Formatting a two-dimensional list...

Question

Is there a way to create a two-dimensional ArrayList of Integers with all cells containing certain values, for example zeros? -I will be storing changing Integer values in certain x- and y-coordinates!!
Something like:
ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();But my problem is that the .get commands cause an Exception if a cell value is not set:
list.get(x).get(y) and list.get(x).set(y,int)How could I get around the "null" values without Exceptions? Would you first replace the null values with zeros? (Or is there some other way to store such information?)

Answer 1

If you want to a fill a List up with values, use java.util.Collections.fill().
But do you really want it full of some kind of value, or do you just want to indicate "no value filled in" for a bunch of positions in the grid? What percentage of positions will not be filled in, or will be filled in with the default?
Here's a simple sparse grid:
public class Pair {
  final int x;
  final int y;
  public boolean equals(Object o) {
    Pair p = (Pair) o;
    return p.x == x && p.y == y;
  public int hashcode() {
    return x + y;
public class SparseGrid {
  private Map<Pair, Integer> grid = new HashMap<Pair, Integer>();
  public Integer get(Pair p) {
    Integer i = grid.get(p);
    if (i == null)
      return 42; // the default.  or just skip this test if you don't want a default
    return i;
  // also a put method, left as exercise
}That's all untested.