如何将 Java8流收集到 GuavaImmutableCollection 中？

我想做以下事情:

List<Integer> list = IntStream.range(0, 7).collect(Collectors.toList());

但是在某种程度上，所得到的列表是 Guava 的 ImmutableList的一个实现。

我知道我能做到

List<Integer> list = IntStream.range(0, 7).collect(Collectors.toList());
List<Integer> immutableList = ImmutableList.copyOf(list);

但我想直接收钱，我试过了

List<Integer> list = IntStream.range(0, 7)
.collect(Collectors.toCollection(ImmutableList::of));

但它抛出了一个例外:

不支持的操作异常 收集。 ImmutableCollection.add (ImmutableCollection.java: 96)

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This is where the collectingAndThen collector is useful:

List<Integer> list = IntStream.range(0, 7).boxed()
.collect(collectingAndThen(toList(), ImmutableList::copyOf));

It applies the transformation to the List you just built; resulting in an ImmutableList.

Or you could directly collect into the Builder and call build() at the end:

List<Integer> list = IntStream.range(0, 7)
.collect(Builder<Integer>::new, Builder<Integer>::add, (builder1, builder2) -> builder1.addAll(builder2.build()))
.build();

If this option is a bit-verbose to you and you want to use it in many places, you can create your own collector:

class ImmutableListCollector<T> implements Collector<T, Builder<T>, ImmutableList<T>> {
@Override
public Supplier<Builder<T>> supplier() {
return Builder::new;
}


@Override
public BiConsumer<Builder<T>, T> accumulator() {
return (b, e) -> b.add(e);
}


@Override
public BinaryOperator<Builder<T>> combiner() {
return (b1, b2) -> b1.addAll(b2.build());
}


@Override
public Function<Builder<T>, ImmutableList<T>> finisher() {
return Builder::build;
}


@Override
public Set<Characteristics> characteristics() {
return ImmutableSet.of();
}
}

and then:

List<Integer> list = IntStream.range(0, 7)
.boxed()
.collect(new ImmutableListCollector<>());

Just in case the link disappears in the comments; my second approach could be defined in a static utility method that simply uses Collector.of. It's simpler than creating your own Collector class.

public static <T> Collector<T, Builder<T>, ImmutableList<T>> toImmutableList() {
return Collector.of(Builder<T>::new, Builder<T>::add, (l, r) -> l.addAll(r.build()), Builder<T>::build);
}

and the usage:

 List<Integer> list = IntStream.range(0, 7)
.boxed()
.collect(toImmutableList());
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While not a direct answer to my question (it does not use collectors), this is a fairly elegant approach which doesn't use intermediate collections:

Stream<Integer> stream = IntStream.range(0, 7).boxed();
List<Integer> list = ImmutableList.copyOf(stream.iterator());

Source.

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FYI, there's a reasonable way to do this in Guava without Java 8:

ImmutableSortedSet<Integer> set = ContiguousSet.create(
Range.closedOpen(0, 7), DiscreteDomain.integers());
ImmutableList<Integer> list = set.asList();

If you don't actually need the List semantics and can just use a NavigableSet, that's even better since a ContiguousSet doesn't have to actually store all the elements in it (just the Range and DiscreteDomain).

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最佳答案

The toImmutableList() method in the accepted answer of Alexis is now included in Guava 21 and can be used as:

ImmutableList<Integer> list = IntStream.range(0, 7)
.boxed()
.collect(ImmutableList.toImmutableList());

Edit: Removed @Beta from ImmutableList.toImmutableList along with other frequently used APIs in Release 27.1 (6242bdd).

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BTW: since JDK 10 it can be done in pure Java:

List<Integer> list = IntStream.range(0, 7)
.collect(Collectors.toUnmodifiableList());

Also toUnmodifiableSet and toUnmodifiableMap available.

Inside collector it was done via List.of(list.toArray())


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