程序员的小窝

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<String, Choice> nameMap(List<Choice> choices) {
        final Map<String, Choice> hashMap = new HashMap<>();
        for (final Choice choice : choices) {
            hashMap.put(choice.getName(), choice);
        }
        return hashMap;
}

I can accomplish this easily using Java 8 and Guava but I would like to know how to do this without Guava.

In Guava:

private Map<String, Choice> nameMap(List<Choice> choices) {
    return Maps.uniqueIndex(choices, new Function<Choice, String>() {

        @Override
        public String apply(final Choice input) {
            return input.getName();
        }
    });
}

And Guava with Java 8 lambdas.

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

Based on Collectors documentation it's as simple as:

Map<String, Choice> result =
    choices.stream().collect(Collectors.toMap(Choice::getName,
                                              Function.identity()));

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));