Chapter 30 Exercise 18, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

30.18 (Parallel Eight Queens) Revise Listing 22.11, EightQueens.java, to develop a parallel algorithm that finds all solutions for the Eight Queens problem. (Hint:
Launch eight subtasks, each of which places the queen in a different column in
the first row.)

import java.awt.*;
import java.awt.event.*;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import javax.swing.*;

public class Exercise18 extends JApplet{

 private static final long serialVersionUID = 1L; 
 private JLabel label = new JLabel(" ");
 
 public Exercise18() {  
  add(new QueensPanel(12), BorderLayout.CENTER);
  label.setHorizontalAlignment(SwingConstants.CENTER);
  add(label, BorderLayout.SOUTH);
 }

 public static void main(String[] args) {
  JFrame frame = new JFrame();
  frame.add(new Exercise18());
  frame.setTitle("Exercise18");
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.pack();
  frame.setMinimumSize(new Dimension(frame.getWidth(), frame.getHeight()));
  frame.setLocationRelativeTo(null);
  frame.setVisible(true);
 }
 
 class QueensPanel extends JPanel {
  private static final long serialVersionUID = 1L;
  private int size;
  int currentBoard = -1;
  
  private List<String> queensList = Collections.synchronizedList(new ArrayList<String>());
  
  public QueensPanel(int size) {
   this.size = size;
   getBoard(size);
   addMouseListener(new MouseAdapter() {
    
    @Override
    public void mouseReleased(MouseEvent e) {
     currentBoard++;
     if (currentBoard == queensList.size())
      currentBoard = 0;
     repaint();
    }
    
   });
  }
  
  @Override
  protected void paintComponent(Graphics g) {
   super.paintComponent(g);
   double stepX = getWidth() / (double)size;
   double stepY = getHeight() / (double)size;
   
   if (currentBoard >= 0) {
    label.setText((currentBoard + 1) + " of " + queensList.size());
    Image queenImage = new ImageIcon("image/20_34/queen.jpg").getImage();
    for (int i = 0; i < size; i++) {
     
     g.drawImage(queenImage, (int) (queensList.get(currentBoard).charAt(i) * 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);
  }
 

  private void getBoard(int size) {
   queensList.clear();

   ExecutorService executor = Executors.newFixedThreadPool(size);
   for (int i = 0; i < size; i++) {
    StringBuilder s1 = new StringBuilder();
    s1.append((char)(i));
    
    StringBuilder s2 = new StringBuilder();
    for (int j = 0; j < size; j++) {
     if(i != j) {
      s2.append((char)(j));
     }
    }
    executor.execute(new GetBoard(s1, s2));    
   }
   executor.shutdown();   
   while(queensList.size() < 20) {    
   }
   
   currentBoard++;
  }
  
  private class GetBoard implements Runnable {
   private StringBuilder s1;
   private StringBuilder s2;
   
   public GetBoard(StringBuilder s1, StringBuilder s2) {
    this.s1 = s1;
    this.s2 = s2;
   }

   @Override
   public void run() {
    getBoard(s1, s2);    
   }
   
  }
  
  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)).toString());
    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))).toString());
   } 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; 
  }


 }
}