Streams & CompletableFuture

- Streams are single-use — can't reuse after terminal operation, create new stream - `findFirst()` vs `findAny()` — findAny may return different element in parallel - `flatMap()` flattens and maps — `Optional.flatMap()` unwraps nested Optionals - `peek()` for debugging only — may not execute with short-circuit ops like `findFirst()` - `toList()` (Java 16+) returns unmodifiable — `Collectors.toList()` is modifiable