Breaking List of list to a single list java

Question

I have a java method that returns below output:

List<CompletableFuture<List<String>>> futureResponselist;

How can I construct a single list from this data? As I have to iterate the final output and do some operation with the "string" param that is present inside.

I want all the string values present inside these lists in a final single list.

It should be as: List<String> My futureResponselist size is 3.so the conversion TO List should happen in nonblocking way

Answer 1

You have many ways to do this. To flatten nested lists for Java 8 and higher, you can use a structure like below to do it functionally. If you want to process in parallel you can use Collection::parallelStream instead of Collection::stream.

List<String> onlyStringValues = futureResponseList.stream()
                                            .map(CompletableFuture::join)
                                            .flatMap(Collection::stream)
                                            .collect(Collectors.toList());

Answer 2

It's actually quite simple: Iterate the outer list using an enhanced for loop, get the inner list from the CompletableFuture and add all the elements to the result list.

List<CompletableFuture<List<String>>> futureResponselist = ...

List<String> result = new ArrayList<>();
for (CompletableFuture<List<String>> future : futureResponselist)
    result.addAll(future.get());

Answer 3

you can use this code, if the order of list is not important consider parallelizsing it

List<CompletableFuture<List<String>>> futureResponselist = new ArrayList<>();
futureResponselist.add(CompletableFuture.supplyAsync(() -> List.of("1" , "2")));
futureResponselist.add(CompletableFuture.supplyAsync(() -> List.of("3" , "4", "5")));
futureResponselist.add(CompletableFuture.supplyAsync(() -> List.of("6")));
List<String> result = futureResponselist
        .stream()
        .map(CompletableFuture::join)
        .flatMap(List::stream)
        .collect(Collectors.toList());

update: more general approach it is adopted from the Modern Java in Action

Executor executor =
        Executors.newFixedThreadPool(5);
List<CompletableFuture<List<String>>> futureResponselist = new ArrayList<>();
futureResponselist.add(CompletableFuture.supplyAsync(() -> List.of("1" , "2"), executor));
futureResponselist.add(CompletableFuture.supplyAsync(() -> List.of("3" , "4", "5"), executor));
futureResponselist.add(CompletableFuture.supplyAsync(() -> List.of("6"), executor));

List<CompletableFuture<List<String>>> futureResult = futureResponselist
        .stream()
        .map(future -> future.thenApply(Function.identity())) // do some processing with list here we just return it but you can do something else
        .map(future -> future.thenApply(Function.identity()))
        .collect(Collectors.toList());
futureResult
                    .parallelStream()
                    .map(CompletableFuture::join)
                    .flatMap(List::stream)
                    .forEach(System.out::println); // do something when the result is ready here we just print it

Answer 4

Iterate futureResponselist and then iterate list of String for each CompletableFuture and add the string to list declared outside of both for loop

List<CompletableFuture<List<String>>> futureResponselist;

// List of string to store combined result
List<String> mergedList = new ArrayList<>();

for (CompletableFuture com : futureResponselist) {
    for (String string : com.getYourListOfString()) {
        mergedList.add();
    }
}