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 NullPointerException 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 NullPointerException 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 NullPointerException 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 NullPointerException 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:

while(curr.next != null){

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

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:

while(curr.next != null){

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

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!