Abstract Base Classes

/**
 * Abstract implementation of the Graph interface designed to simplify 
 * implementation of new graph classes.
 */
public abstract class AbstractGraph<V,E> implements Graph<V,E>, Serializable {
    
    // Edge addition methods
    public boolean addEdge(E edge, Collection<? extends V> vertices);
    public boolean addEdge(E edge, Collection<? extends V> vertices, EdgeType edgeType);
    public boolean addEdge(E e, V v1, V v2);
    public boolean addEdge(E e, V v1, V v2, EdgeType edge_type);
    public boolean addEdge(E edge, Pair<? extends V> endpoints);
    public abstract boolean addEdge(E edge, Pair<? extends V> endpoints, EdgeType edgeType);
    
    // Vertex degree and relationship queries
    public int inDegree(V vertex);
    public int outDegree(V vertex);
    public boolean isPredecessor(V v1, V v2);
    public boolean isSuccessor(V v1, V v2);
    public int getPredecessorCount(V vertex);
    public int getSuccessorCount(V vertex);
    public boolean isNeighbor(V v1, V v2);
    public boolean isIncident(V vertex, E edge);
    public int getNeighborCount(V vertex);
    public int degree(V vertex);
    
    // Edge queries and navigation
    public int getIncidentCount(E edge);
    public V getOpposite(V vertex, E edge);
    public E findEdge(V v1, V v2);
    public Collection<E> findEdgeSet(V v1, V v2);
    public Collection<V> getIncidentVertices(E edge);
    
    // Utility methods
    public String toString();
    
    // Protected helper methods
    protected Pair<V> getValidatedEndpoints(E edge, Pair<? extends V> endpoints);
}

// AbstractGraph is extended by concrete implementations
public class CustomGraph<V,E> extends AbstractGraph<V,E> {
    @Override
    public boolean addEdge(E edge, Pair<? extends V> endpoints, EdgeType edgeType) {
        // Implementation specific logic
        return true;
    }
    
    // Implement other required abstract methods
}

// Common operations available on all graph implementations
Graph<String, String> graph = new DirectedSparseGraph<>();
graph.addVertex("A");
graph.addVertex("B");
graph.addEdge("edge1", "A", "B");

// These methods work on any AbstractGraph subclass
int degree = graph.degree("A");               // Get vertex degree
boolean connected = graph.isNeighbor("A", "B"); // Check if vertices are neighbors
String opposite = graph.getOpposite("A", "edge1"); // Get opposite vertex of edge

/**
 * An abstract class for graphs whose edges all have the same EdgeType.
 * Intended to simplify the implementation of such graph classes.
 */
public abstract class AbstractTypedGraph<V,E> extends AbstractGraph<V,E> {
    
    /**
     * Creates an instance with the specified edge type
     * @param edge_type The edge type for all edges in this graph
     */
    public AbstractTypedGraph(EdgeType edge_type);
    
    /**
     * Returns this graph's edge type
     * @return The default edge type for this graph
     */
    public EdgeType getDefaultEdgeType();
    
    /**
     * Returns this graph's edge type, or null if e is not in this graph
     * @param e The edge to check
     * @return Edge type or null if edge not found
     */
    public EdgeType getEdgeType(E e);
    
    /**
     * Returns the edge set for this graph if edgeType matches default type
     * @param edge_type The edge type to filter by
     * @return Collection of edges of specified type
     */
    public Collection<E> getEdges(EdgeType edge_type);
    
    /**
     * Returns the edge count for this graph if edge_type matches default type
     * @param edge_type The edge type to count
     * @return Number of edges of specified type
     */
    public int getEdgeCount(EdgeType edge_type);
    
    // Protected fields and methods
    protected final EdgeType edge_type;
    protected boolean hasEqualEdgeType(EdgeType edge_type);
    protected void validateEdgeType(EdgeType edge_type);
}

// AbstractTypedGraph ensures all edges have the same type
DirectedSparseGraph<String, String> directedGraph = new DirectedSparseGraph<>();
// All edges in this graph will be DIRECTED

UndirectedSparseGraph<String, String> undirectedGraph = new UndirectedSparseGraph<>();
// All edges in this graph will be UNDIRECTED

// Query edge type information
EdgeType type = directedGraph.getDefaultEdgeType(); // Returns EdgeType.DIRECTED
Collection<String> directedEdges = directedGraph.getEdges(EdgeType.DIRECTED);
int directedCount = directedGraph.getEdgeCount(EdgeType.DIRECTED);

// This would return empty/0 since graph only has directed edges
Collection<String> undirectedEdges = directedGraph.getEdges(EdgeType.UNDIRECTED);