nullpointerexception - Weird Java Null Pointer Exception -

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• what nullpointerexception, , how fix it? 12 answers

why getting nullpointerexception?? instantiate node object in board.setstartposition. call driver using b.getstartposition(). have simplified code leave relavant information.

driver.java

static board b;  public static void main(string[] args){      b = new board(3,3);      b.setstartposition(1, 1);      b.createpath(b.getstartposition()); } 

board.java

public class board {     private int length;     private int width;     private node[][] board;     private arraylist<node> openlist;     private arraylist<node> closedlist;     private node startposition;     private node endposition;      board(int length, int width){         board = new node[length][width];          random rand = new random();          for(int row=0; row < board.length; row++){             for(int col=0; col < board[row].length; col++){                  int randomnum = rand.nextint(10);                  if(randomnum == 0){                     board[row][col] = new node('x',row,col);                 }                 else{                     board[row][col] = new node('o',row,col);                 }             }         }     }      public node getstartposition() {         return this.startposition;     }      public void setstartposition(int x, int y) {         board[x][y].setstatus('s');         board[x][y].setparent(null);         startposition = new node('s', x, y);         startposition.setg(0);     }      public void createpath(node n){          //continue until cant find path or exhaust our searches          //this nullpointer. seems think node         //n null, eventhough when pass it, pass getting startposition node         //add startposition closed list         closedlist.add(n);          //add things around start square openlist if applicable         addtoopenlist(n);            }  } 

you never initialized closedlist or openlist. change

private arraylist<node> openlist; private arraylist<node> closedlist; 

to like

private arraylist<node> openlist = new arraylist<>(); private arraylist<node> closedlist = new arraylist<>(); 

or, better yet, suggest program list interface. like,

private list<node> openlist = new arraylist<>(); private list<node> closedlist = new arraylist<>();