CompletableFuture supplyAsync with Stream.map()

Question

guys! I have a question: what does this code do:

Collection<Contract.class> contracts = fillTheCollectionFromDb();
contracts.stream().filter(condition)
                  .map(contractItem ->
                       CompletableFuture.supplyAsync(() -> 
                           {T result = getAnotherDataFromDb(contractItem); 
                            return result;}, Executor.class)
                  )//end .map
                  .map(CompletableFuture::join).collect(Collectors.toList());

Answer 1

this code is equivalent to:

Collection<Contract> contracts = fillTheCollectionFromDb();
contracts.stream().filter(condition)
              .map(this::getAnotherDataFromDb)
              .collect(Collectors.toList());

To make this program truly parallel, it should be modified. First, launch all the requests to database in parallel:

Collection<Contract> contracts = fillTheCollectionFromDb();
List<CompletableFuture> futures = contracts.stream().filter(condition)
              .map(contractItem ->
                   CompletableFuture.supplyAsync(
                         ()->getAnotherDataFromDb(contractItem),
                       executor)
              )//end .map
              .collect(Collectors.toList());

and only after that collect all the results:

List results = futures.stream
      .map(CompletableFuture::join)
      .collect(Collectors.toList());