Saving value in a field using Java 8 - java-8

Question

I have a list of callables
KNNQuery<O> knnq; //field
List<Callable<KNNQuery>> callables = Arrays.asList(
task1(database, relation),
task2(database, relation));
executor.invokeAll(callables) //List<Future<KNNQery>>
.stream() //stream<Future<KNNQuery>>
.map(future -> {
try{
return future.get();
}catch (Exception e){
throw new IllegalStateException(e);
}
}) //stream<KNNQuery>
.forEach(System.out::println);
Instead of printing output of two Futures to the screen i want to combing and save both future's output to the knnq field how can i do that?

Answer 1

You have to use a reducing operation to combine the stream elements.
A stream collect, that is the transformation of the stream elements into a specific accumulated result (List, Map, KNNQuery, and so for...) is a reducing operation.
Supposing that KNNQuery accepts a String parameter as constructor that is KNNQuery(String value) and you would like to combine the KNNQuery toString() value, you could do the following that uses the Collectors.joining() reduction :
KNNQuery<O> knnq =
new KNNQuery(
executor.invokeAll(callables) //List<Future<KNNQery>>
.stream() //stream<Future<KNNQuery>>
.map(future -> {
try{
return future.get();
}catch (Exception e){
throw new IllegalStateException(e);
}
})
.map(Object::toString)
.collect(Collectors.joining(","))
);
With Collectors.reducing() you would have more freedom to combine KNNQuery instances.
For example you could delegate it to a KNNQuery method : KNNQuery combine(KNNQuery other).
KNNQuery<O> knnq =
executor.invokeAll(callables) //List<Future<KNNQery>>
.stream() //stream<Future<KNNQuery>>
.map(future -> {
try{
return future.get();
}catch (Exception e){
throw new IllegalStateException(e);
}
})
.reducing(KNNQuery::combine) // add it to combine two KNNQuery instances
.orElse(null);