Working with Optional

1. Creating Optional

Method	Returns
`isPresent()`	boolean
`isEmpty()` JAVA 11+	boolean
`ifPresent(c)`	void
`ifPresentOrElse(c, runnable)` JAVA 9+	void

User user = repo.findById(id)
    .orElseThrow(() -> new UserNotFoundException(id));

Pattern	Use
Specific exception	Service-layer not-found
Default exception	`orElseThrow()`

Method	Use
`map(fn)`	Transform if present
`flatMap(fn)`	fn returns Optional

String name = Optional.ofNullable(input)
    .filter(s -> !s.isBlank())
    .map(String::trim)
    .orElse("anonymous");

Method	Detail
`filter(p)`	Empty if predicate false

Method	Use
`ifPresent(c)`	Side-effect when present
`ifPresentOrElse(c, r)`	Side-effect or fallback

Method	Detail
`or(supplier)` JAVA 9+	Alternative Optional
`map / flatMap`	Transform chain

Method	Result
`opt.stream()` JAVA 9+	0 or 1 element stream
Use case	`list.stream().flatMap(Optional::stream)` filters/unwraps

Practice	Reason
Use as return type	Express absence in API
Don't use as field	Not Serializable; adds overhead
Don't use as parameter	Pass T or overload
No collection	Empty collection > `Optional<List>`