Java Graphics Programming

Assignment - Sudoku

Rules of Game

You could wiki "Sudoku" to understand the rules of the game.

Sudoku is a single-player mind game. "The objective is to fill a 9×9 grid with digits 1 to 9, so that each column, each row, and each of the nine 3×3 sub-grids (also called "boxes", "blocks", "regions", or "sub-squares") contains all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a unique solution."

Graphical Display


Start with the GUI for display. The GUI codes is simple. You can simply use a 9x9 JTextFields arranged in a 9x9 GridLayout.

The steps for producing the display are:

  • Set the JFrame's content-pane to 9×9 GridLayout. Create 9×9 JTextFields (called tfCells) and add to the content-pane. The JTextFields shall contain the string "1" to "9" (the number from the puzzle or the number guessed) or empty string (for blank cells).
  • Initialize the game by reading in an input puzzle (int[9][9] puzzle) with blank cells (handled by boolean[9][9] masks), and populate the tfCells arrays. Set the non-empty cells to non-editable containing the number from the puzzle; and set the empty cells to editable containing an empty string.

Study the following codes. Fill in the main() method ([TODO 1]), run the program, which shall produce the display.

import java.awt.*;        // Uses AWT's Layout Managers
import java.awt.event.*;  // Uses AWT's Event Handlers
import javax.swing.*;     // Uses Swing's Container/Components
 * The Sudoku game.
 * To solve the number puzzle, each row, each column, and each of the
 * nine 3×3 sub-grids shall contain all of the digits from 1 to 9
public class Sudoku extends JFrame {
   // Name-constants for the game properties
   public static final int GRID_SIZE = 9;    // Size of the board
   public static final int SUBGRID_SIZE = 3; // Size of the sub-grid
   // Name-constants for UI control (sizes, colors and fonts)
   public static final int CELL_SIZE = 60;   // Cell width/height in pixels
   public static final int CANVAS_WIDTH  = CELL_SIZE * GRID_SIZE;
   public static final int CANVAS_HEIGHT = CELL_SIZE * GRID_SIZE;
                                             // Board width/height in pixels
   public static final Color OPEN_CELL_BGCOLOR = Color.YELLOW;
   public static final Color OPEN_CELL_TEXT_YES = new Color(0, 255, 0);  // RGB
   public static final Color OPEN_CELL_TEXT_NO = Color.RED;
   public static final Color CLOSED_CELL_BGCOLOR = new Color(240, 240, 240); // RGB
   public static final Color CLOSED_CELL_TEXT = Color.BLACK;
   public static final Font FONT_NUMBERS = new Font("Monospaced", Font.BOLD, 20);
   // The game board composes of 9x9 JTextFields,
   // each containing String "1" to "9", or empty String
   private JTextField[][] tfCells = new JTextField[GRID_SIZE][GRID_SIZE];
   // Puzzle to be solved and the mask (which can be used to control the
   //  difficulty level).
   // Hardcoded here. Extra credit for automatic puzzle generation
   //  with various difficulty levels.
   private int[][] puzzle =
      {{5, 3, 4, 6, 7, 8, 9, 1, 2},
       {6, 7, 2, 1, 9, 5, 3, 4, 8},
       {1, 9, 8, 3, 4, 2, 5, 6, 7},
       {8, 5, 9, 7, 6, 1, 4, 2, 3},
       {4, 2, 6, 8, 5, 3, 7, 9, 1},
       {7, 1, 3, 9, 2, 4, 8, 5, 6},
       {9, 6, 1, 5, 3, 7, 2, 8, 4},
       {2, 8, 7, 4, 1, 9, 6, 3, 5},
       {3, 4, 5, 2, 8, 6, 1, 7, 9}};
   // For testing, open only 2 cells.
   private boolean[][] masks =
      {{false, false, false, false, false, true, false, false, false},
       {false, false, false, false, false, false, false, false, true},
       {false, false, false, false, false, false, false, false, false},
       {false, false, false, false, false, false, false, false, false},
       {false, false, false, false, false, false, false, false, false},
       {false, false, false, false, false, false, false, false, false},
       {false, false, false, false, false, false, false, false, false},
       {false, false, false, false, false, false, false, false, false},
       {false, false, false, false, false, false, false, false, false}};
    * Constructor to setup the game and the UI Components
   public Sudoku() {
      Container cp = getContentPane();
      cp.setLayout(new GridLayout(GRID_SIZE, GRID_SIZE));  // 9x9 GridLayout
      // Allocate a common listener as the ActionEvent listener for all the
      //  JTextFields
      // ... [TODO 3] (Later) ....
      // Construct 9x9 JTextFields and add to the content-pane
      for (int row = 0; row < GRID_SIZE; ++row) {
         for (int col = 0; col < GRID_SIZE; ++col) {
            tfCells[row][col] = new JTextField(); // Allocate element of array
            cp.add(tfCells[row][col]);            // ContentPane adds JTextField
            if (masks[row][col]) {
               tfCells[row][col].setText("");     // set to empty string
               // Add ActionEvent listener to process the input
               // ... [TODO 4] (Later) ...
            } else {
               tfCells[row][col].setText(puzzle[row][col] + "");
            // Beautify all the cells
      // Set the size of the content-pane and pack all the components
      //  under this container.
      cp.setPreferredSize(new Dimension(CANVAS_WIDTH, CANVAS_HEIGHT));
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  // Handle window closing
   /** The entry main() entry method */
   public static void main(String[] args) {
      // [TODO 1] (Now)
      // Check Swing program template on how to run the constructor

   // Define the Listener Inner Class
   // ... [TODO 2] (Later) ...

Event Handling

Next, we shall program the event handling.

