Hi all,
I am having some problems with my implementation. what i want to do is
this, i have some class called nodes, edges and selection graph. I
instantiated a node in the selection graph  in order to create a
selection graph with only one node and no edges. When i to run the
program so that the output will be this selection graph i got a null
pointerexception error. Could anyone tell me how to solve this problem?
I have include the code for the class below.  of es make up a selection
graph. here are the codes:

Nodes class:
public class Node {

   private  double nodeID = 0;
   private  String tableName = null;
   private  String primaryKey = null;
   private  Vector condition = new Vector();
   private  boolean sFlag = false;
   private  boolean fFlag = false;

   /** Creates a new instance of SelGrapNode
   public Node(String table_name, Vector condition, boolean node_flag,
String primary_key) {
       this.tableName = table_name;
       this.condition = condition;
       this.sFlag = sFlag;
       this.fFlag = fFlag;

   }
   */

   public Node(){
       //nodeID = 0;
   }
           //modifier methods

           public double getNodeID () { return nodeID; }
           public void setNodeID(double node ) {
               nodeID = node;
           }

           public String getTableName () { return tableName; }
           public void setTableName(String tName ) {
               tableName = tName;
           }

           public Vector getCondition () { return condition; }

           public void addCondition(String cond){
               condition.addElement(cond);
           }

           public void remCondition(String indx){
               condition.remove(indx);
           }

           public boolean getSFlag () { return sFlag;}
           public void setSFlag(boolean sFlag){
               sFlag = sFlag;
           }

           public boolean getFFlag () { return fFlag;}
           public void setFFlag(boolean fFlag){
               fFlag= fFlag;
           }

           public String getPrimaryKey()
           {
               return primaryKey;
           }

           public void setPrimaryKey(String name)
           {
               primaryKey = name;
           }
}

Selection graph class:
public class SelGraph {
   public Node[] Nodes;
   public Edge[] Edges;

   public SelGraph() {

      Nodes = new Node[10];
      for (int i =0; i<10; i++)
       {
          Nodes[i].setNodeID(i);
          Nodes[i].setFFlag(false);
          Nodes[i].setPrimaryKey("");
          Nodes[i].setSFlag(false);
          Nodes[i].setTableName("");
       }
       //Edges[] = new Edge[100];
   }

   //public jarray getNodes(){

   //}

   public void addNode(double node, String tblname, String prKey,
boolean sFlg, boolean fFlg, String cdt){
       // always add new node to the end of array
       int lastIndex = Nodes.length;

       Nodes[lastIndex].setNodeID(node);
       Nodes[lastIndex].setTableName(tblname);
       Nodes[lastIndex].setPrimaryKey(prKey);
       Nodes[lastIndex].setSFlag(sFlg);
       Nodes[lastIndex].setFFlag(fFlg);
       Nodes[lastIndex].addCondition(cdt);
   }

   public void addNode(int idx, double node, String tblname, String
prKey, boolean sFlg, boolean fFlg, String cdt){

       Nodes[idx].setNodeID(node);
       Nodes[idx].setTableName(tblname);
       Nodes[idx].setPrimaryKey(prKey);
       Nodes[idx].setSFlag(sFlg);
       Nodes[idx].setFFlag(fFlg);
       Nodes[idx].addCondition(cdt);
   }

   public void addEdge(double fNode, double bNode, String fColNm,
String bColNm, boolean eFlg) {
       int lastIndex = Edges.length;

       Edges[lastIndex].setFrontNode(fNode);
       Edges[lastIndex].setBackNode(bNode);
       Edges[lastIndex].setFrontColName(fColNm);
       Edges[lastIndex].setBackColName(bColNm);
       Edges[lastIndex].setEFlag(eFlg);
   }

   public void addEdge(int idx, double fNode, double bNode, String
fColNm, String bColNm, boolean eFlg) {

       Edges[idx].setFrontNode(fNode);
       Edges[idx].setBackNode(bNode);
       Edges[idx].setFrontColName(fColNm);
       Edges[idx].setBackColName(bColNm);
       Edges[idx].setEFlag(eFlg);
   }

   public void addNodeAndEdge(double node, String tblname, String
prKey, boolean sFlg, boolean fFlg, String cdt, double fNode, double
bNode, String fColNm, String bColNm, boolean eFlg){
       this.addNode(node, tblname, prKey, sFlg, fFlg, cdt);
       this.addEdge(fNode, bNode, fColNm, bColNm, eFlg);
   }

   public void addPositiveCondition(int idx, String cdt){
       this.Nodes[idx].addCondition(cdt);
   }

  // FIXME: This Are the remaining Methods for the refinement: We need
to know where to include them
   public void addNegativeCondition(int idx,String cdt){

        int size = Nodes.length;

        for (int i =0; i < size; i++) {
           if (Nodes[i].getNodeID()==0)
           {
               this.Nodes[i].addCondition(cdt);
           }
           if (Nodes[idx].getNodeID()!=0){
            this.Edges[idx].setBackNode(idx-1);
            this.Nodes[idx].addCondition(cdt);
            this.Nodes[idx].setSFlag(false);
           }
        }

   }

   public void addPresentEdgeAndOpenNode(int idx, String cdt){
       int size = Edges.length;
       for (int i = 0; i< size; i++){
           for (int j = 0 ; j < i; j++){
       if(Edges[j].getBackColName()== Edges[i].getFrontColName()) {
               Edges[j].setFrontNode(i);
               Edges[i].setEFlag(true);
       }
       }
       }

   }

   public void addAbsentEdgeAndclosesNode(int idx){
       int size = Edges.length;
       for (int i = 0; i< size; i++){
           for (int j = 0 ; j < i; j++){
       if(Edges[j].getBackColName()== Edges[i].getFrontColName()) {
               Edges[j].setFrontNode(i);
               Edges[i].setEFlag(false);
       }
       }
       }
   }

   public Vector refinement(){
       Vector entropyVec = new Vector();
       return entropyVec;
   }
}

edge class:
package DMengine;

/**
*
* @author Adesola + Jorge + Tu Ba
*/
public class Edge {
       private double frontNode = 0;
       private double backNode = 0;
       private String frontColName = new String();
       private String backColName = new String();
       private boolean eFlag = false;

   /**
    * Creates a new instance of Edge

   public Edge(double frontNode, double backNode, String frontColName,
String backColName, boolean eFlag ) {
       this.frontNode = frontNode;
       this.backNode = backNode;
       this.frontColName = frontColName;
       this.backColName = backColName;
       this.eFlag = eFlag;
   }
    */
   public Edge(){
       }

   public double getFronNode() {
       return this.frontNode;
   }

   public void setFrontNode(double fNode){
       this.frontNode = fNode;
   }

   public double getBackNode() {
       return this.backNode;
   }

   public void setBackNode(double bNode){
       this.backNode = bNode;
   }

   public String getFrontColName() {
       return this.frontColName;
   }

   public void setFrontColName(String fColname){
       this.frontColName = fColname;
   }

   public String getBackColName() {
       return this.backColName;
   }

   public void setBackColName(String bColname){
       this.backColName = bColname;
   }

   public boolean getEFlag () {
       return eFlag;
   }

   public void setEFlag(boolean eFl){
       eFlag = eFl;
   }
 
}