Bad caret position message

Question

Here's a message I am plagued by, which occurs when I attempt to programmatically "select" cells (whether empty or not) in a grid via keypress combo shift-rightarrow or shift-leftarrow:

Exception in thread "AWT-EventQueue-0" javax.swing.text.StateInvariantError: 
Bad caret position

(Note that there is NO problem if I "select" via shift-uparrow or shift-downarrow.)

It happens when I attempt to change the font of the "selected" cells:

  static Font fontSelected = new Font("Serif", Font.BOLD , POINTSIZE);
  static Font fontNormal = new Font("Serif", Font.PLAIN, POINTSIZE);

(If I make the Font.type the SAME (both BOLD, both PLAIN, both ITALIC), no problem.)

The error occurs near code where I push a "selected" JTextField onto a stack (named stack), which is defined like so:

class GenericStack<E>:
  public LinkedList <E> stack = new LinkedList<>();

Here's the class declaration where the stack and fonts are used:

public class Grid  extends GenericStack<JTextField> implements ActionListener, KeyListener, KeyCodes, Serializable

Here's what's pushed onto stack:

 public static JTextField[][] cells = new JTextField[N][N];

Here's how cells are created:

    guiFrame.add(textPanel);
    for (int i = 0; i < N; i++) 
      for (int j = 0; j < N; j++) 
        cells[i][j] = addCell(textPanel, i, j);

  private JTextField addCell (Container parent, int row, int col) {
    JTextField cell;
    cell = new JTextField();
    cell.setFont(fontNormal);                  // 'default' font set
    cell.setText("x");                         // for debugging
    String r, c;                               // 11x11 grid
    if(row < N-1) r = "" + row; else r = "A";  // rows  r: 0,1,2,...A
    if(col < N-1) c = "" + col; else c = "A";  // cols  c: 0,1,2,...A
    cell.setActionCommand(r + c);              // cell rc: 00..0A;10..1A;...A0..AA;
    cell.addKeyListener(this);
    cell.setHorizontalAlignment(JTextField.CENTER);
    parent.add(cell);
    return cell;
  }  

Here's main:

  public static void main(String[] args)
  {    
     javax.swing.SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run() {
        new Grid();
      }
    });
  }

Here's where the font is changed (for any "selected" cell):

if(currentCell.selected){
  Grid.cells[currentCell.row][currentCell.col].setBackground(Color.RED);
  Grid.cells[currentCell.row][currentCell.col].setFont(fontSelected);
  stack.push(Grid.cells[currentCell.row][currentCell.col]);
}

The error occurs in this block of code--if I comment out the setFont line, no problem; if I instead change the font declarations to involve the same font, no problem.

Especially puzzling me is that the stack trace doesn't specify which line of code caused the error.

Answer 1

I'm not sure why your exception is occurring, but it can be solved by queuing the font change on the Swing event thread:

@Override
public void keyPressed(KeyEvent evt) {
  final JComponent comp = (JComponent) evt.getSource();
  int keyCode = evt.getKeyCode();
  boolean shiftIsDown = evt.isShiftDown();
  currentCell.selected = ((shiftIsDown & (keyCode == RIGHT | keyCode == UP
        | keyCode == LEFT | keyCode == DOWN)));
  if (currentCell.selected) {
     SwingUtilities.invokeLater(new Runnable() {
        public void run() {
           comp.setFont(fontSelected);
        }
     });
  }
}

Myself, I try to avoid KeyListeners with Swing applications but instead prefer key bindings. For example:

import java.awt.Font;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;

import javax.swing.*;

@SuppressWarnings("serial")
public class SSCCE2 extends JPanel {
   private static final int ROW_COUNT = 11;
   private static final int colCount = 3;
   private static final Font NORMAL_FONT = new Font("Serif", Font.PLAIN, 18);
   private static final Font SELECTED_FONT = NORMAL_FONT.deriveFont(Font.BOLD);

   private JTextField[][] fields = new JTextField[ROW_COUNT][ROW_COUNT];

   public SSCCE2() {
      FontAction fontAction = new FontAction();
      int condition = WHEN_FOCUSED;

      setLayout(new GridLayout(ROW_COUNT, ROW_COUNT));
      for (int i = 0; i < fields.length; i++) {
         for (int j = 0; j < fields[i].length; j++) {
            JTextField cell = new JTextField(colCount);
            InputMap inputMap = cell.getInputMap(condition);
            ActionMap actionMap = cell.getActionMap();
            int[] arrowKeyCodes = {KeyEvent.VK_UP, KeyEvent.VK_DOWN, 
                  KeyEvent.VK_LEFT, KeyEvent.VK_RIGHT};
            for (int keyCode : arrowKeyCodes) {
               KeyStroke keyStroke = KeyStroke.getKeyStroke(keyCode, 
                     KeyEvent.SHIFT_DOWN_MASK);
               inputMap.put(keyStroke, keyStroke.toString());
               actionMap.put(keyStroke.toString(), fontAction);
            }
            cell.setFont(NORMAL_FONT);
            cell.setHorizontalAlignment(JTextField.CENTER);
            add(cell);
            fields[i][j] = cell;
         }
      }


   }

   private class FontAction extends AbstractAction {
      @Override
      public void actionPerformed(ActionEvent evt) {
         for (JTextField[] row : fields) {
            for (JTextField textField : row) {
               if (textField.hasFocus()) {
                  textField.setFont(SELECTED_FONT);
               } else {
                  textField.setFont(NORMAL_FONT);
               }
            }
         }
      }
   }

   private static void createAndShowGui() {
      SSCCE2 mainPanel = new SSCCE2();

      JFrame frame = new JFrame("SSCCE2");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}