Streams :Collect with supplier and accumulator

Question

I have been trying to work with collect method of Stream, to convert list into HashMap. I am using below code as reference :

String result = list.parallelStream().collect(StringBuilder::new,
            (response, element) -> response.append(" ").append(element),
            (response1, response2) -> response1.append(",").append(response2.toString()))
            .toString();

while I write the below incomplete statement in eclipse and do a ctrl+space at ?

Map<String,Choice> map1=  list1.stream().collect(()-> new HashMap<String,Choice>(), 
        (r,s) -> r.?,(r,s) -> r.putAll(s));

considering the code snippet that uses StringBuilder, my expectation was that, the first argument of accumulator function should be a HashMap as I have used HashMap::new as supplier function. As per this understanding, eclipse should suggest me methods of HashMap, but that's not the case.

However, this seems to work fine

list1.stream().collect(ArrayList::new, ArrayList::add, ArrayList::addAll);

Already referred Java 8 Int Stream collect with StringBuilder but, not much luck with this.

Please read [Why not to upload images of code on SO when asking a question?](https://meta.stackoverflow.com/questions/285551/why-not-to-upload-images-of-code-on-so-when-asking-a-question), then edit your question accordingly. — Joe C, Jul 14 '17 at 20:30
Do you understand your own question actually? What does list contain? What do you want to obtain? — Eugene, Jul 14 '17 at 22:01
Explain what is your input and what you expect as output rather than explaining your code. — user3734782, Jul 15 '17 at 06:30
my input is a list of Choice Object (int id,String string) and I want to convert that into a map having string as key and object as value — Rajeev Akotkar, Jul 15 '17 at 06:31
What’s your question about, Eclipse’s code completion or how to solve this Stream collect task? — Holger, Jul 17 '17 at 07:47
initially I was not getting code completion , methods of hasmap were not showing up,but later on ode got compiled with eclipse.Also,I wanted to follow this particular approach to collect (Stream) — Rajeev Akotkar, Jul 17 '17 at 09:10

Eugene · Answer 1 · 2017-07-16T11:18:39.200

If you wanted to explore more the supplier, accumulator, combiner you should have written that a bit more clear to begin with.

Suppose you have a class like this:

static class Foo {
    private final int id;

    private final String s;

    public Foo(int id, String s) {
        super();
        this.id = id;
        this.s = s;
    }

    public int getId() {
        return id;
    }

    public String getS() {
        return s;
    }

    @Override
    public String toString() {
        return "" + id;
    }
}

If you know that s in this case will be unique you can simply do this:

HashMap<String, Foo> result = Arrays.asList(new Foo(1, "a"), new Foo(2, "b"))
            .stream()
            .collect(HashMap::new, 
                    (map, foo) -> map.put(foo.getS(), foo), 
                    HashMap::putAll);

    System.out.println(result); // {a=1, b=2}

But if s is not unique you would probably need to collect to a List and that makes things a bit more complicated:

 HashMap<String, List<Foo>> result = Arrays.asList(
            new Foo(1, "a"),
            new Foo(2, "b"),
            new Foo(3, "a"))
            .stream()
            .parallel()
            .collect(HashMap::new,
                    (map, foo) -> {
                        map.computeIfAbsent(foo.getS(), s -> new ArrayList<>()).add(foo);
                    },
                    (left, right) -> {
                        for (HashMap.Entry<String, List<Foo>> e : right.entrySet()) {
                            left.merge(
                                    e.getKey(),
                                    e.getValue(),
                                    (l1, l2) -> {
                                        l1.addAll(l2);
                                        return l1;
                                    });
                        }
                    });

    System.out.println(result); // {a=[1, 3], b=[2]}

Why aren't you using `map.computeIfAbsent(foo.getS(), new ArrayList<>()).add(foo)` in the accumulator? — fps, Jul 16 '17 at 00:15
And your combiner could be written more succintly as `right.forEach((k, v) -> left.merge(k, v, (l1, l2) -> { l1.addAll(l2); return l1; }))` — fps, Jul 16 '17 at 00:18
@FedericoPeraltaSchaffner I sort of missed that (`computeIfAbsent`) :) especially since it is present in the documentation also... thx. — Eugene, Jul 16 '17 at 11:18

score 2 · Answer 2 · answered Jul 14 '17 at 20:22

2

Convertting a list into a map? Collectors.toMap(..) might be what you need.

the javadocs have a couple good examples: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#toMap-java.util.function.Function-java.util.function.Function-

answered Jul 14 '17 at 20:22

Andreas

4,937
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Yes I am aware of this solution, but want to go this way to explore the things. – Rajeev Akotkar Jul 14 '17 at 20:24
Figured it out myself. Thx Map map= list1.stream().collect(()-> new HashMap(), (r,s) -> r.put(s.getString(),s),(r,s) -> r.putAll(s)); – Rajeev Akotkar Jul 14 '17 at 20:47
@RajeevAkotkar Post an answer instead of a comment then. – Bubletan Jul 14 '17 at 20:50
Done..posted the answer as well – Rajeev Akotkar Jul 15 '17 at 07:04

user3734782 · Answer 3 · 2017-07-15T07:23:23.363

1

Accoring to your comment, I guess you can do it as simple as that.

List<Choice> choices = new ArrayList<>();

Map<String, Integer> map = choices.stream().collect(Collectors.toMap(Choice::getString, Choice::getID);

Rather than writing you own suppliers and accumulators, I recommend using Collectors and its static methods, whose are used to transfer stream to any type of Collection.

EDIT: If value of the map is required to be Choice. Function interface has static method called identity() (type of last stream transformation before collect was called, Choice in this example).

Map<String, Choice> map = choices.stream().collect(Collectors.toMap(Choice::getString, Function.identity());

edited Jul 15 '17 at 07:23

answered Jul 15 '17 at 06:41

user3734782

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thats correct but as mentioned in one of the commnets that I as already aware of this solution , but was trying to explore though the options.i.e no of different possible solutions. – Rajeev Akotkar Jul 15 '17 at 07:02
Why are you aware? However, I am not sure if I understood your question correctly, but value of the map should be `Integer` or `Choice`? You said 'object as value', where both Integer and Choice are objects. – user3734782 Jul 15 '17 at 07:09
should be a choice , key of hashmap should be String and value should be a choice object. – Rajeev Akotkar Jul 15 '17 at 07:14
This is a fairly simple problem and I already found the solution https://stackoverflow.com/questions/20363719/java-8-listv-into-mapk-v. My only constraint was I wanted to do it a particular way. – Rajeev Akotkar Jul 15 '17 at 07:36

score 0 · Answer 4 · edited Apr 14 '20 at 10:05

0

Map<String,Choice> map= list1.stream().collect(()-> new HashMap<String,Choice>(), (r,s) -> r.put(s.getString(),s),(r,s) -> r.putAll(s));

edited Apr 14 '20 at 10:05

TalG

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answered Jul 15 '17 at 06:34

Rajeev Akotkar

1,377
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Streams :Collect with supplier and accumulator

4 Answers4