Hey folks.
I am one of the many newbs who gets a NullPointerException when trying to make an array of objects.
My complete code is at the end; below are the parts that are related to the exception.

Code:

//important system variables
int cellX = 10; //dimensions of a cell
int cellY = 10;
int cellsWide = 5; //dimensions of the world
int cellsTall = 5;

//working variables
Cell[][] world = new Cell[cellsWide][cellsTall]; 

void setup(){  
  size(cellX*cellsWide,cellY*cellsTall);
  background(deadCell);
  DrawGrid();
}

void draw(){
  for(int i=0;i<cellsWide;i++){
    for(int j=0;j<cellsTall;j++){
      //display current state
      if(world[i][j].state){ //exception occurs here
        println("y");
      }
    }
  }
} 

I've looked at several other threads- http://processing.org/discourse/yabb2/num_1251480942_seems_similar__but_the_solutions_don.html't seem to work; initializing the world[][] array before setup() doesn't fix it, nor does declaring it outside setup and initializing the object inside.  Where is the right place for the line

Code:

Cell[][] world = new Cell[cellsWide][cellsTall]; 

?

Thanks!

Full code below.  First, conway2.pde:
Code:

//important system variables
int cellX = 10; //dimensions of a cell
int cellY = 10;
int cellsWide = 5; //dimensions of the world
int cellsTall = 5;
int generation =3000 ; //time length to display a generation (ms)

//aesthetic values
color deadCell = #CCCCCC;
color liveCell = #FFFFFF;
color gridColor = #000000;

//working variables
Cell[][] world = new Cell[cellsWide][cellsTall]; //dimensions are gridX,gridY
int time;

void setup(){
  /* //important system variables
  cellX = 10; //dimensions of a cell
  cellY = 10;
  cellsWide = 5; //dimensions of the world
  cellsTall = 5;
  generation = 3; //time length to display a generation
  
  //aesthetic values
  deadCell = #CCCCCC;
  liveCell = #FFFFFF;
  gridColor = #FFFFFF;*/
  
  size(cellX*cellsWide,cellY*cellsTall);
  background(deadCell);
  //world = new Cell[cellsWide][cellsTall]; //initializes all the cells
  time=0;
  DrawGrid();
}

void DrawGrid(){
  stroke(gridColor);
  for(int i=1;i<cellsWide;i++){
    line(i*cellX,0,i*cellX,height);
  }
  for(int i=1;i<cellsTall;i++){
    line(0,i*cellY,width,i*cellY);
  }
  
}

void draw(){
  for(int i=0;i<cellsWide;i++){
    for(int j=0;j<cellsTall;j++){
      //display current state
      if(world[i][j].state){
        println("y");
      }
    }
  }
  DrawGrid();
} 

and then cell.pde:
[code]class Cell{
 boolean state; //0 is dead, 1 is alive
 int gridX; //X coordinate on the grid
 int gridY; //Y coordinate on the grid
 boolean nextState;
 
 void showState(int myX, int myY){
   if(state){
     stroke(liveCell);
   }else{
     stroke(deadCell);
   }
   rect(myX*cellsWide,myY*cellsTall,cellX,cellY);
 }
 
 Cell(){
   if(round(random(0,1)) == 0){
     state = false;
   }else{
     state = true;
   }
 }
 
}[/cell]

Quark is right, of course.
I just wanted to point out that the arrays are actually similar, initialized with default values: for the array of ints, the default is 0 (zero); for the array of objects, the default of the references is null, ie. no reference (yet).