AssertJ: Retrieving First Items From Extracting List Returns List Instead

Question

I'm trying to verify first item in an extracted List. However AssertJ returns a List instead of expected object from .first().

Given the following sample

public class AssertJTests
{
    @Test
    @DisplayName("Region Asia should only have one Country, Malaysia")
    public void assertRegion()
    {
        Region asia = new Region()
            .setName("Asia")
            .setCountries(Lists.newArrayList("Malaysia"));

        Consumer<Region> regionRequirement = region ->
            assertThat(region)
                .extracting(Region::getName)
                .containsOnly("Asia");

        Consumer<String> countryRequirement = country ->
            assertThat(country)
                .isEqualToIgnoringCase("Malaysia");

        assertThat(asia)
            .isInstanceOfSatisfying(Region.class, regionRequirement)
            .extracting(Region::getCountries)
            .hasSize(1)
            .first()
            .isInstanceOfSatisfying(String.class, countryRequirement);
}

    protected static class Region
    {
        private String name;

        private List<String> countries;

        List<String> getCountries()
        {
            return countries;
        }

        Region setCountries(final List<String> pCountries)
        {
            countries = pCountries;
            return this;
         }

         String getName()
         {
            return name;
         }

         Region setName(final String pName)
         {
             name = pName;
             return this;
          }
    }
}

I'm expecting .first() to return String but instead it returned List<String>. The following error appeared:

java.lang.AssertionError: [List check first element] Expecting: <["Malaysia"]> to be an instance of: <java.lang.String> but was instance of: <java.util.ArrayList>

Answer 1

extracting for Object assertion extracts the object under test (here asia) properties resulting in a List of extracted property values. Your code results in extracting only one property so the result is a list of one value, it turns out that the value is also a list, after extracting you end up having is a List of one element being asia.countries.