if (reference == null) {
      throw new NullPointerException();
    }
    return reference;
  }

  /**
   * Ensures that an object reference passed as a parameter to the calling method is not null.
   *
   * @param reference an object reference
   * @param errorMessage the exception message to use if the check fails; will be converted to a

      }
    }
  }

  /** A weakly referenced value that also has a reference to its containing entry. */
  interface WeakValueReference<K, V, E extends InternalEntry<K, V, E>> {
    /**
     * Returns the current value being referenced, or {@code null} if there is none (e.g. because

    if (reference == null) {
      throw new NullPointerException();
    }
    return reference;
  }

  /**
   * Ensures that an object reference passed as a parameter to the calling method is not null.
   *
   * @param reference an object reference
   * @param errorMessage the exception message to use if the check fails; will be converted to a

  /*
   * We avoid using a method reference or lambda here for now:
   *
   * - method reference: Inside Google, CacheBuilder is used from the implementation of a custom
   *   ClassLoader that is sometimes used as a system classloader. That's a problem because
   *   method-reference linking tries to look up the system classloader, and it fails because there
   *   isn't one yet.
   *

   * accessed directly after this method returns. These conditions are ensured if the map is created
   * empty, passed directly to this method, and no reference to the map is retained, as illustrated
   * in the following code fragment:
   *
   * <pre>{@code
   * Set<Object> identityHashSet = Sets.newSetFromMap(
   *     new IdentityHashMap<Object, Boolean>());

import static com.google.common.truth.Truth.assertThat;
import static com.google.common.truth.Truth.assertWithMessage;
import static java.io.ObjectStreamConstants.TC_REFERENCE;
import static java.io.ObjectStreamConstants.baseWireHandle;
import static java.util.Collections.emptySet;
import static java.util.Collections.singleton;

import com.google.common.annotations.GwtCompatible;

                // This means a module which is build has a dependency which has the same
                // groupId, artifactId, version and classifier coordinates. This is in consequence
                // a self reference or in other words a circular reference which can not being resolved.
                addViolation(
                        problems,
                        Severity.FATAL,
                        Version.V31,

   * errs on the side of returning {@code false} (that is, it tends to assume a character is
   * double-width).
   *
   * <p><b>Note:</b> as the reference file evolves, we will modify this matcher to keep it up to
   * date.
   *
   * <p>See also <a href="http://www.unicode.org/reports/tr11/">UAX #11 East Asian Width</a>.
   *

                        .orElse(null);
                if (cycle != null) {
                    throw new RuntimeException(new ProjectCycleException(
                            "The projects in the reactor contain a cyclic reference: " + cycle.getMessage(),
                            (CycleDetectedException) cycle.getException()));
                }
                throw new ProjectBuildingException(results);
            }

   * errs on the side of returning {@code false} (that is, it tends to assume a character is
   * double-width).
   *
   * <p><b>Note:</b> as the reference file evolves, we will modify this matcher to keep it up to
   * date.
   *
   * <p>See also <a href="http://www.unicode.org/reports/tr11/">UAX #11 East Asian Width</a>.
   *