Check for null entries in Java Map<T, ? extends E>

Question

I have a Map<String, ? extends Collection<SomeClass>>. I would like to ensure that there are no entries where collection's size is 0. Therefore I tried myMap.values().stream().map(Collection::size).allMatch(size -> size > 0). However, as it turned out, myMap contains null entries.

I wanted to check if my map contains null entries: resourceMap.values().stream().noneMatch(null) but I got an Exception java.util.Objects.requireNonNull.

I decided to do it in a relatively old fashioned way, with a for-in loop for (Map.Entry<String, ? extends Collection> entry : myMap) However, I got a compilation error telling me that foreach is not applicable to my Map.

What is an elegant way to check if all entries in my map are not null and with size > 0?

For map you should use map.entrySet() and then iterate on map entry objects. — swapnil
– swapnil, Commented Aug 7, 2017 at 17:18

Holger · Accepted Answer · 2017-08-07 18:07:37Z

You can just check for nulls using myMap.values().contains(null)—no Stream API required.

If your map was consistently using List or Set, you could also check for empty values with myMap.values().contains(Collections.emptySet()) or myMap.values().contains(Collections.emptyList()), but since there is no general equality contract between arbitrary collections, that doesn’t work for your Map<String, ? extends Collection<SomeClass>>.

So for this specific test, you could use

boolean violations = myMap.values().contains(null) || myMap.values().stream().anyMatch(Collection::isEmpty);

or

boolean violations = myMap.values().stream().anyMatch(col -> col==null || col.isEmpty());

When you want to iterate with a for loop, you have to specify a Collection view over the Map, e.g.

Map<String, ? extends Collection<SomeClass>> myMap; … for (Map.Entry<String, ? extends Collection<?>> entry: myMap.entrySet()) // loop body

or, much better for this use case

for(Collection<?> value: myMap.values()) if(value == null || value.isEmpty()) // violation detected, perform your action

Grzegorz Piwowarek · Accepted Answer · 2017-08-07 17:25:52Z

3

If you have a look at the actual method signature:

boolean noneMatch(Predicate<? super T> predicate)

You will see that noneMatch(null) accepts a Predicate instance and not an actual value. Null predicates are forbidden here.

Try:

boolean result = resourceMap.values().stream() .noneMatch(Objects::isNull)

You can merge those two into one chain:

myMap.values().stream() .allMatch(col -> col != null && col.size() > 0);

edited Aug 7, 2017 at 17:25

answered Aug 7, 2017 at 17:18

Grzegorz Piwowarek

14.2k8 gold badges73 silver badges97 bronze badges

4 Comments

3yakuya Over a year ago

Thank you! I cannot combine it, as then if my map contains null entries, but the non-null ones have size > 0 it would evaluate to true. I will just conjunct them with classic &&.

Grzegorz Piwowarek Over a year ago

@3yakuya sure, I get it now. I think it's doable to use one stream for that. Let me try.

Grzegorz Piwowarek Over a year ago

@3yakuya you can simply .allMatch(col -> col != null && col.size() > 0);

3yakuya Over a year ago

Nice! Thank you!

Collectives™ on Stack Overflow

Check for null entries in Java Map<T, ? extends E>

2 Answers 2

Comments

4 Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

4 Comments

Related