CODEWITHSUNDEEP
javajava-8java-stream
Daniel Kelley
Daniel Kelley

Reputation: 7737

Complex aggregation using Java 8 streams

Given a class Item:

public class Item {
    private String field1;
    private String field2;
    private String field3;
    private Integer field4;

    // getters, constructor...
}

And another class Group (field1 and field2 store the equivalent fields from Item):

public class Group {
    private String field1;
    private String field2;
}

I have a List<Item> which I need to aggregate into a map of the following structure:

Map<Group, Map<Field3, List<Field4>>>

Example data:

Field1 | Field2 | Field3 | Field4
------ | ------ | ------ | ------
"f1"   | "f2"   | "a"    | 1
"f1"   | "f2"   | "a"    | 2
"f1"   | "f2"   | "a"    | 3
"f1"   | "f2"   | "b"    | 4
"f1"   | "f2"   | "b"    | 5
"f1"   | "f2"   | "c"    | 6
"f1a"  | "f2a"  | "a"    | 7
"f1a"  | "f2a"  | "a"    | 8

The expected result would look like:

Group(field1=f1a, field2=f2a)={b=[7, 8]}, Group(field1=f1, field2=f2)={a=[1, 2, 3], b=[4, 5], c=[6]}

So far I have been able to aggregate by Field1, Field2 and Field3, such that I have the following structure (where GroupEx represents a POJO that holds Field1, Field2 and Field3):

Map<GroupEx, List<Field4>>

The code to aggregate in this way is:

Map<GroupEx, List<Integer>> aggregated = items.stream()
    .collect(Collectors.groupingBy(item -> new GroupEx(x.getField1(), x.getField2(), x.getField3())
           , Collectors.mapping(Item::getField4, Collectors.toList())));

I'm struggling to get the syntax correct to allow me to group by Field1 and Field2, and to then group by Field3 and Field4 into a map in the way I need.

The "long hand" syntax is:

Map<Group<String, String>, Map<String, List<Integer>>> aggregated = new HashMap<>();
for (Item item : items) {
    Group key = new Group(item.getField1(), item.getField2());
    Map<String, List<Integer>> field3Map = aggregated.get(key);
    if (field3Map == null) {
        field3Map = new HashMap<>();
        aggregated.put(key, field3Map);
    }

    List<Integer> field4s = field3Map.get(item.getField3());
    if (field4s == null) {
        field4s = new ArrayList<>();
        field3Map.put(item.getField3(), field4s);
    }

    field4s.add(item.getField4());
}

Is someone able to show me how my target grouping can be achieved?

Upvotes: 3

Views: 1306

Answers (1)

Alexis C.
Alexis C.

Reputation: 93842

This is where the downstream collectors feature comes handy.

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.toList;

...

List<Item> list = ....
Map<Group, Map<String, List<Integer>>> map =
    list.stream().collect(groupingBy(i -> new Group(i.getField1(), i.getField2()),
                                     groupingBy(Item::getField3, mapping(Item::getField4, toList()))));

First, you group the items by their Group field (you a Map<Group, List<Item>> at this point), then you map each value (List<Item>) to a map again where you group by the field3 (Map<Group, Map<Field3, List<Item>>).

Then you map the values in the second map by the field4 and you collect them into a list, to finally get a Map<Group, Map<Field3, List<Field4>>.

Given your input, it outputs:

{Group{field1='f1a', field2='f2a'}={a=[7, 8]}, Group{field1='f1', field2='f2'}={a=[1, 2, 3], b=[4, 5], c=[6]}}

Upvotes: 8

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