How to assert property of objects at a list?

Question

I am trying to have proper assertion (with implicit null checks) for a property of a list element.

The first assertion is working as expected, except that it will generate no proper error message if actual is null.
The second is supposed to provide proper null check for actual, but it's not compiling.

Is there an option tweak the second assertion to make it work?

import java.util.List;

import org.junit.jupiter.api.Test;
import static org.assertj.core.api.Assertions.assertThat;

class ExampleTest {

    private static class Sub {

        private String value;

        public String getValue() {
            return value;
        }
    }

    private static class Example {

        private List<Sub> subs;

        public List<Sub> getSubs() {
            return subs;
        }
    }

    @Test
    void test() {
        Example actual = null;

        assertThat(actual.getSubs())//not null safe
                .extracting(Sub::getValue)
                .contains("something");

//        assertThat(actual)
//                .extracting(Example::getSubs)
//                .extracting(Sub::getValue)//not compiling
//                .contains("something");
    }
}

score 4 · Accepted Answer · answered Oct 18 '22 at 08:09

For type-specific assertions, extracting(Function, InstanceOfAssertFactory) should be used:

assertThat(actual)
    .extracting(Example::getSubs, as(list(Sub.class)))
    .extracting(Sub::getValue) // compiles
    .contains("something");

Assertions.as(InstanceOfAssertFactory) is an optional syntax sugar to improve readability
InstanceOfAssertFactories.list(Class) provides the list-specific assertions after the extracting call

How to assert property of objects at a list?

1 Answers1