Sudoku in Java?

import java.util.Scanner;

public class Sudoku { 
  public static void main(String[] args) {
    // Read a Sudoku puzzle
    int[][] grid = readAPuzzle();

    if (!isValid(grid))
      System.out.println("Invalid input");
    else if (search(grid)) {
      System.out.println("The solution is found:");
      printGrid(grid);
    }   
    else
      System.out.println("No solution");
  }
  
  /** Read a Sudoku puzzle from the keyboard */
  public static int[][] readAPuzzle() {
    // Create a Scanner
    Scanner input = new Scanner(System.in);

    System.out.println("Enter a Sudoku puzzle:");
    int[][] grid = new int[9][9];
    for (int i = 0; i < 9; i++) 
      for (int j = 0; j < 9; j++)
        grid[i][j] = input.nextInt();
    
    return grid;
  }

  /** Obtain a list of free cells from the puzzle */
  public static int[][] getFreeCellList(int[][] grid) {
    // Determine the number of free cells 
    int numberOfFreeCells = 0;   
    for (int i = 0; i < 9; i++)
      for (int j = 0; j < 9; j++) 
        if (grid[i][j] == 0) 
          numberOfFreeCells++;
    
    // Store free cell positions into freeCellList 
    int[][] freeCellList = new int[numberOfFreeCells][2];
    int count = 0;
    for (int i = 0; i < 9; i++)
      for (int j = 0; j < 9; j++) 
        if (grid[i][j] == 0) {
          freeCellList[count][0] = i;
          freeCellList[count++][1] = j;
        }
    
    return freeCellList;
  }

  /** Print the values in the grid */
  public static void printGrid(int[][] grid) {
    for (int i = 0; i < 9; i++) {
      for (int j = 0; j < 9; j++)
        System.out.print(grid[i][j] + " ");
      System.out.println();
    }
  }

  /** Search for a solution */
  public static boolean search(int[][] grid) {
    int[][] freeCellList = getFreeCellList(grid); // Free cells
    int k = 0; // Start from the first free cell    
    boolean found = false; // Solution found?
    
    while (!found) {
      int i = freeCellList[k][0];
      int j = freeCellList[k][1];
      if (grid[i][j] == 0) 
        grid[i][j] = 1; // Start with 1

      if (isValid(i, j, grid)) {
        if (k + 1 == freeCellList.length) { // No more free cells 
          found = true; // A solution is found
        }
        else { // Move to the next free cell
          k++;
        }
      }
      else if (grid[i][j] < 9) {
        grid[i][j] = grid[i][j] + 1; // Check the next possible value
      }
      else { // grid[i][j] is 9, backtrack
        while (grid[i][j] == 9) {
          grid[i][j] = 0; // Reset to free cell
          if (k == 0) {
            return false; // No possible value
          }
          k--; // Backtrack
          i = freeCellList[k][0];
          j = freeCellList[k][1];
        }

        grid[i][j] = grid[i][j] + 1; // Check the next possible value
      }
    }

    return true; // A solution is found
  }

  /** Check whether grid[i][j] is valid in the grid */
  public static boolean isValid(int i, int j, int[][] grid) {
    // Check whether grid[i][j] is valid at the i's row
    for (int column = 0; column < 9; column++)
      if (column != j && grid[i][column] == grid[i][j])
        return false;

    // Check whether grid[i][j] is valid at the j's column
    for (int row = 0; row < 9; row++)
      if (row != i && grid[row][j] == grid[i][j])
        return false;

    // Check whether grid[i][j] is valid in the 3 by 3 box
    for (int row = (i / 3) * 3; row < (i / 3) * 3 + 3; row++)
      for (int col = (j / 3) * 3; col < (j / 3) * 3 + 3; col++)
        if (row != i && col != j && grid[row][col] == grid[i][j])
          return false;

    return true; // The current value at grid[i][j] is valid
  }

  /** Check whether the fixed cells are valid in the grid */
  public static boolean isValid(int[][] grid) {
    for (int i = 0; i < 9; i++)
      for (int j = 0; j < 9; j++)
        if (grid[i][j] != 0 && !isValid(i, j, grid)) return false;

    return true; // The fixed cells are valid
  }
}

0 6 0 1 0 4 0 5 0  
0 0 8 3 0 5 6 0 0
2 0 0 0 0 0 0 0 1
8 0 0 4 0 7 0 0 6
0 0 6 0 0 0 3 0 0
7 0 0 9 0 1 0 0 4
5 0 0 0 0 0 0 0 2
0 0 7 2 0 6 9 0 0
0 4 0 5 0 8 0 7 0