How to combine logic using Java 8 streams to add to a list based on list size condition

Question

Is it possible to combine below using Java 8 streams that if there is some value for “getCount()” then keep on adding elements to same list to till size of the list is more than getCount().

List<RequestObject> li = results().map(req -> new RequestObject(req())).collect(Collectors.toList());

if(Objects.nonNull(getCount())){
    while(li.size() <= getCount()) {
        li.addAll(li);
    }
}

You do know that you keep doubling the list in size, right? E.g. if the list initially has 3 values, and `getCount()` returns `6`, then you end up with a list with 12 values, i.e. each initial value will now appear 4 times each. — Andreas, Jan 07 '20 at 13:58
Yes, I am aware of that. I need to create a payload having list's size greater than or less than based on count value. — Abhinav, Jan 07 '20 at 14:06

Steyrix · Answer 1 · 2020-01-07T14:07:15.517

0

If you are allowed to use Java 9, it provides dropWhile(predicate) and takeWhile(predicate) for streams.

int count = 0;
List<RequestObject> li = results()
                         .map(mapping)
                         .takeWhile(obj -> {count++ <= getCount()});

EDIT: in case of Java 8 you can take a look at this answer

edited Jan 07 '20 at 14:07

answered Jan 07 '20 at 14:01

Steyrix

2,796
1
9
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Sadly, we are still on Java 8. Thanks for your response. – Abhinav Jan 07 '20 at 14:06
@Abhinav and also I believe there are some 3rd party tools for that. Take a look at this article http://dubravsky.com/b/how-to-use-dropwhile-and-takewhile-in-java-8 – Steyrix Jan 07 '20 at 14:08

score 0 · Answer 2 · answered Jan 07 '20 at 14:09

This should do it.

        List<Integer> list = IntStream.iterate(0, i -> i < getCount(), i -> i+1)
                .map(b -> getNext()).boxed()
                .collect(Collectors.toList());
        System.out.println(list);


        public static int getCount() {
           return 10;
        }
        k = 0;

        public static int getNext() {
          k += 10;
          return k;
        }

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

0

Try with create new collection based on li:

List<RequestObject> collect = IntStream.range(0, getCount())
        .mapToObj(i -> li.get(i % li.size()))
        .collect(Collectors.toList());

UPDATE

I assumed that what you want can be shown on this example (where getCount() is count):

List<String> li = new ArrayList<>(List.of("a", "b", "c"));
int count = 2;
List<String> collect = IntStream.range(0, count)
        .mapToObj(i -> li.get(i % li.size()))
        .collect(Collectors.toList());

Then you get:

[a, b]

and for example count = 10:

[a, b, c, a, b, c, a, b, c, a]

edited Jun 20 '20 at 09:12

Community

1
1

answered Jan 07 '20 at 14:16

lczapski

4,026
3
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list size remains same as initial size. It does not adds in the list – Abhinav Jan 07 '20 at 14:30
@Abhinav try now – lczapski Jan 07 '20 at 15:27

How to combine logic using Java 8 streams to add to a list based on list size condition

3 Answers3

UPDATE