Using an operator (a method of Mono/Flux/Flowable object that returns a Mono/Flux/Flowable) produces a new instance (Publisher). The operator won't be applied until you subscribe to the created Publisher by using subscribe().

Using a factory (for example, Mono.just) without subscribing to the Publisher creates an object that is never used and is treated as an unnecessary memory allocation.

For example, Mono.just(1, 2, 3).map(i -> i + 3) won't be executed unless you subscribe to this Publisher, or unless you produce a new Publisher by applying operators and subscribe to it.

New in 2019.3