With Guava you can use Lists.newArrayList(Iterable) or Sets.newHashSet(Iterable), among other similar methods. This will of course copy all the elements in to memory. If that isn't acceptable, I think your code that works with these ought to take Iterable rather than Collection. Guava also happens to provide convenient methods for doing things you can do on a Collection using an Iterable (such as Iterables.isEmpty(Iterable) or Iterables.contains(Iterable, Object)), but the performance implications are more obvious.

只要你调用contains, containsAll, equals, hashCode, remove, retainAll, size或toArray，你就必须遍历元素。

如果您偶尔只调用isEmpty或clear等方法，我认为您最好是惰性地创建集合。例如，你可以有一个支持数组列表来存储先前迭代的元素。

我不知道任何一个库中有这样的类，但是编写它应该是一个相当简单的练习。

两个评论

There is no need to convert Iterable to Collection to use foreach loop - Iterable may be used in such loop directly, there is no syntactical difference, so I hardly understand why the original question was asked at all. Suggested way to convert Iterable to Collection is unsafe (the same relates to CollectionUtils) - there is no guarantee that subsequent calls to the next() method return different object instances. Moreover, this concern is not pure theoretical. E.g. Iterable implementation used to pass values to a reduce method of Hadoop Reducer always returns the same value instance, just with different field values. So if you apply makeCollection from above (or CollectionUtils.addAll(Iterator)) you will end up with a collection with all identical elements.

我使用我的自定义实用程序强制转换现有的集合(如果可用)。

主要:

public static <T> Collection<T> toCollection(Iterable<T> iterable) {
    if (iterable instanceof Collection) {
        return (Collection<T>) iterable;
    } else {
        return Lists.newArrayList(iterable);
    }
}

理想情况下，上面将使用immutabelist，但ImmutableCollection不允许空值，这可能会产生不希望看到的结果。

测试:

@Test
public void testToCollectionAlreadyCollection() {
    ArrayList<String> list = Lists.newArrayList(FIRST, MIDDLE, LAST);
    assertSame("no need to change, just cast", list, toCollection(list));
}

@Test
public void testIterableToCollection() {
    final ArrayList<String> expected = Lists.newArrayList(FIRST, null, MIDDLE, LAST);

    Collection<String> collection = toCollection(new Iterable<String>() {
        @Override
        public Iterator<String> iterator() {
            return expected.iterator();
        }
    });
    assertNotSame("a new list must have been created", expected, collection);
    assertTrue(expected + " != " + collection, CollectionUtils.isEqualCollection(expected, collection));
}

我为集合的所有子类型(Set,List等)实现了类似的实用程序。我以为这些已经是番石榴的一部分了，但我还没找到。

有点晚了，但我创建了一个非常优雅的Java 8解决方案，允许将T的Iterable转换为T的任何集合，而不需要任何库:

public static <T, C extends Collection<T>> C toCollection(Iterable<T> iterable, Supplier<C> baseSupplier) 
{
    C collection = baseSupplier.get();
    
    iterable.forEach(collection::add);
    
    return collection;
}

使用的例子:

Iterable<String> iterable = ...;
List<String> list = toCollection(iterable, ArrayList::new);

这不是对你问题的回答，但我相信这是解决你问题的办法。org.springframework.data.repository.CrudRepository接口确实有返回java.lang.Iterable的方法，但是你不应该使用这个接口。而是使用子接口，在你的例子中是org.springframework.data.mongodb.repository.MongoRepository。该接口具有返回java.util.List类型对象的方法。