import java.util.*;

/**
     An abstract class for the representation of graphs with Objects
     as entries.
*/


public abstract class Graph implements Combiner
{

   protected int size;

   /**
       Constructs a graph with a given adjacency matrix, assuming
       that the matrix is not null and is not of size 0
       The entries are not initialized; this is the job of the
       concrete subclasses.
       @param entries the desired adjacency matrix
   */

   public Graph(Object[][] entries)                 {
     if (entries.length > 0)
       size = entries[0].length;
     else
       size = 0;                                    }


   /**
       @return the number of vertices in the graph
   */

   public int size()                                                   {
     return size;                                                      }


   /**
       return the weight for a given edge
       @param row the source of the edge
       @param col the destination of the edge
   */

   public abstract Object getEntry(int row, int col);


   /**
       find the edges with a given source vertex
       @param i the vertex
       @return a List of the edges
   */

   protected abstract List getRow(int i);


   /**
       find the edges with a given destination vertex
       @param i the vertex
       @return a List of the edges
   */
   protected abstract List getColumn(int i);


   /**
       an abstract method for applying a row of an adjacency matrix
       of a graph 
       to a column -- that is, a method for processing all sets
       of pairs of edges {(x,i),(i,y)}.
       @param g the graph
       @return a new graph resulting from the processing
   */
   
   protected abstract Object[][] combine(Graph g);

}  // end abstract class Graph