how to update a Map,List> from Java's 8 Stream?

Question

What i'm trying to accomplish is to run a method in my stream's map, based in that if the return is correct, to add it to the success list which would go in the collection's map key, if failure, it would be added to the failure list which would go in the collection's value:

        List<String> success = new ArrayList<>();
        List<String> failure = new ArrayList<>();
        Map<List<String>,List<String>> masterLista = new HashMap<>();
        masterLista.put(success, failure);

        masterLista = list.stream().map(l -> {
            String newS=serviceApi.getClearSongName(l);
            if(newS!=null){    
                //Here the return should update the sucess list
                return serviceApi.getClearSongName(l);
            } else{
                //Here the return should update the failure list
                return l;
            }
        }).collect(Collectors.toList());

Actually the collection's map is not something necessary, the only thing i'd like to have is to inside the Stream's map already update two lists with successes and failures to avoid having to do a further processing inside a single list.

Edit: I'm using stream because actually the goal is to use parallelStream

It really looks like what you are wanting is a `Pair` or maybe even `Seq>` (Vavr). — chrylis -cautiouslyoptimistic-, Jul 15 '19 at 20:12
Not sure why you would need a HashMap in this case. I think you have misunderstood the use of maps — EDToaster, Jul 15 '19 at 20:13
Just use a normal for loop or for-each loop. Why do you need to use Streams here? It a straightforward for loop over the list with if-else inside it and adding an element of the list to either success or failure depending on the condition. — vatsal gosar, Jul 15 '19 at 20:46
Forgot to mention that i want this to run parallelStream as i'd do several API calls and would like to collect the success response and the failed attempts with the info that was sent in the call — sylleryum, Jul 15 '19 at 21:36

Yassin Hajaj · Answer 1 · 2019-07-15T20:43:59.953

You don't need a Map and you don't even need a Stream for this purpose. A simple for loop will do the work, or a List::forEach if you prefer

For loop

for (String str : list) {
    String newS = serviceApi.getClearSongName(str);
    if (newS != null)
        success.add(newS);
    else
        failure.add(newS);
}

Foreach

list.forEach(str -> {
    String newS = serviceApi.getClearSongName(str);
    if (newS != null)
        success.add(newS);
    else
        failure.add(newS);
});

PartitioningBy

You can also use the Collector::partitioningBy which returns a Map<Boolean, List<String>>

Map<Boolean, List<String>> map = 
    list.stream()
        .collect(Collectors.partitioningBy(str -> serviceApi.getClearSongName(str) != null));

map.get(true) will return the ones that classify ass success in your logic

score 0 · Answer 2 · answered Jul 15 '19 at 20:41

I am guessing you need to stream a list, call an api and gather the success and failures separately. Below is a small snippet on how this can be done (Its in groovy not java)

def a = [1,2,3,4,5,6,7,8,9]
Map<Boolean,List> x = a.stream().map{it -> new Tuple2<Boolean,Integer>(it>5, it)}.collect(Collectors.partitioningBy{i -> i[0]})
println(x)

Collectors.partitionBy splits the stream into two groups based on a predicate(condition). If you need to split into multiple groups, use the Collectors.groupingBy() method. The below link has more details on this specific use case Can you split a stream into two streams?

Do pay heed to the 'cannot work on infinite streams' warning

Your code would look something like below

Map<Boolean,List> masterLista = list.stream()
.map(l -> {
                String newS=serviceApi.getClearSongName(l)
                return new Tuple2<Boolean,String>(newS!=null, l)

}).collect(Collectors.partitioningBy{i -> i[0]})

how to update a Map,List> from Java's 8 Stream?

2 Answers2

For loop

Foreach

PartitioningBy