Brute force Sudoku solver in Java

I did this Sudoku solver for a school project and I’ve now decided to release it as a proof-of-concept project. It’s nothing fancy. It contains a class used to brute force Sudoku puzzles, you can read more about the algorithm on Wikipedia, and a little Swing GUI. I’ve never used Swing before doing this project so don’t expect anything amazing. The code is released under GPL v2 and I hope it might be useful to someone out there. There is a generated Javadoc so check it out for more detailed info about the classes.

Check out the code in my public Mercurial repository.

Download executable JAR-file.

And heres a overview of the brute force class (got to the Mercurial repository to get the GUI source):

/**
 * Sudoku Solver Proof-of-concept.
 * Copyright (C) 2011 Anton Fagerberg (Kung Foo Code).
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package sudoku;

public class Sudoku {

	private int[][] matrix = new int[9][9];
	private int[][] inputMatrix = new int[9][9];

	/**
	 * Remove all the numbers from the sudoku-matrixes.
	 */
	public void resetMatrix() {
		matrix = new int[9][9];
		inputMatrix = new int[9][9];
	}

	/**
	 * Set value of the X- & Y-coordinate in the matrix.
	 * If x > 8 the y-value will automatically increase. set(0, 1, 1) is the same as set(9, 0, 1).
	 * @param x X-coordinate, 0-80.
	 * @param y Y-coordinate, 0-8.
	 * @param value Number, 1,9.
	 */
	private void fill(int x, int y, int value) {

		while (x < 0) {
			x += 9;
			y--;
		}

		while(x > 8) {
			x -= 9;
			y++;
		}

		matrix[x][y] = value;
	}

	/**
	 * Save value from user input to the matrix which handles user input.
	 * If x > 8 the y-value will automatically increase. set(0, 1, 1) is the same as set(9, 0, 1).
	 * @param x X-coordinate, 0-80.
	 * @param y Y-coordinate, 0-8.
	 * @param value Number, 1,9.
	 */
	public void set(int x, int y, int value) {

		while (x < 0) {
			x += 9;
			y--;
		}

		while(x > 8) {
			x -= 9;
			y++;
		}

		inputMatrix[x][y] = value;
	}

	/**
	 * Copy the values of the matrix containing the user input to the matrix used when trying to solve the puzzle.
	 */
	private void copyMatrix() {
		for (int x = 0; x != 9; x++)
			for (int y = 0; y != 9; y++)
				matrix[x][y] = inputMatrix[x][y];
	}

	/**
	 * Try to solve the sudoku-puzzle.
	 * @return True or false based on if the puzzle was solved or not.
	 */
	public boolean solve() {
		copyMatrix();
		return solve(0);
	}

	/**
	 * Get the value from the specified X- & Y-coordinate in the matrix used to solve the sudoku-puzzles.
	 * If x > 8 the y-value will automatically increase. set(0, 1, 1) is the same as set(9, 0, 1).
	 * @param x X-coordinate, 0-80.
	 * @param y Y-coordinate, 0-8.
	 * @return The value, 0-9. (0 is regarded as not defined).
	 */
	public int getValue(int x, int y) {

		while (x < 0) {
			x += 9;
			y--;
		}

		while(x > 8) {
			x -= 9;
			y++;
		}

		return matrix[x][y];
	}

	/**
	 * Get the value from the specified X- & Y-coordinate from the matrix which contain the user inputs.
	 * If x > 8 the y-value will automatically increase. set(0, 1, 1) is the same as set(9, 0, 1).
	 * @param x X-coordinate, 0-80.
	 * @param y Y-coordinate, 0-8.
	 * @return The value, 0-9. (0 is regarded as not defined).
	 */
	public int getValueOriginal(int x, int y) {

		while (x < 0) {
			x += 9;
			y--;
		}

		while(x > 8) {
			x -= 9;
			y++;
		}

		return inputMatrix[x][y];
	}

	/**
	 * Recursive function which fills a valid number to the X- & Y-coordinate based on the sudoku-rules.
	 * The function will call itself with the next X- & Y-coordinate if it finds a valid number to insert or if the number is already set by the user input.
	 * If x > 8 the y-value will automatically increase.
	 * @param x X-coordinate.
	 * @return True or false if a valid number can be inserted or not.
	 */
	private boolean solve(int x) {

		if (x == 81)
			return true;

		if (getValue(x, 0) != 0 && getValue(x, 0) == getValueOriginal(x, 0)) {
			return solve(x+1);
		}
		else {
			for (int i : getValidNumbers(x, 0)) {
				if (i != 0) {
					fill(x, 0, i);

					if (!solve(x+1))
						fill(x,0,0);
					else
						return true;
				}
			}
		}
		return false;
	}

