This program will solve any sudoku board that is in a text file. Numbers that aren't known should be represented as underscores and there shouldn't be any spaces. If there is more than one solution, then the first solution that is reached will be shown. Have fun.

Main.java

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

public class Main 
{
	public static void main(String[] args)
	{
		JFileChooser fileDialog = new JFileChooser();
		if (fileDialog.showOpenDialog(null) != JFileChooser.APPROVE_OPTION)
			return;
		
		SudokuBoard sudoku = new SudokuBoard();
		if (!sudoku.loadFromFile(fileDialog.getSelectedFile().getPath()))
		{
			System.out.println("File could not be opened.");
			return;
		}
		
		if (!sudoku.solve())
			System.out.println("No solution.\n");
		else
			System.out.println(sudoku);
	}
}

SudokuBoard.java

import java.util.*;
import java.io.*;
import java.awt.*;

public class SudokuBoard 
{
	private int[][] board = new int[9][9];
	/*private int[][] board = 
		{
			{ 1, 2, 3, 4, 5, 6, 7, 8, 9 },
			{ 2, 2, 3, 4, 5, 6, 7, 8, 9 },
			{ 3, 2, 3, 4, 5, 6, 7, 8, 9 },
			
			{ 4, 2, 3, 4, 5, 6, 7, 8, 9 },
			{ 5, 2, 3, 4, 3, 6, 7, 8, 9 },
			{ 6, 2, 3, 4, 5, 6, 7, 8, 9 },
			
			{ 7, 2, 3, 4, 5, 6, 7, 8, 9 },
			{ 8, 2, 3, 4, 5, 6, 7, 8, 9 },
			{ 9, 2, 3, 4, 5, 6, 7, 8, 9 }
		};*/
	
	public SudokuBoard()
	{
		
	}
	
	public boolean loadFromFile(String path) 
	{
		try
		{
			Scanner file = new Scanner(new File(path));
			
			// load the board from the file, assuming that each number
			// is separated by a space
			for (int y = 0; y < 9; y++)
			{
				char[] characters = file.next().toCharArray();
				for (int x = 0; x < 9; x++)
				{
					// verify that the digit is between 1 and 9, inclusive
					char digit = characters[x];
					if (digit == '_')
					{
						board[y][x] = 0;
					}
					else if (digit > '9' || digit < '1')
					{
						file.close();
						return false;
					}
					else
					{
						board[y][x] = (int)(digit - '0');
					}
				}
			}
			file.close();
			
			return true;
		}
		catch (FileNotFoundException exception)
		{
			return false;
		}
	}
	
	@Override
	public String toString()
	{
		// convert the board into a string
		String toReturn = "";
		for (int y = 0; y < 9; y++)
		{
			for (int x = 0; x < 9; x++)
			{
				// add an extra space every 3, so it looks like an actual board
				toReturn += board[y][x] + (((x + (x + 2)) % 3 == 0) ? "  " : " ");
			}
			// add an extra line every 3
			toReturn = toReturn.trim() + (((y + (y + 2)) % 3 == 0) ? "\r\n\r\n" : "\r\n");
		}
		
		return toReturn;
	}

	public boolean solve()
	{
		return solveHelper(new Point(0, 0));
	}
	
	private boolean solveHelper(Point p)
	{
		// we've reached the end
		if (p.x == 0 && p.y == 9)
			return true;
		
		// if the value is already solved for, then skip it
		if (board[p.y][p.x] != 0)
			return solveHelper(getNextPoint(p));
		
		// brute force the value
		for (int i = 1; i <= 9; i++)
		{
			if (validPoint(p, i))
			{
				board[p.y][p.x] = i;
				if (solveHelper(getNextPoint(p)))
					return true;
			}
		}
		
		// if it didn't find a solution then we need to reset this to 0
		// so that somewhere later it doesn't think that this was already solved for
		board[p.y][p.x] = 0; 
		
		return false;
	}
	
	private boolean validPoint(Point p, int value)
	{
		// check the row
		for (int x = 0; x < 9; x++)
		{
			if (board[p.y][x] == value && x != p.x)
				return false;
		}
		
		// check the column
		for (int y = 0; y < 9; y++)
		{
			if (board[y][p.x] == value && y != p.y)
				return false;
		}
		
		// check the box
		int topY = (p.y / 3) * 3;
		int topX = (p.x / 3) * 3;
		for (int y = 0; y < 3; y++)
		{
			for (int x = 0; x < 3; x++)
			{
				Point toCheck = new Point(topX + x, topY + y);
				if (board[toCheck.y][toCheck.x] == value && !toCheck.equals(p))
					return false;
			}
		}
		
		return true;
	}
	
	private Point getNextPoint(Point p)
	{
		if (p.x == 8)
			return new Point(0, p.y + 1);
		else
			return new Point(p.x + 1, p.y);
	}
}