Why Doesn't My JFormattedTextField Clear Invalid Sudoku Entries?

JFormattedTextField Not Clearing Properly

When using JFormattedTextField as the base class for text boxes in a Sudoku game, the text box does not properly clear if an invalid value is entered after a valid one. This issue is not encountered when using JTextField instead, but JTextField does not provide the desired layout and input restrictions.

Understanding the Behavior

When JFormattedTextField is used, it maintains a MaskFormatter to enforce the input format (in this case, only one integer per text box). When an invalid value is entered, JFormattedTextField sets the text to null. However, subsequent interactions with the text box, such as tabbing forward, may trigger an auto-completion mechanism that fills in the previously entered valid value.

Alternative Solution

To resolve this issue, consider using a custom component that handles input and rendering more precisely. Here's an example of such a component: CellTest, which extends JPanel and includes a JDigit button for displaying the digits.

JDigit Button

The JDigit button:

1. Accepts input from the mouse, menu, or keyboard (including the number keys and space bar).
2. Updates its digit based on user selection.
3. Provides a consistent rendering of the digits, including an anti-aliased font and a centered position.
4. Displays a pop-up menu with options to select digits.

Improved Input Handling

By using CellTest instead of JFormattedTextField, you can separate the input validation from the rendering process. This allows you to have more control over the behavior of the text boxes, ensuring that invalid values are properly cleared and that tabbing does not cause unexpected values to appear.

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