Java spring controller method to trigger e-mail verification procedure [closed]

Your code is missing some important context. Based on the context, it looks to me like .map(verificationTokenService::findByUser) would return an Optional<VerificationToken> or a similar optional, which would mean that if verificationToken is not present, the .map(verificationToken -> { operation will never be executed. But that lambda executes verificationToken.ifPresent(...) which indicates that it's expected to be executed even ehen the token is not present.

If this is correct, it may be a sign that splitting the code into separate operations (instead of one single optional stream) would make it more readable. It would also mean that your code wasn't tested and did not work as expected and would thus be off-topic for codereview...

Continuing on from @TorbenPutkonen:

• The variable optionalUser is not useful. Just chain onto .findByEmail().
• It may make sense to move filtering for a disabeld user onto the database layer and thus having a method .findDisabledUserByEmail() instead if .findByEmail().
• .stream().findFirst() seems completely pointless.
• Since verificationTokenService::findByUser returns an Optional itself that would make this an Optional<Optional<...>> which is an anti-pattern. It would make more sense to use .flatMap() instead of .map() to "flatten" the type to a simple Optional<...> here.
• The return value of the second .map() has nothing to do with the Optional value. You sould move those last three lines out of the lambda. Instead use .idPresentOrElse().

Possible simplified version:

@PostMapping("/verify-email")
public ResponseEntity<String> verifyAccount(@RequestBody String email) {
    appUserService.findByEmail(email)
            .filter(user -> !user.isEnabled())
            .flatMap(verificationTokenService::findByUser)
            .ifPresentOrElse(
                verificationTokenService::delete, 
                () -> throw new VerificationTokenException()
            );

    verificationTokenService.createVerificationToken(optionalUser.get(), VerificationTokenType.VERIFY_ACCOUNT);
    String message = String.format("Account verification link has been sent to %s", email);
    return ResponseEntity.ok(message);
}