An 8-Puzzle application using the framework must have a class,
say
PuzzleState, that implements the
State interface.
In order for PuzzleState objects to act like graph vertices,
the PuzzleState class must implement the
Vertex interface.
We already have a class that implements Vertex:
SimpleVertex.
So, make PuzzleState extend SimpleVertex:
package puzzle;
import framework.State;
import graph.SimpleVertex;
public class PuzzleState extends SimpleVertex implements State { ... }