Is there a Hamcrest matcher to check that a Collection is neither empty nor null?

You are welcome to use Matchers:

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.anyOf;

assertThat(collection, anyOf(nullValue(), empty()));

As I posted in the comments, the logical equivalent of collection != null and size != 0 is size > 0, that implies the collection is not null. A simpler way to express size > 0 is there is an (arbitrary) element X in collection. Below a working code example.

import static org.hamcrest.core.IsCollectionContaining.hasItem;
import static org.hamcrest.CoreMatchers.anything;

public class Main {

    public static void main(String[] args) {
        boolean result = hasItem(anything()).matches(null);
        System.out.println(result); // false for null

        result = hasItem(anything()).matches(Arrays.asList());
        System.out.println(result); // false for empty

        result = hasItem(anything()).matches(Arrays.asList(1, 2));
        System.out.println(result); // true for (non-null and) non-empty 
    }
}

You can combine the IsCollectionWithSize and the OrderingComparison matcher:

@Test
public void test() throws Exception {
    Collection<String> collection = ...;
    assertThat(collection, hasSize(greaterThan(0)));
}

• For collection = null you get

  java.lang.AssertionError: 
  Expected: a collection with size a value greater than <0>
      but: was null
  

• For collection = Collections.emptyList() you get

  java.lang.AssertionError: 
  Expected: a collection with size a value greater than <0>
      but: collection size <0> was equal to <0>
  

• For collection = Collections.singletonList("Hello world") the test passes.

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Edit:

Just noticed that the following approch is not working:

assertThat(collection, is(not(empty())));

The more i think about it the more i would recommend a slightly altered version of the statement written by the OP if you want to test explicitly for null.

assertThat(collection, both(not(empty())).and(notNullValue()));