Can someone tell me were the problem is, Please?

[thatos]

I am trying to write an program which performs a breath first search on a graph and returns a tree.

class Alist{
	int x;
	Alist next;
	public Alist (){		
	}
	public Alist(int x){
		this.x = x;
		next = null;
	}	
}

//Under Graph class

protected Alist list;
int parent[];
  public void removeFirst(){
	  list = list.next;
  }
  public int first(){
	  return list.x;
  }
  public void addLast(int x){
	  Alist curr = list;
	  Alist temp = new Alist(x);
	  while(curr.next != null){
		  curr = curr.next;
	  }
	  curr.next = temp;
	  
  }
  public boolean empty(){
	  if(list != null){
		  return false;
	  }
	  return true;
  }
  public void BreathFirst(int v){
	  int parent[] = new int[numvertices];
	  boolean marked[] = new boolean[numvertices];
	  for(int i=0;i<numvertices;i++){
		  marked[i] = false;
	  }
	  list = new Alist(v);
	  marked[v] = true;
	  int w;
	  while (!empty()){
		  w = first();
		  removeFirst();
		  for(int x=0;x<numvertices;x++){
			  if(isedge(w,x) && !marked[x]){
				  marked[x] = true;
				  parent[x] = w;
				  addLast(x);
			  }
		  }
	  }
	  for(int i=0;i<numvertices;i++){
		  System.out.println(parent[i]+" "+i);
	  }
	  
  }

Here is the main method

Graph g;
int n;
		
		
SimpleInp inp = new SimpleInp("E:/graph.txt");
		
n = inp.readInt();
g = new Graph(n, false);
		
g.read(inp);
g.BreathFirst(0);//Everything works except this point

The error which it returns

Exception in thread "main" java.lang.NullPointerException
	at Graph.addLast(Graph.java:292)
	at Graph.BreathFirst(Graph.java:320)
	at Driver.main(Driver.java:19)

If you need additional info just ask?

[BigDaddyLH]

Your error is on line 292, right?

Do you know what a NullPointerExce ption is? It is the most common programming error.

You are trying to dereference a null reference on that line.

[thatos]

Your error is on line 292, right?

Do you know what a NullPointerExce ption is? It is the most common programming error.

You are trying to dereference a null reference on that line.

Can you plsk tell me how can I resolve my problem,I know what NullPointerExce ption is, but I do not know were is it there cause list references something.

[BigDaddyLH]

Can you plsk tell me how can I resolve my problem,I know what NullPointerExce ption is, but I do not know were is it there cause list references something.

We're both in the dark. I don't know what line 292 is!-

[thatos]

We're both in the dark. I don't know what line 292 is!-

Here is line 292

list = new Alist(v);

[thatos]

Here is line 292

list = new Alist(v);

I made a mistake the line 292 is

addLast(x);

[BigDaddyLH]

Here is line 292

list = new Alist(v);

Are you sure it isn't:

[CODE=Java]while(curr.next != null){[/CODE]

...because there is nothing in the line you quote that can cause a NullPointerExce ption!

Suggestion: clean and build. Delete all your *.class files and recompile. This won't fix your bug but maybe something is out of synch.

[thatos]

Are you sure it isn't:

[CODE=Java]while(curr.next != null){[/CODE]

...because there is nothing in the line you quote that can cause a NullPointerExce ption!

Suggestion: clean and build. Delete all your *.class files and recompile. This won't fix your bug but maybe something is out of synch.

You are right, I changed addLast to this

public void addLast(int x){
	  Alist curr = list;
	  Alist temp = new Alist(x);
	  while(curr != null){
		  curr = curr.next;
	  }
	  curr = temp;
	  
  }

[BigDaddyLH]

You are right, I changed addLast to this

public void addLast(int x){
	  Alist curr = list;
	  Alist temp = new Alist(x);
	  while(curr != null){
		  curr = curr.next;
	  }
	  curr = temp;
	  
  }

Woo-hoo! I thought it was because curr was null!