Returns a new CompletionStage that, when this stage completes normally, is executed using the supplied Executor, with this stage's result as the argument to the supplied function. See the CompletionStage documentation for rules covering exceptional completion.

Parameters:

fn		the function to use to compute the value of the returned CompletionStage
executor		the executor to use for asynchronous execution
<U>		the function's return type

Returns:  the new CompletionStage