	/**
	 * Validate if a specified number is valid at the X- & Y-coordinate based on the sudoku-rules.
	 * If x > 8 the y-value will automatically increase. set(0, 1, 1) is the same as set(9, 0, 1).
	 * @param x X-coordinate, 0-80.
	 * @param y Y-coordinate, 0-8.
	 * @param number Number (Only 1-9 can be valid).
	 * @return True or false based on if the number is valid according to the sudoku-rules.
	 */
	public boolean validNumberOriginal (int x, int y, int number) {

		if (number < 1 || number > 9)
			return false;

		while (x < 0) {
			x += 9;
			y--;
		}

		while(x > 8) {
			x -= 9;
			y++;
		}

		int[] validNumbers = {1,2,3,4,5,6,7,8,9};

		// Check vertical and horizontal
		for (int i = 0; i != 9; i++) {
			if (inputMatrix[i][y] != 0)
				validNumbers[inputMatrix[i][y]-1] = 0;
			if (inputMatrix[x][i] != 0)
				validNumbers[inputMatrix[x][i]-1] = 0;
		}

		// Check the "squares"
		int squareX, squareY;

		if (x < 3)
			squareX = 0;
		else if (x < 6)
			squareX = 3;
		else
			squareX = 6;

		if (y < 3)
			squareY = 0;
		else if (y < 6)
			squareY = 3;
		else
			squareY = 6;

		for (int i = 0; i != 3; i++) {
			for (int j = 0; j != 3; j++) {
				if (inputMatrix[i+squareX][j+squareY] != 0) {
					validNumbers[inputMatrix[i+squareX][j+squareY]-1] = 0;
				}
			}
		}

		return (validNumbers[number-1] == 0) ? false : true;
	}

	/**
	 * Returns an array of the valid numbers at the X- & Y-coordinate according to the sudoku-rules.
	 * If the number n is valid the array will have array[n-1] = n. If n is invalid array[n-1] = 0.
	 * If x > 8 the y-value will automatically increase. set(0, 1, 1) is the same as set(9, 0, 1).
	 * @param x X-coordinate, 0-80.
	 * @param y Y-coordinate, 0-8.
	 * @return Array of valid numbers (size 9).
	 */
	private int[] getValidNumbers(int x, int y) {

		while (x < 0) {
			x += 9;
			y--;
		}

		while(x > 8) {
			x -= 9;
			y++;
		}

		int[] validNumbers = {1,2,3,4,5,6,7,8,9};

		// Check vertical and horizontal
		for (int i = 0; i != 9; i++) {
			if (matrix[i][y] != 0) {
				validNumbers[matrix[i][y]-1] = 0;
			}
			if (matrix[x][i] != 0) {
				validNumbers[matrix[x][i]-1] = 0;
			}
		}

		// Check the "squares"
		int squareX, squareY;

		if (x < 3)
			squareX = 0;
		else if (x < 6)
			squareX = 3;
		else
			squareX = 6;

		if (y < 3)
			squareY = 0;
		else if (y < 6)
			squareY = 3;
		else
			squareY = 6;

		for (int i = 0; i != 3; i++) {
			for (int j = 0; j != 3; j++) {
				if (matrix[i+squareX][j+squareY] != 0) {
					validNumbers[matrix[i+squareX][j+squareY]-1] = 0;
				}
			}
		}

		return validNumbers;
	}

	/**
	 * Print a ascii-version of the sudoku-puzzle to the console.
	 */
	public void printTable() {
		System.out.print("\n");
		for (int x = 0; x != 9; x++) {
			for (int y = 0; y != 9; y++) {
				System.out.print(" " + inputMatrix[x][y] + " ");
				if (y == 2 || y == 5)
					System.out.print("|");
			}
			if (x == 2 || x == 5)
				System.out.print("\n---------+---------+--------");
			System.out.print("\n");
		}
	}
}

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