* computation is complete.
   *
   * <p>Example:
   *
   * {@snippet :
   * future.addCallback(
   *     new FutureCallback<QueryResult>() {
   *       public void onSuccess(QueryResult result) {
   *         storeInCache(result);
   *       }
   *       public void onFailure(Throwable t) {
   *         reportError(t);
   *       }
   *     }, executor);
   * }

   * and logged.
   *
   * <p>Example:
   *
   * {@snippet :
   * ListenableFuture<QueryResult> future = ...;
   * Executor e = ...
   * addCallback(future,
   *     new FutureCallback<QueryResult>() {
   *       public void onSuccess(QueryResult result) {
   *         storeInCache(result);
   *       }
   *       public void onFailure(Throwable t) {