Java N queen implements problem resolution

The N queen problem is a typical constraint problem, which can be solved quickly by using the recursive mechanism.
Description of queen N problem:
On an n by n chessboard, n queens are placed. No other queens are allowed in the rows, columns, and diagonals of each queen, or they will attack each other. As shown in the following figure.

The n queen problem can be solved easily by using the recursive mechanism. For the eight queens, there are 92 solutions. The following is the general solution code for the n-queen problem, where the code is coded in Java.
A total of three classes are designed, one is the Queen class, one is the Board class, one is the main program class (NQueens). The specific code is as follows:

 
import java.util.ArrayList; 
import java.util.List; 
public class NQueens { 
private int numSolutions;//Number of results
private int queenSize;//How many queens
private Board board;//The board
private List<Queen> queens = new ArrayList<Queen>();//The queen
//private List<Queen> nQueens = new ArrayList<Queen>(); 
public NQueens(){ 
} 
public NQueens(int size){ 
numSolutions = 0; 
queenSize = size; 
board = new Board(queenSize); 
for (int i = 0; i < queenSize; i++) { 
Queen queen = new Queen(); 
queens.add(queen); 
} 
} 
public void solve(){ 
System.out.println("Start solve...."); 
putQueen(0); 
} 
private void putQueen(int index){ 
int row = index; 
//If this column is available
for (int col = 0; col < board.getQueenSize(); col++) { 
if (board.getLayout()[row][col] == 0) { 
//Place the position of the queen in this column position
queens.get(row).setPosition(col); 
//Then add 1 to the corresponding position (right below the column and two diagonals) (even if it's not available)
for (int i = row + 1; i < board.getQueenSize(); i++) { 
board.getLayout()[i][col] ++; 
if (row + col - i >= 0) { 
board.getLayout()[i][row + col - i] ++; 
} 
if (i + col - row < board.getQueenSize()) { 
board.getLayout()[i][i + col - row] ++; 
} 
} 
if (row == board.getQueenSize()-1) { 
numSolutions++; 
printSolution(numSolutions); 
}else { 
putQueen(row + 1); 
} 
//Go back and subtract 1 from the corresponding position (right below the column and two diagonals)
for (int i = row + 1; i < board.getQueenSize(); i++) { 
board.getLayout()[i][col] --; 
if (row + col - i >= 0) { 
board.getLayout()[i][row + col - i] --; 
} 
if (i + col - row < board.getQueenSize()) { 
board.getLayout()[i][i + col - row] --; 
} 
} 
} 
} 
} 
//Print solution
private void printSolution(int i){ 
System.out.println("The "+i+ " solution is:"); 
for (int j = 0; j < board.getQueenSize(); j++) { 
for (int k = 0; k < board.getQueenSize(); k++) { 
System.out.print(queens.get(j).getPosition() == k ? " * ":" - "); 
} 
System.out.println(); 
} 
System.out.println(); 
} 
 
public static void main(String[] args) { 
//You can change the parameters
NQueens nQueens = new NQueens(8); 
nQueens.solve(); 
} 
} 
import java.io.Serializable; 
//The queen's class
public class Queen implements Serializable, Cloneable { 
 
private static final long serialVersionUID = 7354459222300557644L; 
//Queen's position
private int position; 
public Queen(){ 
} 
public int getPosition() { 
return position; 
} 
public void setPosition(int position) { 
this.position = position; 
} 
public Object clone() throws CloneNotSupportedException { 
return super.clone(); 
} 
} 
import java.io.Serializable; 
//A board
public class Board implements Serializable,Cloneable { 
 
private static final long serialVersionUID = -2530321259544461828L; 
//Size of board
private int queenSize; 
//Checkerboard layout
private int[][] layout; 
public Board(int size){ 
this.queenSize = size; 
this.layout = new int[queenSize][queenSize]; 
//Initialize so that all positions on the board are available, that is, set all to 0
for (int i = 0; i < queenSize; i++) { 
for (int j = 0; j < queenSize; j++) { 
layout[i][j] = 0; 
} 
} 
} 
public int getQueenSize() { 
return queenSize; 
} 
public void setQueenSize(int queenSize) { 
this.queenSize = queenSize; 
} 
public int[][] getLayout() { 
return layout; 
} 
public void setLayout(int[][] layout) { 
this.layout = layout; 
} 
public Object clone() throws CloneNotSupportedException { 
return super.clone(); 
} 
}