Grouping Objects from Different classes using java 8 streams

Question

I have the classes below

class Pojo1 {
    pojo2 item;
}

class pojo2 {
    pojo3 item;
}

class pojo3 {
   pojo4 item;
}

I want to accumulate them as below from a stream of pojo1. (Stream multiple)

Map<pojo4,Map<pojo3,List<pojo2>>>  k=new HashMap()<>;

Can anyone help me do this in streams? And also I want to sort the map on attributes below:

   (pojo4 - some property)
   (pojo3 - some property)
   (pojo2- some property)

I would like to do this using both java 8 stream and parallel streams.

Nowhere Man · Accepted Answer · 2021-11-14 10:15:59Z

1

First, the mentioned POJO classes Pojo4 and Pojo3 need to implement hashCode and equals in order to be used as map keys as well as relevant getters for the properties.

The conversion should use common Collectors.groupingBy / Collectors.mapping collectors, and initial remapping to a plain container of two objects such as Pair is needed.

The example below uses Java 11 Map.entry as an intermediate container.

static Map<Pojo4, <Map<Pojo3, List<Pojo2>>>> convert(Stream<Pojo1> ones) {
    return ones              // Stream<Pojo1>
        .map(Pojo1::getItem) // Stream<Pojo2>
        .map(p2 -> Map.entry(p2, p2.getItem())) // Stream<Map.Entry<Pojo2, Pojo3>>
        .collect(Collectors.groupingBy(
            me -> me.getValue().getItem(), // key: Pojo4
            Collectors.groupingBy(
                me -> me.getValue(), // key: Pojo3
                Collectors.mapping(
                    Map.Entry::getKey, Collectors.toList()
                )
            )
        ));
}

Similarly, sorted version may be implemented using Collectors.groupingBy with map supplier; additionally List<Pojo2> should be sorted using collectingAndThen.

Assuming that Pojo2, Pojo3, Pojo4 have getter getField for the property to sort, the following update is provided:

static Map<Pojo4, <Map<Pojo3, List<Pojo2>>>> convert(Stream<Pojo1> ones) {
    return ones
        .map(Pojo1::getItem) // Stream<Pojo2>
        .map(p2 -> Map.entry(p2, p2.getItem())) // Stream<Map.Entry<Pojo2, Pojo3>>
        .collect(Collectors.groupingBy(
            me -> me.getValue().getItem(), // key: Pojo4
            () -> new TreeMap(Comparator.comparing(Pojo4::getField)),
            Collectors.groupingBy(
                me -> me.getValue(), // key: Pojo3
                () -> new TreeMap(Comparator.comparing(Pojo3::getField)),
                Collectors.mapping(
                    Map.Entry::getKey, Collectors.collectingAndThen(
                        Collectors.toList(), // List<Pojo2>
                        list -> {
                            list.sort(Comparator.comparing(Pojo2::getField)); 
                            return list;
                        }
                    )
                )
            )
        ));
}

Disclaimer: the code samples above have not been tested, so additional tweaking may be needed.

edited Nov 14, 2021 at 10:15

answered Nov 14, 2021 at 9:00

Nowhere Man

19.7k9 gold badges21 silver badges45 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

karthik Over a year ago

Thanks but its not working can it get any simpler to understand :( Type mismatch: cannot convert from Map<Object,Map<Object,List<Object>>> to Map<pojo4,Map<pojo3,List<pojo2>>>

Nowhere Man Over a year ago

What is not working? There are plenty things to define in the pojo classes.

Nowhere Man Over a year ago

can it get any simpler I'm afraid not.

karthik Over a year ago

new TreeMap(Comparator.comparing(Pojo3::getField)),new TreeMap(Comparator.comparing(Pojo4::getField)) what could be Type arguments for both treemaps? as in <K,V> info?

karthik Over a year ago

can you please tell what the groupBy and mapping operations result to?

|

Collectives™ on Stack Overflow

Grouping Objects from Different classes using java 8 streams

1 Answer 1

7 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

7 Comments

Your Answer

Sign up or log in

Post as a guest

Related