This is an early revision of a Sudoku applet. The "Solve" button doesn't do anything yet, but user input allows you to fill in the board. Moves are checked to make sure they are legal.

To add a number to a square, click on it. A red underline should appear in the cell. Then press 1 through 9 on your keyboard, and the number should appear in red in the cell.

To remove a number, click on a cell, and use any of space, delete, backspace, or 0 to clear the cell.

Source code: Main applet and Cell object