How to sort a jtable with null values always at the end

Question 1

I solved the problem thanks to @MadProgrammer and @kleopatra through a follow-up question (Java Comparator always reading values as Strings) I had.

The answer is to create a custom Comparator that can determine which way the table is being sorted, then return the right value accordingly (if the sort is ASCENDING then o1 needs to return 1, and -1 visa versa, and o2 is exactly the opposite).

In my opinion, which is not worth much, this is not TOO hackish and does solve my problem, so I'm satisfied, even if others are not.

Here is the complete code to my solution:

import java.awt.Component;
import java.util.Comparator;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.SortOrder;
import javax.swing.table.DefaultTableCellRenderer;
import javax.swing.table.DefaultTableModel;
import javax.swing.table.TableModel;
import javax.swing.table.TableRowSorter;

public class Test {

    public static void main(String args[]) {
        JFrame frame = new JFrame();
        Object[][] data = new Object[8][3];
        data[0][0] = 6.5d; data[0][1] = "Name1";
        data[1][0] = new NullClassFiller(); data[1][1] = "Name2";
        data[2][0] = 2.6d; data[2][1] = "Name3";
        data[3][0] = 0d; data[3][1] = "Name4";
        data[4][0] = new NullClassFiller(); data[4][1] = "Name5";
        data[5][0] = -4d; data[5][1] = "Name6";
        data[6][0] = 0d; data[6][1] = "Name7";
        data[7][0] = -4.3d; data[7][1] = "Name8";
        JTable table = new JTable();
        table.setModel(new DefaultTableModel(data, new String[]{"One", "Two"}) {
            @Override
            public Class<?> getColumnClass(int column) {
                return column == 0 ? Double.class : String.class;
            }
            @Override
            public boolean isCellEditable(int row, int column) {
                return false;
            }
        });

        TableRowSorter<TableModel> sorter = new TableRowSorter<TableModel>(table.getModel()) {
            @Override
            public Comparator<?> getComparator(final int column) {
                Comparator c = new Comparator() {
                    @Override
                    public int compare(Object o1, Object o2) {
                        boolean ascending = getSortKeys().get(column).getSortOrder() == SortOrder.ASCENDING;
                        System.out.println(o1.getClass() + " - " + o2.getClass());
                        if (o1 instanceof NullClassFiller) {
                            if(ascending)
                                return 1;
                            else
                                return -1;
                        } else if (o2 instanceof NullClassFiller) {
                            if(ascending)
                                return -1;
                            else
                                return 1;
                        } else {
                            return ((Comparable<Object>) o1).compareTo(o2);
                        }

                    }
                };
                return c;
            }
        };
        table.setRowSorter(sorter);
        table.getColumnModel().getColumn(0).setCellRenderer(new CustomRenderer());
        JScrollPane pane = new JScrollPane(table);
        frame.add(pane);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setSize(500, 500);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    static class NullClassFiller {}

    static class CustomRenderer extends DefaultTableCellRenderer {

        @Override
        public Component getTableCellRendererComponent(JTable table, Object value, boolean isSelected, boolean hasFocus, int row, int column) {
            DefaultTableCellRenderer renderer = (DefaultTableCellRenderer) super.getTableCellRendererComponent(table, value, isSelected, hasFocus, row, column);

            if(value instanceof NullClassFiller)
                renderer.setText("");

            return renderer;
        }

    }
}

In my original post, I was creating a custom Comparator, but it had no access to getSortKeys(). So, I overwrote the getComparator() method in the TableRowSorter so that I could access getSortKeys(). It works.

Question 2

The call to getSortKeys().get() must use 0 (i.e. the main sort key) and not column:

boolean ascending = getSortKeys().get(0).getSortOrder() == SortOrder.ASCENDING;

If the user clicks, for example, over the column number 12 or 7, you want the first sort key (there will be 3 at most, if the user clicked before over other columns, to perform multi column sort). This sort key will have two attributes:

Sort Order (ASCENDING, DESCENDING, UNORDERED)
```
getSortKeys().get(0).getSortOrder()
```
Column Index (12 or 7, for example)
```
getSortKeys().get(0).getColumn()
```