A graph implementation in Java, based on The Algorithm Design Manual by Steven S. Skiena

EdgeNode.java

package impl.graph;

class EdgeNode {

    Integer y; /* adjacency info */
    Integer weight; /* edge weight, if any */
    EdgeNode next; /* next edge in list */

    public EdgeNode(Integer y) {
        this.y = y;
        this.weight = null;
        this.next = null;
    }

    public EdgeNode(Integer y, Integer weight) {
        this.y = y;
        this.weight = weight;
        this.next = null;
    }

    public EdgeNode(Integer y, EdgeNode next) {
        this.y = y;
        this.weight = null;
        this.next = next;
    }

    public EdgeNode(Integer y, Integer weight, EdgeNode next) {
        this.y = y;
        this.weight = weight;
        this.next = next;
    }

}

Graph.java

package impl.graph;

import java.util.Arrays;

public class Graph {

    public static final Integer MAXV = 1000;

    EdgeNode[] edges; /* adjcency info */
    Integer[] degree; /* outdegree of each vertex */
    Integer nvertices; /* number of vertices in graph */
    Integer nedges; /* number of edges in graph */
    boolean directed; /* is the graph directed? */

    public Graph(Integer nvertices, boolean directed) {
        this.edges = new EdgeNode[MAXV + 1];
        this.degree = new Integer[MAXV + 1];
        this.nvertices = nvertices;
        this.nedges = 0;
        this.directed = directed;

        Arrays.fill(edges, null);
        Arrays.fill(degree, 0);
    }

    public void insert(Integer x, Integer y, boolean directed) {
        EdgeNode tempNode = new EdgeNode(y, null, edges[x]);

        edges[x] = tempNode;
        degree[x]++;

        if (!directed) {
            insert(y, x, !directed);
        } else {
            nedges++;
        }
    }

    public void print() {
        EdgeNode pointer;

        System.out.println("No. of Vertices: " + nvertices);
        System.out.println("No. of Edges:    " + nedges);
        System.out.println("Directed:        " + directed);

        System.out.println();
        System.out.println("Edges:");
        for (int i = 1; i <= nvertices; i++) {
            System.out.print("[" + i + "]");
            pointer = edges[i];

            while (pointer != null) {
                System.out.print(" -> " + pointer.y);
                pointer = pointer.next;
            }
            System.out.print(" -> null");

            System.out.println();
        }

        System.out.println();
        System.out.println("Degrees:");

        System.out.printf("V: ");
        for (int i = 1; i <= nvertices; i++) {
            System.out.printf("% 2d", i);
        }
        System.out.println();

        System.out.printf("D: ");
        for (int i = 1; i <= nvertices; i++) {
            System.out.printf("% 2d", degree[i]);
        }
        System.out.println();
    }

}

GraphTest.java

package impl.graph;

class GraphTest {

    public static void main(String[] args) {
        boolean directed = true;
        var graph = new Graph(4, directed);

        graph.insert(1, 2, directed);
        graph.insert(2, 3, directed);
        graph.insert(3, 4, directed);
        graph.insert(4, 1, directed);

        graph.print();
    }

}