Chapter 22 Exercise 23, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

22.23 (Game: multiple Eight Queens solution) Write a program to display all possible solutions for the Eight Queens puzzle in a scroll pane, as shown in Figure 22.16. For each solution, put a label to denote the solution number. (Hint: Place all solution panes into an HBox and place this one pane into a ScrollPane .)

import java.awt.*;
import java.util.ArrayList;

import javax.swing.*;

public class Exercise23 extends JApplet{

 private static final long serialVersionUID = 1L; 
 private ArrayList<StringBuilder> queensList = new ArrayList<>();
 
 public Exercise23() { 
  setLayout(new BorderLayout());
  getBoard(8);
  
  JPanel panel1 = new JPanel(new GridLayout(1, queensList.size(), 5, 5));
  for (int i = 0; i < queensList.size(); i++) {
   JPanel tmpPanel = new JPanel(new BorderLayout(5, 5));
   tmpPanel.add(new QueensPanel(queensList.get(i).toString()), BorderLayout.CENTER);
   JLabel tmpLabel = new JLabel("Solution " + (i + 1)); 
   tmpLabel.setHorizontalAlignment(SwingConstants.CENTER );
   tmpPanel.add(tmpLabel, BorderLayout.NORTH);
   panel1.add(tmpPanel);
  }
  JScrollPane jScrollPane = new JScrollPane(panel1);
  add(jScrollPane, BorderLayout.CENTER);
 }

 private void getBoard(int size) {
  queensList.clear();
  StringBuilder s = new StringBuilder();
  for (int i = 0; i < size; i++) {
   s.append(i);
  }
  getBoard(new StringBuilder(), new StringBuilder(s));
 }
 
 private void getBoard(StringBuilder s1, StringBuilder s2) {
  if (s2.length() == 2) {   
   if (isDiagonalFine(new StringBuilder(s1).append(s2)))
    queensList.add(new StringBuilder(s1).append(s2));
   if (isDiagonalFine(new StringBuilder(s1).append(s2.charAt(1)).append(s2.charAt(0))))
    queensList.add(new StringBuilder(s1).append(s2.charAt(1)).append(s2.charAt(0)));
  } else {
   for (int i = 0; i < s2.length(); i++) {
    StringBuilder newS2 = new StringBuilder();
    for (int j = 0; j < s2.length(); j++) {
     if (j != i)
      newS2.append(s2.charAt(j));
    }    
    getBoard(new StringBuilder(s1).append(s2.charAt(i)), newS2);
   }
  }
 }
 
 private boolean isDiagonalFine(StringBuilder chessboard) {
  for (int i = 0; i < chessboard.length(); i++) {
   for (int j = i + 1; j < chessboard.length(); j++) {
    if (chessboard.charAt(j) - chessboard.charAt(i) == j - i) 
     return false;
    if (chessboard.charAt(i) - chessboard.charAt(j) == j - i) 
     return false;
   }
  }
  return true; 
 }
 
 public static void main(String[] args) {
  JFrame frame = new JFrame();
  frame.add(new Exercise23());
  frame.setTitle("Exercise23");
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.pack();
  frame.setSize(new Dimension(frame.getWidth() - 100, frame.getHeight() + 20));
  frame.setLocationRelativeTo(null);
  frame.setVisible(true);
 }
 
 class QueensPanel extends JPanel {
  private static final long serialVersionUID = 1L;
  private int size;
  private String board;
  
  public QueensPanel(String board) {
   this.size = board.length();
   this.board = board;
   getBoard(size);
  }
  
  @Override
  protected void paintComponent(Graphics g) {
   super.paintComponent(g);
   double stepX = getWidth() / (double)size;
   double stepY = getHeight() / (double)size;
   
   Image queenImage = new ImageIcon("image/queen.jpg").getImage();
   for (int i = 0; i < size; i++) {    
    g.drawImage(queenImage, (int) ((board.charAt(i) - '0') * stepX), (int) (i * stepY), (int)stepX, (int)stepY, this); 
   }    

   
   g.setColor(Color.DARK_GRAY);
   for (int i = 0; i < size; i++) {
    g.drawLine(0, (int) (i * stepY), getWidth(), (int) (i * stepY));
    g.drawLine((int) (i * stepX), 0, (int) (i * stepX), getHeight());
   }

   g.drawLine(0, getHeight() - 1, getWidth(), getHeight() - 1);
   g.drawLine(getWidth() - 1, 0, getWidth() - 1, getHeight());

  }
  
  @Override
  public Dimension getPreferredSize() {
   return new Dimension(size * 50, size * 50);
  }
 }
}