split items of batch into multiple categories I have a batch job that contains a lot of items. I need to split these items into 4 categories - 1,2 category should be moved to a separate jobs as is; 3,4 categories job be additionally checked on validness and split into separate jobs - invalid items should be moved to a separate jobs.

Roughly diagram of the expected flow looks like this



For now I come up the prototype that looks like this

import java.util.*;
import java.util.stream.*;

public class MyClass {
    public static void main(String args[]) {
        Job job = new Job();
        job.items = Arrays.asList(
            new Item(ItemCategory.CAT_1, "1"),
            new Item(ItemCategory.CAT_2, "2"),
            new Item(ItemCategory.CAT_3, "3"),
            new Item(ItemCategory.CAT_1, "4"),
            new Item(ItemCategory.CAT_2, "5"),
            new Item(ItemCategory.CAT_3, "6"),
            new Item(ItemCategory.CAT_4, "7"),
            new Item(ItemCategory.CAT_4, "8")
        );

        /*
            [1:CAT_1, 4:CAT_1]
            [2:CAT_2, 5:CAT_2]
            [6:CAT_3]
            [8:CAT_4]
            [3:CAT_3, 7:CAT_4]
        */
        List&lt;List&lt;Item&gt;&gt; categorizedItems = job.items.parallelStream().collect(
            () -&gt; Arrays.asList(new ArrayList&lt;&gt;(), new ArrayList&lt;&gt;(), new ArrayList&lt;&gt;(), new ArrayList&lt;&gt;(), new ArrayList&lt;&gt;()), // last item is error aggregator
            (list, item) -&gt; {
                switch (item.category) {
                    case CAT_1: list.get(0).add(item); break;
                    case CAT_2: list.get(1).add(item); break;
                    case CAT_3: {

                        if (isValid(item.id)) {
                            list.get(2).add(item);
                        } else {
                            list.get(4).add(item);
                        }

                        break;
                    }
                    case CAT_4: {

                        if (isValid(item.id)) {
                            list.get(3).add(item);
                        } else {
                            list.get(4).add(item);
                        }

                        break;
                    }
                }
            },
            (list1, list2) -&gt; {
                list1.get(0).addAll(list2.get(0));
                list1.get(1).addAll(list2.get(1));
                list1.get(2).addAll(list2.get(2));
                list1.get(3).addAll(list2.get(3));
                list1.get(4).addAll(list2.get(4));
            }
        );
    }

    public static boolean isValid(String id) {
        return Integer.valueOf(id) % 2 == 0;
    }

    public static class Item {
        ItemCategory category;
        String id;

        Item(ItemCategory category, String id) {
            this.id = id;
            this.category = category;
        }

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

    public enum ItemCategory {
        CAT_1, CAT_2, CAT_3, CAT_4;
    }

    public static class Job {
        List&lt;Item&gt; items;
    }
}


Is it a good a idea to do everything in a single stream? Or for example it is better split into 4 categories first and then process 3,4 categories separately?
Your Collector is reinventing the wheel.  There already exists a collector which partitions the collected items into groups: Collectors.groupingBy, which you could use something like:

.collect(
    Collectors.groupingBy(
        item -&gt; (item.category == CAT_3 || item.category == CAT_4) &amp;&amp; !isValid(item.id)
                     ? CAT_5 : item.category)
    )
)


Of course, this will return a Map&lt;ItemCategory, List&lt;Item&gt;&gt;.  You can transform this back to a list of lists, if needed. 

And of course, you would need to add the CAT_5 to the ItemCategory enum.  Alternately, you could use null at the category 5 key, if you don't mind null as a key value, but be warned that it will make some people's skin crawl.



If the order of the items in the all of the groups (as opposed to just the Category 5 group) does not matter, then groupingByConcurrent will give better parallel stream performance.