Use ArchUnit to inspect interfaces and their subclasses

InterfacesAndImplementors.java

import com.tngtech.archunit.core.domain.JavaClass;
import com.tngtech.archunit.core.importer.ClassFileImporter;
import org.junit.jupiter.api.Test;

import static java.util.function.Function.identity;
import static java.util.function.Predicate.not;
import static java.util.stream.Collectors.joining;
import static java.util.stream.Collectors.toMap;

class InterfacesAndImplementors {

    @Test
    void list() {
        var myCode = new ClassFileImporter().importPackages("my.package");
        var interfacesAndImplementors = myCode.stream()
                .filter(JavaClass::isInterface)
                .filter(not(JavaClass::isAnnotation))
                .collect(toMap(identity(), JavaClass::getAllSubclasses));

        interfacesAndImplementors.forEach((ifc, implementors) -> {
            if (implementors.size() == 0) {
                System.out.println("Nothing implements " + ifc.getName() + "!");
            } else if (implementors.size() == 1) {
                System.out.println("Interface " + ifc.getName() + " is implemented by a single class: " + implementors.stream().map(JavaClass::getName).collect(joining()));
            } else {
                System.out.println("Interface " + ifc.getName() + " is implemented by multiple classes");
            }
        });
    }

}

https://twitter.com/nikolaiii/status/1441308118017724424?s=20