Checking that a List is not empty in Hamcrest

I was wondering if anyone knew of a way to check if a List is empty using assertThat() and Matchers?

Best way I could see just use JUnit:

assertFalse(list.isEmpty());

But I was hoping that there was some way to do this in Hamcrest.

Well there's always

assertThat(list.isEmpty(), is(false));

... but I'm guessing that's not quite what you meant :)

Alternatively:

assertThat((Collection)list, is(not(empty())));

empty() is a static in the Matchers class. Note the need to cast the list to Collection, thanks to Hamcrest 1.2's wonky generics.

The following imports can be used with hamcrest 1.3

import static org.hamcrest.Matchers.empty;
import static org.hamcrest.core.Is.is;
import static org.hamcrest.core.IsNot.*;

This is fixed in Hamcrest 1.3. The below code compiles and does not generate any warnings:

// given
List<String> list = new ArrayList<String>();
// then
assertThat(list, is(not(empty())));

But if you have to use older version - instead of bugged empty() you could use:

hasSize(greaterThan(0))
(import static org.hamcrest.number.OrderingComparison.greaterThan; or
import static org.hamcrest.Matchers.greaterThan;)

Example:

// given
List<String> list = new ArrayList<String>();
// then
assertThat(list, hasSize(greaterThan(0)));

The most important thing about above solutions is that it does not generate any warnings. The second solution is even more useful if you would like to estimate minimum result size.

If you're after readable fail messages, you can do without hamcrest by using the usual assertEquals with an empty list:

assertEquals(new ArrayList<>(0), yourList);

E.g. if you run

assertEquals(new ArrayList<>(0), Arrays.asList("foo", "bar");

you get

java.lang.AssertionError
Expected :[]
Actual   :[foo, bar]

Create your own custom IsEmpty TypeSafeMatcher:

Even if the generics problems are fixed in 1.3 the great thing about this method is it works on any class that has an isEmpty() method! Not just Collections!

For example it will work on String as well!

/* Matches any class that has an <code>isEmpty()</code> method
 * that returns a <code>boolean</code> */ 
public class IsEmpty<T> extends TypeSafeMatcher<T>
{
    @Factory
    public static <T> Matcher<T> empty()
    {
        return new IsEmpty<T>();
    }

    @Override
    protected boolean matchesSafely(@Nonnull final T item)
    {
        try { return (boolean) item.getClass().getMethod("isEmpty", (Class<?>[]) null).invoke(item); }
        catch (final NoSuchMethodException e) { return false; }
        catch (final InvocationTargetException | IllegalAccessException e) { throw new RuntimeException(e); }
    }

    @Override
    public void describeTo(@Nonnull final Description description) { description.appendText("is empty"); }
}

This works:

assertThat(list,IsEmptyCollection.empty())