It looks like you're looking for catchThrowableOfType, which allows you to receive the correct class:
import static org.assertj.core.api.Assertions.catchThrowableOfType;
SomeException throwable = catchThrowableOfType(() -> service.doSomething(), SomeException.class);
assertThat(throwable.getSomething()).isNotNull();
by 
Jamie Tanna
.
#java
#assertj.