* writer each time it is called.
   *
   * <p>The caller is responsible for ensuring that the returned writer is closed.
   *
   * @throws IOException if an I/O error occurs while opening the writer
   */
  public abstract Writer openStream() throws IOException;

  /**
   * Opens a new buffered {@link Writer} for writing to this sink. The returned stream is not

   * value if the expiration time has not passed. After the expiration time, a new value is
   * retrieved, cached, and returned. See: <a
   * href="http://en.wikipedia.org/wiki/Memoization">memoization</a>
   *
   * <p>The returned supplier is thread-safe. The supplier's serialized form does not contain the
   * cached value, which will be recalculated when {@code get()} is called on the reserialized

 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)
 *   <li>methods whose return type is a type variable but which can return {@code null} regardless
 *       of the type argument supplied by the user of the class: For example, {@code
 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}

 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)
 *   <li>methods whose return type is a type variable but which can return {@code null} regardless
 *       of the type argument supplied by the user of the class: For example, {@code
 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}

 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)
 *   <li>methods whose return type is a type variable but which can return {@code null} regardless
 *       of the type argument supplied by the user of the class: For example, {@code
 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}

 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)
 *   <li>methods whose return type is a type variable but which can return {@code null} regardless
 *       of the type argument supplied by the user of the class: For example, {@code
 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}

 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)
 *   <li>methods whose return type is a type variable but which can return {@code null} regardless
 *       of the type argument supplied by the user of the class: For example, {@code
 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}

 * x is an integer, the result is the value which would appear at index x in the sorted dataset
 * (unless there are {@link Double#NaN NaN} values, see below); otherwise, the result is the average
 * of the values which would appear at the indexes floor(x) and ceil(x) weighted by (1-frac(x)) and

   *
   * @throws UnsupportedOperationException if this {@link EndpointPair} is not ordered
   */
  public abstract N source();

  /**
   * If this {@link EndpointPair} {@link #isOrdered()}, returns the node which is the target.
   *
   * @throws UnsupportedOperationException if this {@link EndpointPair} is not ordered
   */
  public abstract N target();

  /**
   * Describes why the annotated element is incompatible with GWT. Since this is generally due to a
   * dependence on a type/method which GWT doesn't support, it is sufficient to simply reference the
   * unsupported type/method. E.g. "Class.isInstance".
   *
   * <p>As of Guava 20.0, this value is optional. We encourage authors who wish to describe why an
   * API is {@code @GwtIncompatible} to instead leave an implementation comment.