Simplify Reactor switchIfEmpty(Mono.just(...)) using defaultIfEmpty

[EnricSala]

Problem

Specifying default behavior in a Reactor publisher can be written like switchIfEmpty(Mono.just(42)) or defaultIfEmpty(42).

The behavior is identical, but the latter is more concise and may have slight performance benefits due to reduced overhead.

Description of the proposed new feature

• Support a stylistic preference.
• Avoid a common gotcha, or potential problem.

I would like to rewrite the following code:

Mono<Integer> mono = ...;
return mono.switchIfEmpty(Mono.just(42));

to:

Mono<Integer> mono = ...;
return mono.defaultIfEmpty(42);

Considerations

• The rewrite operation is applicable for both Mono and Flux.
• This rewrite is only suitable when the default is a {Mono|Flux}.just(...). Cases like Mono.fromSupplier cannot be rewritten because they would lose the lazy-evaluation behavior.
• It could be possible to rewrite also cases like switchIfEmpty(Mono.empty()), in which case the switchIfEmpty operator can simply be dropped.

Participation

• I am willing to submit a pull request to implement this improvement.

[giall]

Opened a PR: #370. The third point in considerations seems to already be covered by the MonoSwitchIfEmptyOfEmptyPublisher rule.