Returns a new CompletionStage that, when this stage completes either normally or exceptionally, is executed using the supplied executor, with this stage's result and exception as arguments to the supplied function.

When this stage is complete, the given function is invoked with the result (or null if none) and the exception (or null if none) of this stage as arguments, and the function's result is used to complete the returned stage.

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