Java 8 List into Map

Question

I want to translate a List of objects into a Map using Java 8\'s streams and lambdas.

This is how I would write it in Java 7 and below.

private Map&l         


        

Answer 1

Map<String, Set<String>> collect = Arrays.asList(Locale.getAvailableLocales()).stream().collect(Collectors
                .toMap(l -> l.getDisplayCountry(), l -> Collections.singleton(l.getDisplayLanguage())));

Answer 2

Another possibility only present in comments yet:

Map<String, Choice> result =
choices.stream().collect(Collectors.toMap(c -> c.getName(), c -> c)));

Useful if you want to use a parameter of a sub-object as Key:

Map<String, Choice> result =
choices.stream().collect(Collectors.toMap(c -> c.getUser().getName(), c -> c)));

Answer 3

I use this syntax

Map<Integer, List<Choice>> choiceMap = 
choices.stream().collect(Collectors.groupingBy(choice -> choice.getName()));

Answer 4

As an alternative to guava one can use kotlin-stdlib

private Map<String, Choice> nameMap(List<Choice> choices) {
    return CollectionsKt.associateBy(choices, Choice::getName);
}

Answer 5

Here's another one in case you don't want to use Collectors.toMap()

Map<String, Choice> result =
   choices.stream().collect(HashMap<String, Choice>::new, 
                           (m, c) -> m.put(c.getName(), c),
                           (m, u) -> {});

Answer 6

If your key is NOT guaranteed to be unique for all elements in the list, you should convert it to a Map<String, List<Choice>> instead of a Map<String, Choice>

Map<String, List<Choice>> result =
 choices.stream().collect(Collectors.groupingBy(Choice::getName));