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

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

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

   *
   * <p>If the function throws an exception, that exception is used as the result of the derived
   * {@code ClosingFuture}.
   *
   * <p>Example usage:
   *
   * <pre>{@code
   * ClosingFuture<QueryResult> queryFuture =
   *     queryFuture.catching(
   *         QueryException.class, (closer, x) -> Query.emptyQueryResult(), executor);
   * }</pre>
   *