Question

There is a list of objects I need to add to a grid without receiving an IndexOutOfBoundsException. Each object has two numbers associated with it which corresponds to it’s index and column position in a grid. There can be 3 columns and unlimited rows. I need to call add() method for this grid but only in the correct order, so :

(0,0),(0,1),(0,2),(1,0)…

The grid would therefore look like this:

  0 1 2
0 x x x
1 x x x
2 x x x
3 x x x

I must also take into account the chance that no object exists for a certain position. For example:

A) x x x  B) x   x   C) x x x
   x x x     x   x        x x
   x   x         x        x
   x   x         x        x
   x   x

Can this be done? I’m not sure where to start.

Answer 1

Maybe you should think about another datastructure that stores objects by (row,column). It’s interface would look like

public interface GridModel {
  void set(int row, int column, Object o);
  Object get(int row, int column)
}

And than you can use a list of lists to store the data. List<List<Object>> or, as Mark Peters suggests, a sparse matrix

If working with the cell values is important, add a cell iterator method. A simple implementation would look like:

public Iterable<Object> cellIterator() {
    final List<Object> items = new java.util.ArrayList<Object>();
    for(final List<Object> row : cells) {
        for(final Object cell: row) {
            items.add(cell);
        }
    }
    return items;
}