ArrayList of arrays vs. array of ArrayLists vs. something similar

Question

I'm creating a TableModel which will have a fixed number of columns, but the number of rows will be changing (mostly, increasing as function of time). Which would be better approach to store the data,

ArrayList[] columns = new ArrayList[numberOfColumns];
// Each array element is one column. Fill each of them with a new ArrayList.
...
public Object getValueAt(int row, int column) {
    return columns[column].get(row);
}

i.e. creating an array of ArrayLists, each ArrayList representing one column, or:

ArrayList<Object[]> rows = new ArrayList<Object[]>();
// Each ArrayList element is one row.

public Object getValueAt(int row, int column) {
    return rows.get(row)[column];
}

i.e. creating one ArrayList that holds arrays, each of which represent one row.

Any ideas which of these is more efficient in terms of speed or storage? Alternative 1 requires extending N ArrayLists with each added row, while alternative 2 requires extending just one ArrayList but also creating a new array of length N (to represent the new row). Or is there an obvious, better solution?

Answer 1

If the number of columns is fixed then your data is probably row-oriented or at least row-variable at which point each row should be an array. Fixed numbers of columns means you don't need to reallocate your array.

So your structure is:

List<Object[]> rows;

where the array element is one row.

There are several options for what your row object should be however:

1. An array;
2. A List or other Collection; or
3. A custom object.

(3) can probably be done by using some kind of interface that allows you to query the number, type and name of columns.