Java nested list to array conversion

For JTable in particular, I'd suggest subclassing AbstractTableModel like so:

class MyTableModel extends AbstractTableModel {
    private List<List<String>> data;
    public MyTableModel(List<List<String>> data) {
        this.data = data;
    }
    @Override
    public int getRowCount() {
        return data.size();
    }
    @Override
    public int getColumnCount() {
        return data.get(0).size();
    }
    @Override
    public Object getValueAt(int row, int column) {
        return data.get(row).get(column);
    }
    // optional
    @Override
    public void setValueAt(Object aValue, int row, int column) {
        data.get(row).set(column, aValue);
    }
}

Note: this is the most basic implementation possible; error-checking is omitted for brevity.

Using a model like this, you don't have to worry about pointless conversions to Object[][].

//defined somewhere
List<List<String>> lists = ....

String[][] array = new String[lists.size()][];
String[] blankArray = new String[0];
for(int i=0; i < lists.size(); i++) {
    array[i] = lists.get(i).toArray(blankArray);
}

I don't know anything about JTable, but converting a list of lists to array can be done with a few lines.