How to workaround this JDK limitation?

Question

I'm looking for a class from the Java Collection Framework that would not allow null elements.

Do you know one?

What should happen? If you just want to ignore list.add(null) I would make a subclass of your favourite implementation, overwrite add() and check for null value — leifg, Aug 09 '11 at 13:50
Perhaps the better way to do it is to never insert nulls in the first place? Tolerating (and thus guarding against) nulls is usually a poor practice. — Konrad Garus, Aug 09 '11 at 13:51
@leifg Don't just overwrite add but also addAll! Otherwise you may be in for an interesting surprise. — Voo, Aug 09 '11 at 14:35

score 47 · Answer 1 · edited Jun 20 '20 at 09:12

47

Use Constraints:

import com.google.common.collect.Constraints;
...
Constraints.constrainedList(new ArrayList(), Constraints.notNull())

from Guava for maximum flexibility.

UPDATE: Guava Constraints has been deprecated in Release 15 - apparently without replacement.

UPDATE 2: As of now (Guava 19.0-rc2) Constrains is still there and not deprecated anymore. However, it's missing from the Javadoc.

I'm afraid that the Javadoc is right as MapConstraint have been deprecated in Release 19, too

edited Jun 20 '20 at 09:12

Community

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answered Aug 09 '11 at 14:01

maaartinus

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+1 for your answer but I want to stick with Java Framework ;) – Stephan Aug 09 '11 at 14:02
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OK, then I can offer you e.g. `java.util.concurrent.ConcurrentSkipListSet` from JDK. Or `Collections.newSetFromMap(new ConcurrentHashMap – maaartinus Aug 09 '11 at 14:09
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Unfortunately `Constraints` is deprecated and scheduled for removal in Guava 16.0 :-( – Joachim Sauer Jun 07 '13 at 08:50
That's sad. Funnily, `MapConstaints` are not. – maaartinus Jun 15 '13 at 13:39
3

`Constraints` is still there and it is no longer marked as deprecated. :-) – Fabian Barney Nov 11 '14 at 10:08

score 20 · Accepted Answer · answered Aug 09 '11 at 13:48

20

Most Queue implementations (with the notable exception of LinkedList) don't accept null.

EnumSet is a special-purpose Set implementation that doesn't allow null values.

answered Aug 09 '11 at 13:48

Joachim Sauer

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Cengiz · Answer 3 · 2021-08-23T03:24:36.210

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Apache Commons Framework - CollectionUtils.addIgnoreNull

Adds to myList if myObj is not null.

org.apache.commons.collections.CollectionUtils.addIgnoreNull(myList, myObj)

edited Aug 23 '21 at 03:24

answered Feb 20 '13 at 14:52

Cengiz

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score 12 · Answer 4 · answered Aug 11 '11 at 01:18

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There's a roundup of such collections here.

answered Aug 11 '11 at 01:18

Kevin Bourrillion

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For my future projects I'll definitively have a look at Guava ! +1 – Stephan Aug 11 '11 at 04:07

Didac Montero · Answer 5 · 2013-12-12T11:14:58.040

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Using Google Guava Predicates (the answer from @Joachim Sauer is deprecated)

//list is the variable where we want to remove null elements
List nullRemovedList=Lists.newArrayList(Iterables.filter(list, Predicates.notNull()));

//Or convert to Immutable list
List nullRemovedList2=ImmutableList.copyOf(Iterables.filter(list, Predicates.notNull()));

edited Dec 12 '13 at 11:14

answered Dec 12 '13 at 11:09

Didac Montero

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score 1 · Answer 6 · answered Aug 09 '11 at 13:50

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Hashtable does not allow null keys or values.

answered Aug 09 '11 at 13:50

Steve B.

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my goodness, that's a blast from the past! It isn't even a `Collection`, never mind a `List`. – fommil Oct 03 '12 at 17:51

score 1 · Answer 7 · answered Aug 09 '11 at 14:06

Start here, the Collections API Page. Check out the "See Also" section. Follow the links. The decision to allow or disallow null is made by the implementing class; just follow the links in the various "See Also" sections to get to the implementing classes (for example, HashMap) then look at the insertion methods (generally a variation on add, push, or put) to see if that implementation permits null.

How to workaround this JDK limitation?

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