How to Implement Functional If-Then-Else in Java

Question

Given the following Function implementations...

Get User Credentials From Master Database

private Function<Long, Optional<UserCredentials>>
    getUserCredentialsFromMaster() {
        return userId -> Optional.ofNullable(userId)
            .flatMap(masterUserRepository::findById)
            .map(User::getCredentials);
}

Get User Credentials From Secondary Database

private Function<Long, Optional<UserCredentials>>
    getUserCredentialsFromSecondary() {
        return userId -> Optional.ofNullable(userId)
            .flatMap(secondaryUserRepository::findById)
            .map(User::getCredentials);
}

I need to execute either getUserCredentialsFromMaster or getUserCredentialsFromSecondary depending on where userId comes from.

Here below is my attempt:

Consider domain class UserProfile

public class UserProfile {
    Long id;
    Long internalUserId; // if internalUserId is null then externalUserId is not
    Long externalUserId; // and vice-versa
}

Attempt to obtain UserCredentials:

final UserProfile userProfile = userProfileRepository.findById(userProvileId);

final UserCredentials userCredentials =
    Optional.ofNullable(userProfile.internalUserId)
        .flatMap(getUserCredentialsFromMaster())
        .orElse(
            Optional.ofNullable(userProfile.externalUserId)
                .flatMap(getUserCredentialsFromSecondary())
                .orElseThrow(UserCredentialsNotFound::new));

internalUserId is not null but the statements above always throw UserCredentialsNotFound. I've tried to rewrite getUserCredentialsFromMaster and getUserCredentialsFromSecondary as plain Java methods invoked from an if-then-else block, and it worked as expected.

Am I missing something?

Answer 1

TL;DR

You're getting an exception because the argument of the Optional.orElse() is evaluated eagerly. I.e. it would be evaluated even if the Optional on which it was invoked is not empty.

But as you have said, either "internalUserId is null then externalUserId is not null and vice-versa" and orElseThrow() produces an exception.

Avoid using Optional to replace Null-checks

Firstly, note that Optional wasn't designed to perform null-checks. The design goal of the Optional is to serve as a return type, and its method ofNullable() is supposed to wrap a nullable return value, not to substitute a null-check.

You might be interested in reading:

• Should Optional.ofNullable() be used for null check?

• Valid usage of Optional type in Java 8

And there's nothing wrong with implicit null-checks, it's a bit of a problem when there are a lot of them in the code, but it's rather an immediate consequence of how particular behavior in the application was implemented, then the issue related to the tools offered by the language.

The cleaner way to address this problem would be to get read of the functions and define a method producing a Supplier based on the provided id and UserRepository:

private Supplier<Optional<UserCredentials>> getUserCredentials(Long id, UserRepository repository) {
    
    return id == null ?
        Optional::empty :
        () -> repository.findById(id).map(User::getCredentials);
}

Which can be used in the following way:

UserCredentials userCredentials = Stream.of(
        getUserCredentials(userProfile.internalUserId, masterUserRepository),
        getUserCredentials(userProfile.internalUserId, secondaryUserRepository)
    )
    .map(Supplier::get)
    .flatMap(Optional::stream)
    .findFirst()
    .orElseThrow(UserCredentialsNotFound::new);

If you still want to keep using Optional

As a remedy you can use Java 9 Optional.or() which expects a Supplier of Optional which would be evaluated only if needed.

UserCredentials userCredentials = Optional.ofNullable(userProfile.internalUserId)
    .flatMap(getUserCredentialsFromMaster())
    .or(() ->
        Optional.ofNullable(userProfile.externalUserId)
            .flatMap(getUserCredentialsFromSecondary())
    )
    .orElseThrow(UserCredentialsNotFound::new);

Alternatively, you can make use Optional.orElseGet() which takes a Supplier of resulting value:

UserCredentials userCredentials = Optional.ofNullable(userProfile.internalUserId)
    .flatMap(getUserCredentialsFromMaster())
    .orElseGet(() ->
        Optional.ofNullable(userProfile.externalUserId)
            .flatMap(getUserCredentialsFromSecondary())
            .orElseThrow(UserCredentialsNotFound::new)
    );