[java] How should we manage jdk8 stream for null values

Hello fellow Java developers,

I know the subject may be a bit in advance as the JDK8 is not yet released (and not for now anyway..) but I was reading some articles about the Lambda expressions and particularly the part related to the new collection API known as Stream.

Here is the example as given in the Java Magazine article (it is an otter population algorithm..):

Set<Otter> otters = getOtters();
System.out.println(otters.stream()
    .filter(o -> !o.isWild())
    .map(o -> o.getKeeper())
    .filter(k -> k.isFemale())
    .into(new ArrayList<>())
    .size());

My question is what happen if in the middle of the Set internal iteration, one of the otter is null?

I would expect a NullPointerException to be thrown but maybe am I still stuck in the previous development paradigm (non-functional), can someone enlighten me as how this should be handled?

If this really throw a NullPointerException, I find the feature quite dangerous and will have to be used only as below:

• Developer to ensure there is no null value (maybe using a previous .filter(o -> o != null))
• Developer to ensure the application is never generating null otter or a special NullOtter object to deal with.

What is the best option, or any other option?

Thanks!

Stuart's answer provides a great explanation, but I'd like to provide another example.

I ran into this issue when attempting to perform a reduce on a Stream containing null values (actually it was LongStream.average(), which is a type of reduction). Since average() returns OptionalDouble, I assumed the Stream could contain nulls but instead a NullPointerException was thrown. This is due to Stuart's explanation of null v. empty.

So, as the OP suggests, I added a filter like so:

list.stream()
    .filter(o -> o != null)
    .reduce(..);

Or as tangens pointed out below, use the predicate provided by the Java API:

list.stream()
    .filter(Objects::nonNull)
    .reduce(..);

From the mailing list discussion Stuart linked: Brian Goetz on nulls in Streams

Although the answers are 100% correct, a small suggestion to improve null case handling of the list itself with Optional:

 List<String> listOfStuffFiltered = Optional.ofNullable(listOfStuff)
                .orElseGet(Collections::emptyList)
                .stream()
                .filter(Objects::nonNull)
                .collect(Collectors.toList());

The part Optional.ofNullable(listOfStuff).orElseGet(Collections::emptyList) will allow you to handle nicely the case when listOfStuff is null and return an emptyList instead of failing with NullPointerException.

If you just want to filter null values out of a stream, you can simply use a method reference to java.util.Objects.nonNull(Object). From its documentation:

This method exists to be used as a Predicate, filter(Objects::nonNull)

For example:

List<String> list = Arrays.asList( null, "Foo", null, "Bar", null, null);

list.stream()
    .filter( Objects::nonNull )  // <-- Filter out null values
    .forEach( System.out::println );

This will print:

Foo
Bar

An example how to avoid null e.g. use filter before groupingBy

Filter out the null instances before groupingBy.

MyObjectlist.stream()
            .filter(p -> p.getSomeInstance() != null)
            .collect(Collectors.groupingBy(MyObject::getSomeInstance));