Returns a new CompletionStage that, when this stage completes
normally, is executed using this stage's default asynchronous
execution facility, with this stage's result as the argument to
the supplied function.
See the {@link CompletionStage} documentation for rules
covering exceptional completion.
@param fn the function to use to compute the value of the
returned CompletionStage
@param (U) the function's return type
@return the new CompletionStage
Returns a new CompletionStage that, when this stage completes normally, is executed using this stage's default asynchronous execution facility, with this stage's result as the argument to the supplied function.
See the {@link CompletionStage} documentation for rules covering exceptional completion.
@param fn the function to use to compute the value of the returned CompletionStage @param (U) the function's return type @return the new CompletionStage