Using Java's FutureTask to execute Callable tasks concurrently

In this demo we have 2 Case Providers, one for Activiti and one for WPS. I used Thread.sleep to mimic a time consuming computation (e.g. IO). Now we want to have a composite Case Provider which composes cases from both case providers. However, as we naively implement this in SequentialCaseProvider we will see that the total time it takes is the sum of having the 2 caseproviders fetch the cases (9 seconds). By using a concurrent approach in ConcurrentCaseProvider, we can reduce the time to +- the time it takes the longest CaseProvider to fetch the cases. (5 seconds)