Is there a JDK or Guava method to turn a null into an empty list?

Question

Is there a method like this one in the JDK or Google Guava

public static <T> Collection<T> safe(Collection<T> collection) {
    if (collection == null) {
        return new ArrayList<>(0);
    } else {
        return collection;
    }
}

which makes it easy to not crash on a an enhanced loop if something returns a null list for example

for (String string : CollectionUtils.safe(foo.canReturnANullListOfStrings())) {
    // do something
}

would not crash.

I looked around but could not find any such method, and I am wondering if I missed it or if there is a reason why such a handy method is not handy and therefore not included?

Methods should not return `null` collections in the first place. — SLaks, Feb 15 '13 at 21:26
I believe such a method can be easily written using `Collections.emptyList()`. — Danilo Piazzalunga, Feb 15 '13 at 21:27
@SLaks agreed in principle but you can't control other peoples code :) — ams, Feb 15 '13 at 21:35
One somewhat confusing way to protect yourself from code that returns `null` instead of an empty `Collection` is to use AspectJ and after-advice. Create a pointcut that matches execution of methods returning `Collection`, and return an empty `Collection` if the original return value is `null`. Repeat for `Map`, `List`, etc. Your colleagues may not appreciate this, however. — Eric Jablow, Mar 24 '13 at 03:01

score 28 · Accepted Answer · edited May 23 '17 at 11:47

28

Objects.firstNonNull(list, ImmutableList.<Foo>of());

There's no need for a specialized method, and this is indeed the solution we recommend you use immediately whenever you get a potentially-null collection from a naughty API that ideally shouldn't do that in the first place.

edited May 23 '17 at 11:47

Community

1
1

answered Feb 15 '13 at 21:31

Louis Wasserman

191,574
25
345
413

3

What a way to make simple task look complicated. Use apache collections ListUtils.emptyIfNull from comment below. reads way better. – Vasiliy Dec 18 '19 at 14:37
This is exactly why people make fun of Java. There are far more readable solutions given by others. – saran3h Jan 22 '22 at 06:05

score 27 · Answer 2 · edited Oct 03 '18 at 01:24

27

Apache Collections 4 has a generic method ListUtils.emptyIfNull(List<T> list)

Here is the doc: https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/ListUtils.html

edited Oct 03 '18 at 01:24

palacsint

28,416
10
82
109

answered Aug 03 '16 at 03:50

Jeffrey

1,068
2
15
25

Best non-java 8 solution. – saran3h Jan 22 '22 at 06:03

score 18 · Answer 3 · answered Dec 09 '16 at 13:52

18

In Java 8 it is possible to use this:

Optional.ofNullable(foo.canReturnANullListOfStrings()).orElse(Collections.emptyList());

answered Dec 09 '16 at 13:52

Vojta

1,583
17
19

score 7 · Answer 4 · answered Sep 14 '18 at 11:07

7

Update for Java 9: java.util.Objects.requireNonNullElse(collection, List.<T>of())

The <T> is still required.

answered Sep 14 '18 at 11:07

Mateusz Stefek

3,478
2
23
28

score 1 · Answer 5 · answered Feb 15 '13 at 21:27

1

So there does not exist a function to my knowledge of this sort. However writing one is trivial as you have shown above. The reasoning behind why it might not have been included is because null has a specific meaning and it might not be appropriate to return an empty Collection when one gets passed around. Generally (in my experience) when a null value enters the system something failed higher up the chain or an invalid value was not properly sanitized.

answered Feb 15 '13 at 21:27

Woot4Moo

23,987
16
94
151

Unfortunately I see it quite often in a generated wrapping e.g. some JSON. And construction like `for(String thing : someObjFromJson.getFoo().getBar().getListOfThings()) {...do stuff...}` becomes not trivial without such method. – kan Jul 04 '19 at 10:21

Is there a JDK or Guava method to turn a null into an empty list?

5 Answers5

Linked

Related