Careeroza

Optional Best Practices

Do and Do Not

Do	Do not
Return `Optional` from methods that may find nothing (`findById`)	Use Optional for fields, method parameters or in collections
Use `map`/`orElse`/`orElseThrow`	Call `get()` without checking — that is just a null check in disguise
Return an empty list instead of `Optional<List>`	Return `null` from a method whose return type is Optional
Use `OptionalInt`/`OptionalDouble` for primitives	Use Optional in serialized DTOs / JPA entities

Spring Data Style

Spring Data repositories return Optional from findById. The usual service pattern is orElseThrow with a domain exception, which a @ControllerAdvice turns into HTTP 404.

public ProductDto getProduct(Long id) {
    return productRepository.findById(id)
            .map(ProductDto::from)
            .orElseThrow(() -> new ProductNotFoundException(id));
}

Practice

Exercise: Write a method Optional<Integer> parseAge(String s) that returns empty when s is not a number or is negative.

Show answer

static Optional<Integer> parseAge(String s) {
    try {
        return Optional.of(Integer.parseInt(s.trim())).filter(a -> a >= 0);
    } catch (NumberFormatException e) {
        return Optional.empty();
    }
}