How to transform a Map<String, List<String>> into a List<Map<String, String>> and get Cartesian product?

Question

Using Java I am attempting to create the Cartesian Product of a map which contains String keys and List<String> values. I need to preserve the values of the keys when creating the list like so:

• From a Map<String, List<String>> Object
• To a List<Map<String, String>> Object

I also have the requirement that when a List<String> value in the original map is empty it should still be treated as a single value when creating the Cartesian Product so as to avoid multiplying by 0 and creating no maps. For example the following map:

{
    "Location Number" = {"100", "500"}
    "Department Number" = {"11", "22", "33"}
    "District Number" = {}
    "Preferred Language" = {"en-US"}
}

Is translated to:

{
    {
        "Location Number" = "100"
        "Department Number" = "11"
        "District Number" = {}
        "Preferred Language" = "en-US"
    },
    {
        "Location Number" = "100"
        "Department Number" = "22"
        "District Number" = {}
        "Preferred Language" = "en-US"
    },
    {
        "Location Number" = "100"
        "Department Number" = "33"
        "District Number" = {}
        "Preferred Language" = "en-US"
    },
    {
        "Location Number" = "500"
        "Department Number" = "11"
        "District Number" = {}
        "Preferred Language" = "en-US"
    },
    {
        "Location Number" = "500"
        "Department Number" = "22"
        "District Number" = {}
        "Preferred Language" = "en-US"
    },
    {
        "Location Number" = "500"
        "Department Number" = "33"
        "District Number" = {}
        "Preferred Language" = "en-US"
    }
}

Below is the code I am currently using to accomplish a similar translation, but it does not retain the key, which I need. I do not know if this is possible to accomplish using Java 8 Streams in general.

private static List<List<String>> createRuleListFromMap(Map<String, List<String>> ruleMap) {
    List<List<String>> ruleList = new ArrayList<>();
    cartesianProduct(ruleMap.values()).forEach(ruleList::add);
    return ruleList;
}

private static <T> Stream<List<T>> cartesianProduct(Collection<? extends Collection<T>> collections) {
    return cartesianProduct(new ArrayList<Collection<T>>(collections), Collections.emptyList());
}

private static <T> Stream<List<T>> cartesianProduct(List<? extends Collection<T>> collections, List<T> current) {
    return collections.isEmpty() ? Stream.of(current) : collections.get(0).stream().flatMap(e -> {
        List<T> list = new ArrayList<>(current);
        list.add(e);
        return cartesianProduct(collections.subList(1, collections.size()), list);
    });
}

Answer 1

Here is simple example how to transform Map<String,List<String>> to List<Map<String,String>> :

import lombok.extern.slf4j.Slf4j;
import org.junit.Test;

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

@Slf4j
public class StreamTest {
    @Test
    public void test() {
        var map = new HashMap<String, List<String>>();
        map.put("key1", Arrays.asList("value1", "value2", "value3"));
        map.put("key2", Arrays.asList("value4", "value5", "value6"));

        var list = map.entrySet().stream()
                .flatMap(e -> e.getValue().stream()
                        .map(v -> Map.of(e.getKey(), v)))
                .collect(Collectors.toList());

        log.info(list.toString());
    }
}

Answer 2

You can first represent the elements of lists, each as Map<String,String> , and get a stream of lists of maps , and then reduce this stream to a single list by sequentially summing pairs of inner maps to get the Cartesian product .

Try it online!

Map<String, List<String>> map = new LinkedHashMap<>();
map.put("Location Number", List.of("100", "500"));
map.put("Department Number", List.of("11", "22", "33"));
map.put("District Number", List.of("")); // NOT an empty list!
map.put("Preferred Language", List.of("en-US"));

List<Map<String, String>> list = map.entrySet().stream()
        .map(entry -> entry.getValue().stream()
                // list elements as Map<String,String>
                .map(str -> Map.of(entry.getKey(), str))
                // List<Map<String,String>>
                .collect(Collectors.toList()))
        // intermediate output
        .peek(System.out::println)
        // stream of lists to a single list
        .reduce((list1, list2) -> list1.stream()
                // combinations of inner maps
                .flatMap(map1 -> list2.stream()
                        // join entries of two maps
                        .map(map2 -> {
                            Map<String, String> mp =
                                    new LinkedHashMap<>();
                            mp.putAll(map1);
                            mp.putAll(map2);
                            return mp;
                        }))
                // list of combinations
                .collect(Collectors.toList()))
        // returns List<Map<String,String>
        // otherwise an empty list
        .orElse(Collections.emptyList());

// final output
list.forEach(System.out::println);

Intermediate output:

[{Location Number=100}, {Location Number=500}]
[{Department Number=11}, {Department Number=22}, {Department Number=33}]
[{District Number=}]
[{Preferred Language=en-US}]

Final output:

{Location Number=100, Department Number=11, District Number=, Preferred Language=en-US}
{Location Number=100, Department Number=22, District Number=, Preferred Language=en-US}
{Location Number=100, Department Number=33, District Number=, Preferred Language=en-US}
{Location Number=500, Department Number=11, District Number=, Preferred Language=en-US}
{Location Number=500, Department Number=22, District Number=, Preferred Language=en-US}
{Location Number=500, Department Number=33, District Number=, Preferred Language=en-US}

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^{See also: Convert a map of lists into a list of maps}