How can I map Optional to another Optional if not present?

Question

I have this Java 8 code:

public Optional<User> getUser(String id) {
    Optional<User> userFromCache = cache.getUser(id);
    if (userFromCache.isPresent()) {
        return userFromCache;
    }
    return repository.getUser(id);
}

It works fine but I'm wondering how can I chain the call to not to use if. I have tried with orElseGet but it doesn't allow to return another Optional<User> but a User.

I want something like this:

Optional<User> userFromCache = cache.getUser(id)
    .orElseGet(() -> repository.getUser(id));

See here: https://stackoverflow.com/questions/28818506/optional-orelse-optional-in-java. There are answers for both JDK 8 and 9. See the answer by @Misha, here: https://stackoverflow.com/a/28821352. — Druckles, Dec 17 '18 at 15:11
This really should be closed as a duplicate but I have no more flags .^. — Druckles, Dec 17 '18 at 15:19

score 11 · Accepted Answer · edited Dec 17 '18 at 14:50

11

Since Java 9, there is Optional.or. It accepts a supplier for another Optional.

return cache.getUser(id).or(() -> repository.getUser(id));

edited Dec 17 '18 at 14:50

Michael

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answered Dec 17 '18 at 14:48

khelwood

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1

This is exactly what I want. Unfortunately I'm using Java 8 =( – Héctor Dec 17 '18 at 14:50
3

@Héctor then your approach is good enough and probably is the best way possible in jdk8 – Ousmane D. Dec 17 '18 at 14:50

score 6 · Answer 2 · edited Dec 17 '18 at 15:11

6

You can create an optional based on a nullable value from other optionals:

public Optional<User> getUser(String id) {
    return Optional.ofNullable(
        cache.getUser(id).orElseGet(
            () -> repository.getUser(id).orElse(null)
        )
    );
}

But your current solution is clearly more readable.

edited Dec 17 '18 at 15:11

Michael

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answered Dec 17 '18 at 14:48

ernest_k

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score 2 · Answer 3 · answered Dec 17 '18 at 15:14

2

You can still use ?:

return (userFromCache.isPresent()) ? userFromCache : repository.getUser(id);

It's obviously an if in disguise but so is any other solution.

answered Dec 17 '18 at 15:14

Erwin Smout

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How can I map Optional to another Optional if not present?

3 Answers3

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