Why Does My JFormattedTextField Re-display Valid Input After an Invalid Entry?

JFormattedTextField Issue: Text Re-appearing After Invalid Entry

In your code, you utilize JFormattedTextField within a SudokuTextBox to enforce validation. However, when entering valid and subsequently invalid values, the text box clears, but the previous valid value re-appears when tabbing forward.

Cause and Solution

Your problem lies in the way you clear the text box after an invalid entry. When invalid, you set the text to "null" instead of the empty string. This causes the JFormattedTextField to retain the last valid value as its default value, which reappears when focus changes.

To resolve this, simply set the text to an empty string after an invalid entry:

public void keyReleased(KeyEvent e) {
  //...
  // Corrected line:
  if(sudoku.isValid(row, col, value)) {
   sudoku.set(row, col, value);
  }
  else {
   sudoku.set(row, col, 0);
   tb.setText(""); // Set to empty string, not null
  }
  //...
}

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