android/guava/src/com/google/common/graph/EndpointPairIterator.java
protected final boolean advance() {
    checkState(!successorIterator.hasNext());
    if (!nodeIterator.hasNext()) {
      return false;
    }
    node = nodeIterator.next();
    successorIterator = graph.successors(node).iterator();
    return true;
  }
protected final boolean advance() {
    checkState(!successorIterator.hasNext());
    if (!nodeIterator.hasNext()) {
      return false;
    }
    node = nodeIterator.next();
    successorIterator = graph.successors(node).iterator();
    return true;
  }
Called after {@link #successorIterator} is exhausted. Advances {@link #node} to the next node
and updates {@link #successorIterator} to iterate through the successors of {@link #node}.