@GwtIncompatible
    @J2ktIncompatible
        private void writeObject(ObjectOutputStream stream) throws IOException {
      synchronized (mutex) {
        stream.defaultWriteObject();
      }
    }

    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
  }

  private static <E extends @Nullable Object> Collection<E> collection(

    @GwtIncompatible
    @J2ktIncompatible
        private void writeObject(ObjectOutputStream stream) throws IOException {
      synchronized (mutex) {
        stream.defaultWriteObject();
      }
    }

    @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0;
  }

  private static <E extends @Nullable Object> Collection<E> collection(

      super(
          keyStrength, valueStrength, keyEquivalence, valueEquivalence, concurrencyLevel, delegate);
    }

    private void writeObject(ObjectOutputStream out) throws IOException {
      out.defaultWriteObject();
      writeMapTo(out);
    }

    @J2ktIncompatible // java.io.ObjectInputStream
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {