Sudoku.java

import javax.swing.*;
import java.awt.*;

public class Sudoku extends JFrame {

    public static final int GRID_SIZE = 9;
    public static final int SUBGRID_SIZE = 3;
    public static final int CELL_SIZE = 60;
    public static final int CANVAS_WIDTH = CELL_SIZE * GRID_SIZE;
    public static final int CANVAS_HEIGHT = CELL_SIZE * GRID_SIZE;
    public static final Color OPEN_CELL_BGCOLOR = Color.YELLOW;
    public static final Color OPEN_CELL_TEXT_YES = new Color(0, 255, 0);
    public static final Color OPEN_CELL_TEXT_NO = Color.RED;
    public static final Color CLOSED_CELL_BGCOLOR = new Color(240, 240, 240);
    public static final Color CLOSED_CELL_TEXT = Color.BLACK;
    public static final Font FONT_NUMBERS = new Font("AvantGarde", Font.BOLD, 20);
    private final JTextField[][] tfCells = new JTextField[GRID_SIZE][GRID_SIZE];

    private final 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}
    };
    private final 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}
    };
    private JPanel mainPanel;

    public Sudoku() {
        Container cp = getContentPane();
        cp.setLayout(new GridLayout(GRID_SIZE, GRID_SIZE)); // 9x9 GridLayout

        for (int row = 0; row < GRID_SIZE; ++row) {
            for (int col = 0; col < GRID_SIZE; ++col) {
                tfCells[row][col] = new JTextField();
                cp.add(tfCells[row][col]);
                if (masks[row][col]) {
                    tfCells[row][col].setText("");
                    tfCells[row][col].setEditable(true);
                    tfCells[row][col].setBackground(OPEN_CELL_BGCOLOR);
                } else {
                    tfCells[row][col].setText(puzzle[row][col] + "");
                    tfCells[row][col].setEditable(false);
                    tfCells[row][col].setBackground(CLOSED_CELL_BGCOLOR);
                    tfCells[row][col].setForeground(CLOSED_CELL_TEXT);
                }
                tfCells[row][col].setHorizontalAlignment(JTextField.CENTER);
                tfCells[row][col].setFont(FONT_NUMBERS);
            }
        }

        cp.setPreferredSize(new Dimension(CANVAS_WIDTH, CANVAS_HEIGHT));
        pack();

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setTitle("Sudoku");
        setVisible(true);
    }

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

    }
}