Convert list to map using only two keys and odd or even index indices as values ​​- Java 8 Stream

Question

I would like to convert the list to a map using only two string values ​​as key values. Then, as values, just a list of strings containing items from odd or even index items from the input list. Here is the old fashion code:

Map<String, List<String>> map = new HashMap<>();

List<String> list = Arrays.asList("one", "two", "three", "four");

map.put("evenIndex", new ArrayList<>());
map.put("oddIndex", new ArrayList<>());
for (int i = 0; i < list.size(); i++) {
    if(i % 2 == 0)
        map.get("evenIndex").add(list.get(i));
    else 
        map.get("oddIndex").add(list.get(i));
}

How to convert this code to Java 8 using threads to get this result?

{evenIndex=[one, three], oddIndex=[two, four]}

My current messy attempt requires changing the list items, but there definitely should be a better option.

List<String> listModified = Arrays.asList("++one", "two", "++three", "four");

map = listModified.stream()
           .collect(Collectors.groupingBy(
                               str -> str.startsWith("++") ? "evenIndex" : "oddIndex"));

Or maybe someone will help me with this wrong decision?

IntStream.range(0, list.size())
         .boxed()
         .collect(Collectors.groupingBy( i -> i % 2 == 0 ? "even" : "odd",
                  Collectors.toMap( (i -> i ) , i -> list.get(i) ) )));

which return this:

{even={0=one, 2=three}, odd={1=two, 3=four}}

Answer 1

:

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

IntStream.range(0,list.size())
        .boxed()
        .collect(groupingBy(
                i -> i % 2 == 0 ? "even" : "odd", 
                mapping(list::get, toList())
        ));

boolean, partitioningBy:

IntStream.range(0, list.size())
        .boxed()
        .collect(partitioningBy(
                i -> i % 2 == 0, 
                mapping(list::get, toList())
        ));

Answer 2

Collectors.toMap; :

Map<String, List<String>> map = IntStream.range(0, list.size())
            .boxed()
            .collect(Collectors.toMap(
                    x -> x % 2 == 0 ? "odd" : "even",
                    x -> {
                        List<String> inner = new ArrayList<>();
                        inner.add(list.get(x));
                        return inner;
                    },
                    (left, right) -> {
                        left.addAll(right);
                        return left;
                    },
                    HashMap::new));

    System.out.println(map);