Java 8 collector for Guava immutable collections?

Question

I really like Java 8 streams and Guava\'s immutable collections, but I can\'t figure out how to use the two together.

For example, how do I implement a Java 8 Collec

Answer 1

Here's a version that will support several ImmutableMultimap implementations. Note that the first method is private since it requires an unsafe cast.

@SuppressWarnings("unchecked")
private static > Collector toImmutableMultimap(
        Function keyFunction,
        Function valueFunction,
        Supplier> builderSupplier) {

    return Collector.of(
            builderSupplier,
            (builder, element) -> builder.put(keyFunction.apply(element), valueFunction.apply(element)),
            (left, right) -> {
                left.putAll(right.build());
                return left;
            },
            builder -> (M)builder.build());
}

public static  Collector> toImmutableMultimap(
        Function keyFunction,
        Function valueFunction) {
    return toImmutableMultimap(keyFunction, valueFunction, ImmutableMultimap::builder);
}

public static  Collector> toImmutableListMultimap(
        Function keyFunction,
        Function valueFunction) {
    return toImmutableMultimap(keyFunction, valueFunction, ImmutableListMultimap::builder);
}

public static  Collector> toImmutableSetMultimap(
        Function keyFunction,
        Function valueFunction) {
    return toImmutableMultimap(keyFunction, valueFunction, ImmutableSetMultimap::builder);
}