* created for testing should have Integer node and String edge objects.
 *
 * <p>Test cases that should be handled similarly in any graph implementation are included in this
 * class. For example, testing that {@code nodes()} method returns the set of the nodes in the
 * graph. The following test cases are left for the subclasses to handle:
 *
 * <ul>
 *   <li>Test cases related to whether the graph is directed or undirected.

 * created for testing should have Integer node and String edge objects.
 *
 * <p>Test cases that should be handled similarly in any graph implementation are included in this
 * class. For example, testing that {@code nodes()} method returns the set of the nodes in the
 * graph. The following test cases are left for the subclasses to handle:
 *
 * <ul>
 *   <li>Test cases related to whether the graph is directed or undirected.