plus findAny to get an Optional<Employee>, then unwrap it with orElse for an eager default, orElseGet for a lazy default, or orElseThrow when absence is an error.

mapToDouble(...).average() returns an OptionalDouble, and orElse(0.0) provides a safe default when the stream is empty.