We shall use a common instance of a Named Inner Class (called InputListener) as the ActionEvent listener for "all" the editable JTextFields. Hence, in the actionPerformed(), we need to identify the particular JTextField (in terms of row and col) that trigger the event. You could use the ActionEvent.getSource() method to retrieve the source object that has fired the event and compare the object with all the 9×9 JTextFields.

Place the inner class at [TODO 2].

   // [TODO 2]
   // Inner class to be used as ActionEvent listener for ALL JTextFields
   private class InputListener implements ActionListener {
      public void actionPerformed(ActionEvent e) {
         // All the 9*9 JTextFileds invoke this handler. We need to determine
         // which JTextField (which row and column) is the source for this invocation.
         int rowSelected = -1;
         int colSelected = -1;
         // Get the source object that fired the event
         JTextField source = (JTextField)e.getSource();
         // Scan JTextFileds for all rows and columns, and match with the source object
         boolean found = false;
         for (int row = 0; row < GRID_SIZE && !found; ++row) {
            for (int col = 0; col < GRID_SIZE && !found; ++col) {
               if (tfCells[row][col] == source) {
                  rowSelected = row;
                  colSelected = col;
                  found = true;  // break the inner/outer loops
          * [TODO 5]
          * 1. Get the input String via tfCells[rowSelected][colSelected].getText()
          * 2. Convert the String to int via Integer.parseInt().
          * 3. Assume that the solution is unique. Compare the input number with
          *    the number in the puzzle[rowSelected][colSelected].  If they are the same,
          *    set the background to green (Color.GREEN); otherwise, set to red (Color.RED).
          * [TODO 6] Check if the player has solved the puzzle after this move.
          * You could update the masks[][] on correct guess, and check the masks[][] if
          * any input cell pending.

In's constructor:

  1. Declare and allocate a common instance called listener of the InputListener class:
    // [TODO 3]
    InputListener listener = new InputListener();
  2. All editable JTextField shall add this common instance as its ActionEvent listener:
    // [TODO 4]
    tfCells[row][col].addActionListener(listener);   // For all editable rows and cols

Continue the actionPerformed() to check if the input number is acceptable in that particular (rowSelected, colSelected) ([TODO 5] and [TODO 6]).

Some useful methods of JTextField are as follows. You can check the Java API for more methods.

setBackground(Color c)  // Set the background color of the component
setForeground(Color c)  // Set the text color of the JTextField
setFont(Font f)         // Set the font used by the JTextField
setHorizontalAlignment(int align);  // align: JTextField.CENTER, JTextField.LEFT, JTextField.RIGHT

Common colors are defined via constants such as Color.RED, Color.GREEN, Color.BLUE, and etc.

Hints and Miscellaneous

  • This is a moderately complex program. You need to use the graphics debugger in Eclipse/NetBeans to debug your program logic.
  • Check the JDK API on the classes and methods available under Swing.
  • You can use the following static method to pop up a dialog box with a message:
    JOptionPane.showMessageDialog(null, "Congratulation!");
    Check the API for JOptionPane.
  • To check the sub-grid, you could use InputRow/3 and InputCol/3 to get the subGridRow and subGridCol. The (row, col)'s for the sub-grip are (subGridRow*3 + 0|1|2, subGridCol*3 + 0|1|2).
  • There are many ways to automatically generate a new puzzle, e.g.,
    1. Start with a solved puzzle, add a random number between 1-9 to all the cells and modulo 9.
    2. Start with a solved puzzle, swap rows/columns among 1-2-3, 4-5-6, 7-8-9.
    3. [TODO]
  • There are many ways to set the difficulty level, e.g.,
    1. Control the number of un-revealed cells.
    2. [TODO]

More Credits

  • Reset/Restart the game.
  • A good Sudoku engine shall accept any "valid" number at the time of input (no duplicate in row, column and sub-grid), but signal a conflict whenever it is detected. Highlight the conflicting cells.
  • Choice of puzzles and difficulty levels (e.g,, easy, intermediate, difficult).
  • Create a menu bar for options such as "File" ("New Game", "Reset Game", "Exit"), "Options", and "Help" (Use JMenuBar, JMenu, and JMenuItem classes).
  • Create a status bar (JTextField at the south zone of BorderLayout) to show the messages (e.g., number of cells remaining) (google "java swing statusbar").
  • Re-organize your codes, with methods such as initGame(), getPuzzle(), etc. Better still, apply your OO knowledge to write a separate class called Puzzle, and etc.
  • Beautify your graphical interface, e.g., color, font, layout, etc.
  • Timer (pause/resume), score, progress bar.
  • A side panel for command, display, strategy?
  • Automatic puzzle generation with various difficulty level.
  • The sample program processes ActionEvent of the JTextField, which requires user to push the ENTER key. Try KeyEvent with keyTyped() handler; or other means that does not require pushing of ENTER key.
  • Sound effect, background music, enable/disable sound?
  • High score and player name?
  • Hints and cheats (reveal a cell, or reveal all cells with number n)?
  • Choice of display "theme"?
  • Use of images and icons?
  • (Very difficult) Multi-Player game through networking.
  • ......

(Advanced) Subclassing JTextField

The javax.swing.JTextField does not contain identifiers (such as row and column). You can actually create your subclass of JTextField to provide the additional functionality, which will greatly simplify your programming. E.g,.

import javax.swing.JTextField;
public class Cell extends JTextField {
   // Package access
   int row, col;      // row and col of this JTextField

   // More?! For examples,
   int numberPuzzle;  // The puzzle number
   boolean isOpened;  // to be solved?
   int numberGuess;   // The number entered/display
   boolean isValid;