* elements left in the iteration.
   *
   * @return {@code null}; a convenience so your {@code computeNext} implementation can use the
   *     simple statement {@code return endOfData();}
   */
  @CanIgnoreReturnValue
  protected final @Nullable T endOfData() {
    state = State.DONE;
    return null;
  }

  @Override
  public final boolean hasNext() {
    checkState(state != State.FAILED);
    switch (state) {

   * elements left in the iteration.
   *
   * @return {@code null}; a convenience so your {@code computeNext} implementation can use the
   *     simple statement {@code return endOfData();}
   */
  @CanIgnoreReturnValue
  protected final @Nullable T endOfData() {
    state = State.DONE;
    return null;
  }

  @Override
  public final boolean hasNext() {
    checkState(state != State.FAILED);
    switch (state) {

            switch (rep++) {
              case 0:
                return 0;
              case 1:
                return 1;
              case 2:
                return endOfData();
              default:
                throw new AssertionError("Should not have been invoked again");
            }
          }
        };

    assertTrue(iter.hasNext());

            switch (rep++) {
              case 0:
                return 0;
              case 1:
                return 1;
              case 2:
                return endOfData();
              default:
                throw new AssertionError("Should not have been invoked again");
            }
          }
        };

    assertTrue(iter.hasNext());

                boolean added = seenNodes.add(nodeConnection.node);
                if (added) {
                  return nodeConnection.node;
                }
              }
              return endOfData();
            }
          };
        }

        @Override
        public int size() {
          return adjacentNodeValues.size();
        }

        @Override

            switch (rep++) {
              case 0:
                return 0;
              case 1:
                return 1;
              case 2:
                return endOfData();
              default:
                throw new AssertionError("Should not have been invoked again");
            }
          }
        };

    assertTrue(iter.hasNext());

          return EndpointPair.ordered(requireNonNull(node), successorIterator.next());
        }
        if (!advance()) {
          return endOfData();
        }
      }
    }
  }

  /**
   * If the graph is undirected, each unordered [node, otherNode] pair (except self-loops) will be

    READY,
    NOT_READY,
    DONE,
    FAILED,
  }

  private @Nullable T next;

  protected abstract @Nullable T computeNext();

  @CanIgnoreReturnValue
  protected final @Nullable T endOfData() {
    state = State.DONE;
    return null;
  }

  @Override
  public final boolean hasNext() {
    checkState(state != State.FAILED);
    switch (state) {
      case DONE:
        return false;

                if (entry.getUpperBound().compareTo(subRange.lowerBound) <= 0) {
                  return endOfData();
                }
                return immutableEntry(entry.getKey().intersection(subRange), entry.getValue());
              }
              return endOfData();
            }
          };
        }
      };
    }

    @Override

  }

  @Override
  protected @Nullable T computeNext() {
    // TODO(b/192579700): Use a ternary once it no longer confuses our nullness checker.
    if (queue.isEmpty()) {
      return endOfData();
    }
    return queue.remove();
  }