Error in Implementing a graph in java using simple arrays

Question

I have created 3 classes Edge Vertex and Graph. The graph is the main class. The following is the code for the 3 classes:

Edge

public class Edge {
Vertex vertex1;
Vertex vertex2;
int weight;
String name;
int edgeNumber;

Edge(){
    weight=-1;
    name=null;
    edgeNumber=-1;
}
Edge(Vertex vertex1,int edgeNumber,Vertex vertex2,int weight,String name){
    this.vertex1=vertex1;
    this.vertex2=vertex2;
    this.weight=weight;
    this.name=name;
    this.edgeNumber=edgeNumber;
}

public void printEdge(Edge e){
    System.out.println("Edge Number :"+edgeNumber);
    System.out.println(e.vertex1);
    System.out.println(e.vertex2);
    System.out.println(e.name);
    System.out.println(e.weight);
}

}

Vertex

public class Vertex {
int vertexId;
String name;

Vertex(){
    vertexId=-1;
    name=null;
}
public void createVertex(Vertex ver1,int id,String vName){
    ver1.vertexId=id;
    ver1.name=vName;
}

}

Graph.java

public class Graph {
int numVertices;
int numEdges;
int[][] matrix=new int[20][20];
Vertex[] vertexList =new Vertex[10];
Edge[] edgeList=new Edge[10];
/*
 * @param args the command line arguments
 */
// Intitalise the graph
//All values in matrix are zero
//Vertex names are also taken from name
Graph(int[][] vertexArr,int numVertices){
    this.numVertices=numVertices;

    for(int i=0;i<numVertices;i++){
        for(int j=0;j<numVertices;j++){
            vertexArr[i][j]=0;
        }
    }
}
public int graphDegree(Graph G,int vertex){
    int i,degree=0;
    for(i=0;i<numVertices;i++){
        if(matrix[vertex][i]==1)
            degree++;
    }
    return degree;
}
public void addEdge(Edge e1,int edgeIndex,Vertex v1,Vertex v2,String nameIn,int weightIn){
    int old=matrix[v1.vertexId][v2.vertexId];
    if(old!=0)
        System.out.println("The edge already exists");
    //creating new edge
    matrix[v1.vertexId][v2.vertexId]=1;
    matrix[v2.vertexId][v1.vertexId]=1;
    e1.vertex1=v1;
    e1.vertex2=v2;
    e1.name=nameIn;
    e1.weight=weightIn;
    edgeList[edgeIndex]=e1;
    numEdges++; 
}
public void printGraph(Graph G){
    System.out.println("Number of vertices in the graph : "+G.numVertices);
    for(int i=0;i<numEdges;i++){
        G.printEdge(edgeList[i]);
    }

} 
public static void main(String[] args) {

    int vertexCount=5;

    Vertex v1=new Vertex();
    Vertex v2=new Vertex();
    Vertex v3=new Vertex();
    Vertex v4=new Vertex();
    v1.createVertex(v1,0,"Delhi");
    v2.createVertex(v2,1,"Noida");
    v3.createVertex(v3,2,"Mumbai");
    v4.createVertex(v4,3,"Kanpur");
    int[][] graphStore=new int[vertexCount][vertexCount];
    Graph g1=new Graph(graphStore,vertexCount);

    Edge edge1=new Edge();
    Edge edge2=new Edge();
    g1.addEdge(edge1,0,v1,v2,"DtoN",10);
    g1.addEdge(edge2,1,v2,v3,"NtoM",50);
    g1.printGraph(g1);
}

}

The printGraph() function shows the error:

Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - Erroneous sym type: graph.Graph.printEdge
at graph.Graph.printGraph(Graph.java:63)
at graph.Graph.main(Graph.java:86)

I am not able to make the print graph function. How do i print all the edges in the graph?

Answer 1

1. Change printEdge() method to static

   public static void printEdge(Edge e)
   

2. Your printGraph() method is not correctly implemented. It should be like this:

   public void printGraph(Graph G){
   System.out.println("Number of vertices in the graph : "+G.numVertices);
   for(int i=0;i<numEdges;i++){
   Edge.printEdge(e);
     }
   }
   

3. In addEdge() method, you need to add this also, otherwise edgeNumber will always be -1

   e1.edgeNumber = edgeIndex;
   

4. In class Vertex you have to override toString() method, otherwise you will not be able to print the vertex's information in printEdge()correctly.

   @Override
   public String toString() {
   return "Vertex [vertexId=" + vertexId + ", name=" + name + "]";
   }
   

Answer 2

make your printEdge static:

public static void printEdge(Edge e){
    System.out.println("Edge Number :"+e.edgeNumber);
    System.out.println(e.vertex1);
    System.out.println(e.vertex2);
    System.out.println(e.name);
    System.out.println(e.weight);
}

change you printGraph method:

public void printGraph() {
    System.out.println("Number of vertices in the graph : " + numVertices);
    for (int i = 0; i < numEdges; i++) {
       Edge.printEdge(edgeList[i]);
    }
}

call it like this: g1.printGraph();