builder.append("FAILURE, cause=[").append(e.getCause()).append("]");
    } catch (CancellationException e) {
      builder.append("CANCELLED");
    } catch (RuntimeException e) {
      builder.append("UNKNOWN, cause=[").append(e.getClass()).append(" thrown from get()]");
    }
  }

  private enum State {
    PENDING {
      @Override
      boolean isDone() {
        return false;
      }

    } catch (CancellationException e) {
      builder.append("CANCELLED"); // shouldn't be reachable
    } catch (Exception e) { // sneaky checked exception
      builder.append("UNKNOWN, cause=[").append(e.getClass()).append(" thrown from get()]");
    }
  }

  /**
   * Any object can be the result of a Future, and not every object has a reasonable toString()