this.artifactId = (artifactId != null && !artifactId.isEmpty()) ? artifactId : null;
        this.classifier = (classifier != null && !classifier.isEmpty()) ? classifier : null;
        this.extension = (extension != null && !extension.isEmpty()) ? extension : null;
        this.version = (version != null && !version.isEmpty()) ? version : null;
        this.message = (message != null && !message.isEmpty()) ? message : null;
    }

    @Override

import static org.junit.jupiter.api.Assertions.assertTrue;

/**
 * Tests {@code Extension}.
 *
 */
class ExtensionTest {

    @Test
    void testHashCodeNullSafe() {
        new Extension().hashCode();
    }

    @Test
    void testEqualsNullSafe() {
        assertFalse(new Extension().equals(null));

        new Extension().equals(new Extension());
    }

    @Test
    void testEqualsIdentity() {

    private final CoreExtensionEntry extension;
    private final UnaryOperator<String> callback;
    private final DefaultInterpolator interpolator = new DefaultInterpolator();

    public ExtensionConfigurationModule(CoreExtensionEntry extension, UnaryOperator<String> callback) {
        this.extension = extension;
        this.callback = callback;
    }

    @Override

    public ExtensionPropertiesSummaryRenderer(PropertyTableRenderer propertyTableRenderer) {
        this.propertyTableRenderer = propertyTableRenderer;
    }

    public void renderTo(ClassExtensionDoc extension, Element parent) {
        if (extension.getExtensionProperties().isEmpty()) {
            return;
        }

        Document document = parent.getOwnerDocument();

        Element section = document.createElement("section");

    public String getType() {
        return type;
    }

    @Override
    public String getExtension() {
        if (extension == null) {
            return type;
        }
        return extension;
    }

    public void setExtension(final String extension) {
        this.extension = extension;
    }

    @Override
    public String getClassifier() {
        return classifier;
    }

    private final VagrantExtension extension;
    private final Provider<ReaperService> reaperServiceProvider;
    private ReaperService reaper;
    // pkg private so plugin can set this after construction
    long refs;
    private boolean isVMStarted = false;

    public VagrantMachine(VagrantExtension extension, Provider<ReaperService> reaperServiceProvider) {
        this.extension = extension;

        return create(ArtifactFactoryRequest.build(session, groupId, artifactId, version, extension));
    }

    @Nonnull
    default Artifact create(
            @Nonnull Session session,
            String groupId,
            String artifactId,
            String version,
            String classifier,
            String extension,
            String type) {

    private final String classifier;

    private final String extension;

    private final String packaging;

    TransformedArtifactHandler(String classifier, String extension, String packaging) {
        this.classifier = classifier;
        this.extension = requireNonNull(extension);
        this.packaging = requireNonNull(packaging);
    }

    @Override

    public ExtensionMethodsSummaryRenderer(MethodTableRenderer methodTableRenderer) {
        this.methodTableRenderer = methodTableRenderer;
    }

    public void renderTo(ClassExtensionDoc extension, Element parent) {
        if (extension.getExtensionMethods().isEmpty()) {
            return;
        }

        Document document = parent.getOwnerDocument();

        Element section = document.createElement("section");

 *       {@link java.util.Set} contract.
 *   <dt>{@link SortedSetMultimap}
 *   <dd>An extension of {@link SetMultimap} for which the {@linkplain SortedSetMultimap#get
 *       collection values} associated with a given key is a {@link java.util.SortedSet}.
 *   <dt>{@link BiMap}
 *   <dd>An extension of {@link java.util.Map} that guarantees the uniqueness of its values as well