Create Maps using a reduce operation

createmaps.java

import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.Arrays;
public class MyClass {
    public static void main(String args[]) {
        List<String> values = List.of("1", "2", "3", "4", "5", "5", "2");
        Map<String, List<String>> map = values.stream()
        .collect(Collectors.toMap(
            v -> v, 
            v -> new ArrayList<>(Arrays.asList(v)),
            (left, right) -> {
                left.addAll(right);
                return left;
            }
        ));
        System.out.println(map);
    }
}

java -cp /tmp/p8nc3kSSrJ MyClass
{1=[1], 2=[2, 2], 3=[3], 4=[4], 5=[5, 5]}