JTableの4隅のみ角を丸める

enum Corner { TOP_LEFT, TOP_RIGHT, BOTTOM_LEFT, BOTTOM_RIGHT }

class CalendarTableRenderer extends DefaultTableCellRenderer {
  private final Set<Corner> roundedCorners = EnumSet.noneOf(Corner.class);
  private final LocalDate realDate = LocalDate.now(ZoneId.systemDefault());
  private int row;
  private int column;

  @Override public Component getTableCellRendererComponent(
      JTable table, Object value, boolean selected,
      boolean focused, int row, int column) {
    Component renderer = super.getTableCellRendererComponent(
        table, value, selected, focused, row, column);
    renderer.setBackground(table.getBackground());
    this.row = row;
    this.column = column;
    updateCorners(table, row, column);
    // ...
    return renderer;
  }

  private void updateCorners(JTable table, int row, int col) {
    roundedCorners.clear();
    TableModel model = table.getModel();
    int lastRow = model.getRowCount() - 1;
    int lastCol = model.getColumnCount() - 1;
    if (row == 0 && col == 0) {
      roundedCorners.add(Corner.TOP_LEFT);
    }
    if (row == 0 && col == lastCol) {
      roundedCorners.add(Corner.TOP_RIGHT);
    }
    if (row == lastRow && col == 0) {
      roundedCorners.add(Corner.BOTTOM_LEFT);
    }
    if (row == lastRow && col == lastCol) {
      roundedCorners.add(Corner.BOTTOM_RIGHT);
    }
  }
  // ...
}