i have to create a graph for my project i found an example that was working and followed it first time with graph. i wrote my code got no errors but when i run the code this is what i am getting i don't understand it can some one help me please i am in crunch mode.
Exception in thread "main" java.lang.NullPointerException
at Main.GetAllAdjcentNodes(Main.java:69)
at Main.Dijkstras(Main.java:101)
at Main.main(Main.java:184)
here is the code
import java.util.ArrayList;
import java.util.HashMap;
import java.util.ArrayDeque;
public class Main {
public static ArrayList<node> Nodes = new ArrayList<node>();
public static HashMap<String,Integer> NodesIndex = new HashMap<String,Integer>();
public static ArrayList<edge> Edges = new ArrayList<edge>();
public static int SizeNodes;
public static int SizeEdges;
public static String StartNode;
public static class node {
public String name;
public String Pre;
public int Dist;
public boolean vis;
public node(String NodeName) {
name = NodeName;
Dist = -1;
vis = false;
Nodes.add(this);
NodesIndex.put(name, Nodes.size()-1);
}
}
public static class edge {
String N1;
String N2;
int Dist;
public edge(String Node1, String Node2, int EdgeVal) {
N1 = Node1;
N2 = Node2;
Dist = EdgeVal;
Edges.add(this);
}
}
public static void SetStartNode(String name) {
Nodes.get(NodesIndex.get(name)).Dist=0;
SizeNodes = Nodes.size();
SizeEdges = Edges.size();
StartNode = name;
}
public static int VisitClosestNode() {
int index=0;
int Dist=0;
for(int i=0;i<SizeNodes;++i) {
if(!Nodes.get(i).vis && Nodes.get(i).Dist >=0) {
Dist = Nodes.get(i).Dist;
index=i;
break;
}
}
for(int i=0;i<SizeNodes;++i) {
if(Nodes.get(i).Dist < Dist && !Nodes.get(i).vis && Nodes.get(i).Dist >=0){
Dist = Nodes.get(i).Dist;
index=i;
}
}
Nodes.get(index).vis = true;
return index;
}
public static void GetAllAdjcentNodes(String N, ArrayList<String> AdjNodes) {
//nodes can be conetcted both ways
//can take out the else if if you want them to only conect one way
for(int i=0;i<SizeEdges;++i) {
if(Edges.get(i).N1.equals(N) && !Nodes.get(NodesIndex.get(Edges.get(i).N2)).vis) {
AdjNodes.add(Edges.get(i).N2);
} else if(Edges.get(i).N2.equals(N) && !Nodes.get(NodesIndex.get(Edges.get(i).N1)).vis){
AdjNodes.add(Edges.get(i).N1);
}
}
}
public static boolean EdgeConnectsNodes(edge E, String N1, String N2) {
//again this statment determins weather or not edges go both ways or not
//take out the second half of the condtional if you only want them to
//conect one way
return (E.N1.equals(N1)&&E.N2.equals(N2)||E.N1.equals(N2)&&E.N2.equals(N1)) ? true:false;
}
public static int GetDistance(String N1, String N2) {
for(int i=0;i<SizeEdges;++i) {
if(EdgeConnectsNodes(Edges.get(i),N1,N2)) {
return Edges.get(i).Dist;
}
}
return -1; //they dont conect so we will say that distance is endless
}
public static int GetNumOfUnVisNodes() {
int NOUVN=0;
for(int i=0;i<SizeNodes;++i) {
if(!Nodes.get(i).vis) {
++NOUVN;
}
}
return NOUVN;
}
public static void Dijkstras() {
while(GetNumOfUnVisNodes()>0) {
node ClosetsNode = Nodes.get(VisitClosestNode());
ArrayList<String> AdjNodes = new ArrayList<String>();
GetAllAdjcentNodes(ClosetsNode.name,AdjNodes);
int SizeAdj = AdjNodes.size();
for(int i=0;i<SizeAdj;++i) {
node AdjNode = Nodes.get(NodesIndex.get(AdjNodes.get(i)));
int Distance = ClosetsNode.Dist + GetDistance(ClosetsNode.name,AdjNodes.get(i));
if(AdjNode.Dist >= 0) {
if(Distance<AdjNode.Dist) {
AdjNode.Dist = Distance;
AdjNode.Pre = ClosetsNode.name;
}
} else {
AdjNode.Dist = Distance;
AdjNode.Pre = ClosetsNode.name;
}
}
}
}
public static void GetPathTo(String N, ArrayDeque<String> Path) {
String CN = N;
while(!CN.equals(StartNode)) {
String Temp = CN;
Path.addFirst(Temp);
CN = Nodes.get(NodesIndex.get(CN)).Pre;
}
Path.addFirst(StartNode);
}
public static void main(String[] args) {
new node("Alta Garcia");
new node("Loango");
new node("Castillo");
new node("Waterfall");
new node("Santa Barbara");
new node("Acono");
new node("Caurita");
new node("USC");
new node("Mountain View");
new node("Avondale");
new node("Whaf Trace");
new node("Maracas Gardens");
new node("Poolside");
new node("Tunapuna");
new node("Riverside");
new node("Curepe");
new node("Piarco");
new node("St Joseph");
new node("Port of Spain");
new edge("Alta Garcia","Loango",3);
new edge("Alta Garcia","Castillo",5);
new edge("Loango","Castillo",10);
new edge("Castillo","Waterfall",10);
new edge("Castillo","Santa Barbara",8);
new edge("Santa Barbara","Acono",7);
new edge("Acono","Caurita",6);
new edge("Acono","USC",4);
new edge("USC","Avondale",3);
new edge("USC","Mountain View",5);
new edge("Mountain View","Avondale",4);
new edge("Avondale","Maracas Gardens",8);
new edge("Maracas Gardens","Poolside",10);
new edge("Poolside","Wharf Trace",5);
new edge("Wharf Trace","Mountain View",10);
new edge("Poolside","Riverside",12);
new edge("Poolside","St Joseph",18);
new edge("St Joseph","Riverside",6);
new edge("St Joseph","Port of Spain",30);
new edge("St Joseph","Curepe",6);
new edge("St Joseph","Piarco",30);
new edge("Piarco","Port of Spain",60);
new edge("Piarco","Curepe",25);
new edge("Piarco","Tunapuna",20);
new edge("Curepe","Riverside",10);
new edge("Curepe","Tunapuna",10);
new edge("Riverside","Caurita",25);
new edge("Riverside","Tunapuna",17);
new edge("Tunapuna","Caurita",30);
new edge("Tunapuna","Piarco",20);
SetStartNode("Santa Barbara");
Dijkstras();
ArrayDeque<String> Path = new ArrayDeque<String>();
GetPathTo("Maracas Gardens",Path);
int Size = Path.size();
for(int i=0;i<Size;++i) {
System.out.println(Path.getFirst());
Path.poll();
}
}
}
