Chapter 30 Exercise 22, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

30.22 (Eight Queens animation) Modify Listing 22.11, EightQueens.java, to display the intermediate results of the search. As shown in Figure 30.33, the current row being searched is highlighted. Every one second, a new state of the chess board is displayed.

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class Exercise22 extends JApplet{

 private static final long serialVersionUID = 1L;
 private int currentRow = -1;
 
 public Exercise22() {  
  add(new QueensPanel(8), BorderLayout.CENTER);
  JLabel jLabel = new JLabel("Click the mouse to see the next move");
  jLabel.setHorizontalAlignment(SwingConstants.CENTER);
  add(jLabel, BorderLayout.SOUTH);
  
  addMouseListener(new MouseAdapter() {
   
   @Override
   public void mouseReleased(MouseEvent e) {
    currentRow++;
    repaint();
   }
  });
 }

 public static void main(String[] args) {
  JFrame frame = new JFrame();
  frame.add(new Exercise22());
  frame.setTitle("Exercise22");
  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;
  private StringBuilder queen = null;
  
  public QueensPanel(int size) {
   this.size = size;
   queen = getBoard(size);
  }
  
  @Override
  protected void paintComponent(Graphics g) {
   super.paintComponent(g);
   double stepX = getWidth() / (double)size;
   double stepY = getHeight() / (double)size;
   
   if (queen != null) {
    Image queenImage = new ImageIcon("image/20_34/queen.jpg").getImage();
    for (int i = 0; (i < queen.length()) && (i <= currentRow); i++) {     
     g.drawImage(queenImage, (int) (queen.charAt(i) * stepX), (int) (i * stepY), (int)stepX, (int)stepY, this); 
    }    
   }
   
   if((currentRow >= 0) && (currentRow < size)) {
    g.setColor(Color.GRAY);
    g.fillRect(0, (int) ((currentRow + 1) * stepY), getHeight(), (int)(stepY));
    g.setColor(Color.BLACK);
   }
   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 StringBuilder getBoard(int size) {
   StringBuilder s = new StringBuilder();
   for (int i = 0; i < size; i++) {
    s.append((char)(i));
   }
   return getBoard(new StringBuilder(), s);
  }
  
  private StringBuilder getBoard(StringBuilder s1, StringBuilder s2) {
   if (s2.length() == 2) {   
    if (isDiagonalFine(new StringBuilder(s1).append(s2))) {
     return new StringBuilder(s1).append(s2);
    }
    if (isDiagonalFine(new StringBuilder(s1).append(s2.charAt(1)).append(s2.charAt(0)))) {
     return 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));
     }    
     StringBuilder result = getBoard(new StringBuilder(s1).append(s2.charAt(i)), newS2);
     if(result != null) {
      return result;
     }
    }
   }
   return null;
  }
  
  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; 
  }
 }
}