// ===================================================================================
    //                                                                           Singleton
    //                                                                           =========
    private static final ThumbnailQueueDbm _instance = new ThumbnailQueueDbm();

    private ThumbnailQueueDbm() {
    }

    // ===================================================================================
    //                                                                           Singleton
    //                                                                           =========
    private static final FavoriteLogDbm _instance = new FavoriteLogDbm();

    private FavoriteLogDbm() {
    }

    // ===================================================================================
    //                                                                           Singleton
    //                                                                           =========
    private static final WebAuthenticationDbm _instance = new WebAuthenticationDbm();

    private WebAuthenticationDbm() {
    }

  }

  public void testOrNull_absent() {
    assertThat(Optional.absent().orNull()).isNull();
  }

  public void testAsSet_present() {
    Set<String> expected = Collections.singleton("a");
    assertEquals(expected, Optional.of("a").asSet());
  }

  public void testAsSet_absent() {
    assertTrue("Returned set should be empty", Optional.absent().asSet().isEmpty());
  }

   * the JDK {@link java.util.function.Function} class, see {@link Function}.
   */
  public static Function<Object, String> toStringFunction() {
    return ToStringFunction.INSTANCE;
  }

  // enum singleton pattern
  private enum ToStringFunction implements Function<Object, String> {
    INSTANCE;

    @Override
    public String apply(Object o) {
      checkNotNull(o); // eager for GWT.
      return o.toString();

        super.setUp();

        StandardCrawlerContainer container = new StandardCrawlerContainer();
        container.singleton("mimeTypeHelper", MimeTypeHelperImpl.class)
                .singleton("tikaExtractor", TikaExtractor.class)
                .<ExtractorFactory> singleton("extractorFactory", ExtractorFactory.class, factory -> {
                    TikaExtractor tikaExtractor = container.getComponent("tikaExtractor");

 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.maven.artifact.resolver;

import javax.inject.Inject;
import javax.inject.Named;
import javax.inject.Singleton;

import java.io.File;
import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;

    // ===================================================================================
    //                                                                           Singleton
    //                                                                           =========
    private static final FailureUrlDbm _instance = new FailureUrlDbm();

    private FailureUrlDbm() {
    }

    @Override
    protected void setUp() throws Exception {
        super.setUp();
        StandardCrawlerContainer container = new StandardCrawlerContainer().singleton("mimeTypeHelper", MimeTypeHelperImpl.class)//
                .singleton("ftpClient", FtpClient.class);
        ftpClient = container.getComponent("ftpClient");
    }

    // ===================================================================================
    //                                                                           Singleton
    //                                                                           =========
    private static final KeyMatchDbm _instance = new KeyMatchDbm();

    private KeyMatchDbm() {
    }