Convert Collection to Map

Question

I have two collections like below:

Set<String> attributes = Sets.newHashSet("aaa", "bbb", "ccc", "ddd");
Set<String> activeAttributes = Sets.newHashSet("eee", "lll", "ccc", "mmm");

The idea to convert these collections to a map, given that attributes collection should be used as keys of this map and activeAttributes should be used during calculating value (In case activeAttributes contains value from collection attributes then "true", otherwise "false" parameter should be set):

As example:

({aaa -> false, bbb -> false, ccc -> true, ddd -> false })

I've tried to create a Guava function that converts list to collection of Map.Entry:

private static class ActiveAttributesFunction implements Function<String, Map.Entry<String, Boolean>> {

    private Set<String> activeAttributes;

    public ActiveAttributesFunction (Set<String> activeAttributes) {
        this.activeAttributes = activeAttributes;
    }

    @Override
    public Map.Entry<String, Boolean> apply(String input) {
        return Maps.immutableEntry(input, activeAttributes.contains(input));
    }
}

But, this function will require to convert this list of entries to map.

Please suggest in which way this can be simplified?

you don't want to use a standard "for" loop ? – Pras Sep 23 '15 at 11:30 — Pras, Sep 23 '15 at 11:30

score 4 · Accepted Answer · edited Sep 23 '15 at 16:59

4

If you are using Java 8, you can do the following:

Set<String> attributes = Sets.newHashSet("aaa", "bbb", "ccc", "ddd");
Set<String> activeAttributes = Sets.newHashSet("eee", "lll", "ccc", "mmm");
Map<String, Boolean> map = attributes.stream().collect(Collectors.toMap(s -> s, activeAttributes::contains));
System.out.println(map);

For an earlier version of Java and with Guava, you can use Maps.asMap since Guava 14.0:

Map<String, Boolean> map = Maps.asMap(
    attributes, Functions.forPredicate(Predicates.in(activeAttributes)));

Note that this returns a live copy (any changes to the set will be reflected on the map). If you want an immutable Map, use Maps.toMap.

edited Sep 23 '15 at 16:59

Louis Wasserman

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answered Sep 23 '15 at 11:23

Tunaki

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Tunaki: Thanks for answer, but unfortunately we are using java 7. Will update tags. – fashuser Sep 23 '15 at 11:24
Thanks you very much! – fashuser Sep 23 '15 at 11:54
You could replace `s -> activeAttributes.contains(s)` with `activeAttributes::contains`. You could also replace `s -> s` with `Function.identity()` though that's not so neat. – sprinter Sep 23 '15 at 12:14

Convert Collection to Map

1 Answers1