question: public final < U > ParallelFlux < U > map ( Function < ? super T , ? extends U > mapper ) { Objects . requireNonNull ( mapper , "mapper" ) ; return onAssembly ( new ParallelUnorderedMap < > ( this , mapper ) ) ; }
target:  Maps the source values on each 'rail' to another value. <p> Note that the same mapper function may be called from multiple threads concurrently.