Someone

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

public class Sudoku{

	private JFrame frame;
	private JPanel area;
	private JPanel buttons;
	private JButton solve;
	private JButton clear;
	private int[][] matrix;
	private JTextField[][] JTF;
	private boolean[][] change;
	private int count=0;

	public class Solve implements ActionListener {
		public void actionPerformed(ActionEvent e) {
			if(readMatrix()){
				if(solveNext(matrix,change,0,-1)==true){
					showSolution();
					System.out.println("Done");
				}else{
					System.out.println("Inga lösningar");
				}
			}else{
				System.out.println("Matat in fel");
			}
		}
	}

	public boolean readMatrix(){
		for(int i =0; i<=8; i++){
			for(int j=0; j<=8; j++){
				if(JTF[i][j].getText().equals("")){
					matrix[i][j]=-1;
					change[i][j]=true;
					JTF[i][j].setForeground(Color.black);
					
				}else{
					matrix[i][j]=Integer.parseInt(JTF[i][j].getText());	
					change[i][j]=false;
					JTF[i][j].setForeground(Color.blue);
				}
			}
		}

		for(int i =0; i<=8; i++){
			for(int j=0; j<=8; j++){
				if(chk(matrix, i,j)==false){
					return false;
				}
			}
		}
		return true;
	}

	public void showSolution(){
		String s = "";
		for(int i =0; i<=8; i++){
			for(int j=0; j<=8; j++){
				s = ""+matrix[i][j];
				JTF[i][j].setText(s);
			}
		}
	}

	public class Clear implements ActionListener {
		public void actionPerformed(ActionEvent e) {
			for(int i=0; i<=8;i++){
				for(int j =0; j<=8;j++){
					JTF[i][j].setText("");
				}
			}
		}
	}

	public Sudoku(){

		//Create matrix
		matrix = new int[9][9];
		change = new boolean[9][9];

	}

	public void createWindow(){
		frame = new JFrame("test");
		area = new JPanel(new GridLayout(9, 9));
		frame.setPreferredSize(new Dimension(200, 250));
		frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
		area.setPreferredSize(new Dimension(200, 200));
		JTF = new JTextField[9][9];
		Color C = Color.white;

		for(int i=0; i<=8;i++){

			for(int j =0; j<=8;j++){

				if(j == 3 || j ==6){
					C=changeColor(C);
				}
				if((i == 3|| i == 6) && j==0){
					C=changeColor(C);
				}
				JTF[i][j] = new JTextField(1);
				JTF[i][j].setHorizontalAlignment(0);
				JTF[i][j].setBackground(C);
				area.add(JTF[i][j]);
			}
		}

	
		frame.add(area, BorderLayout.NORTH);

		buttons = new JPanel(new GridLayout(1, 2));
		buttons.setPreferredSize(new Dimension(100,20));
		solve = new JButton("Lös");
		clear = new JButton("Rensa");
		buttons.add(solve);
		buttons.add(clear);
		frame.add(buttons, BorderLayout.SOUTH);
		frame.pack();
		frame.setVisible(true);
		area.setVisible(true);
		solve.addActionListener(new Solve());
		clear.addActionListener(new Clear());

	}

	public static void main(String [] arg) {

		Sudoku S = new Sudoku();
		S.createWindow();

	}


	public static Color changeColor(Color C){
		if (C == Color.lightGray){
			return Color.white;
		}
		return Color.lightGray;
	}

	public boolean solveNext(int[][] matrix,boolean[][]change, int row, int col) {
		
		col++;
		if(col == 9){
			row++;
			col = 0;
		}

		while(change[row][col]== false ){
			col++;
			if(col == 9){
				row++;
				col = 0;
			}
		}

		for(int k=1; k<=9;k++){
			
			matrix[row][col] = k;

			boolean chkk = chk(matrix,row,col);

			if(chkk){
				if(row==8 && col==8){
					return true;
				}
				if(solveNext(matrix,change,row,col)==true){
					return true;
				}
			}
		}
		
		matrix[row][col] = -1;
		return false;
	}

	public boolean chk(int[][] matrix, int row, int col){

		for(int i = 0; i<=8;i++){
			for(int k = 0; k<=8;k++){
				if(matrix[row][i] == matrix[row][k]&& i!=k && matrix[row][i]!=-1)
					return false;

				if(matrix[i][col] == matrix[k][col] && i!=k && matrix[i][col]!=-1){
					return false;
				}

			}
		}

		for (int x = 0; x <= 6; x=x+3){
			for (int y = 0;y <= 6; y=y+3){
				if(row == x || row == x+1 || row == x+2){
					if(col == y|| col == y+1|| col == y+2){
						for(int i = x; i<=(2+x); i++){
							for(int j = y; j<= (2+y); j++){
								if(row != i && col !=j){
									if (matrix[row][col]==matrix[i][j] && matrix[row][col]!=-1){
										return false;
									}	
								}
							}
						}
					}
				}
			}
		}

		return true;

	}
}