How to best create a Java 8 stream from a nullable object?

Question

What is the best/idiomatic way of doing a null check before getting a stream?

I have method that is receiving a List that might be null. So I can\'t jus

Answer 1

The best thing I can think of would be to use an Optional with the orElseGet method.

return Optional.ofNullable(userList)
                .orElseGet(Collections::emptyList)
                .stream()
                .map(user -> user.getName())
                .collect(toList());

Updated with @Misha's suggest to use Collections::emptyList over ArrayList::new

Answer 2

In the other answers, the Optional instance is created and used strictly within the same statement. The Optional class is primarily useful for communicating with the caller about presence or absence of a return value, fused with the actual value if present. Using it wholly within a single method seems unnecessary.

Let me propose the following more prosaic technique:

static <T> Stream<T> nullableListToStream(List<T> list) {
    return list == null ? Stream.empty() : list.stream();
}

I guess the ternary operator is somewhat déclassé these days, but I think this is the simplest and most efficient of the solutions.

If I were writing this for real (that is, for a real library, not just sample code on Stack Overflow) I'd put in wildcards so that that the stream return type can vary from the List type. Oh, and it can be a Collection, since that's where the stream() method is defined:

@SuppressWarnings("unchecked")
static <T> Stream<T> nullableCollectionToStream(Collection<? extends T> coll) {
    return coll == null ? Stream.empty() : (Stream<T>)coll.stream();
}

(The warning suppression is necessary because of the cast from Stream<? extends T> to Stream<T> which is safe, but the compiler doesn't know that.)

Answer 3

Java 8:

Optional.ofNullable(collection)
    .stream()
    .flatMap(Collection::stream)

Java 9:

Stream.ofNullable(collection)
    .flatMap(Collection::stream)

Apache Commons Collections 4:

import org.apache.commons.collections4.CollectionUtils;
CollectionUtils.emptyIfNull(collection)
    .stream()

Answer 4

Personally I consider null deprecated and use Optional wherever possible despite the (tiny) performance overhead. So I use the interface from Stuart Marks with an implementation based on gdejohn, i.e.

@SuppressWarnings("unchecked")
static <T> Stream<T> nullableCollectionToStream(Collection<? extends T> coll)
{
    return (Stream<T>) Optional.ofNullable(coll)
                           .map(Collection::stream)
                           .orElseGet(Stream::empty);
}

Answer 5

apache commons-collections4:

CollectionUtils.emptyIfNull(list).stream()

Answer 6

I agree with Stuart Marks that list == null ? Stream.empty() : list.stream() is the right way to do this (see his answer), or at least the right way to do this pre-Java 9 (see edit below), but I'll leave this answer up to demonstrate usage of the Optional API.

<T> Stream<T> getStream(List<T> list) {
    return Optional.ofNullable(list).map(List::stream).orElseGet(Stream::empty);
}

---

Edit: Java 9 added the static factory method Stream.<T>ofNullable(T), which returns the empty stream given a null argument, otherwise a stream with the argument as its only element. If the argument is a collection, we can then flatMap to turn it into a stream.

<T> Stream<T> fromNullableCollection(Collection<? extends T> collection) {
    return Stream.ofNullable(collection).flatMap(Collection::stream);
}

This doesn't misuse the Optional API as discussed by Stuart Marks, and in contrast to the ternary operator solution, there's no opportunity for a null pointer exception (like if you weren't paying attention and screwed up the order of the operands). It also works with an upper-bounded wildcard without needing SuppressWarnings("unchecked") thanks to the signature of flatMap, so you can get a Stream<T> from a collection of elements of any subtype of T.