java - Getting NullPointerException while adding element to ArrayList -

my code:

import java.util.random; import java.util.arraylist;  public class percolation {     arraylist<int[]> grid;     random dice = new random();     public percolation(int n){         for(int i=0;i<n;i++){             grid.add(new int[n]);         }         output(grid,n);     }      public void output(arraylist<int[]> x,int n){         for(int i=0;i<n;i++)             for(int j=0;j<n;j++)                 system.out.println(x.get(i)[j]);     }     public static void main(string[] args){         percolation p = new percolation(2);     } } 

using code throws nullpointerexception @ grid.add(new int[n]). how can add data grid?

you haven't initialize arraylist.

arraylist<int[]> grid = new arraylist<>();