This method should not be used. Sub-classes may use ReadOnlyCompletableFuture.setFailure(Throwable)
to complete the future exceptionally. Once completed, it cannot be changed. Instead, create a
derived future using various CompletionStage methods to define an alternate value.
This method should not be used. Sub-classes may use ReadOnlyCompletableFuture.setValue(Object) to
complete the future. Once completed, it cannot be changed. Instead, create a derived future
using various CompletionStage methods to define an alternate value.