一个有趣的程序-暂时没有看懂

package test;
import javax.swing.JFrame; 
import javax.swing.JOptionPane;
import java.awt.Graphics;
import java.util.Random;
public class Testsync extends JFrame implements Runnable {      
 private int size;   
 private int squarePixels;   
 private boolean[][] pointsOccupied;   
 private int[] currentPosition = new int[2], nextPosition = new int[2];      
 private Thread thread;   
 private Random random;      
 public Testsync(int gridSize, int gridSquarePixels) {    
  super("Self Avoiding Random Walk");
  size = gridSize;
  squarePixels = gridSquarePixels;
  setSize(size*squarePixels, size*squarePixels);   
  setVisible(true);
  setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  pointsOccupied = new boolean[size][];
  for (int i = 0; i  < size; i++) 
   pointsOccupied[i] = new boolean[size];
  thread = new Thread(this); 
  random = new Random();   
  reset();   
  }
 public void reset() {    
  currentPosition[0] = nextPosition[0] = currentPosition[1] = nextPosition[1] = size/2;
  for (int i = 0; i  < size; i++)     
   java.util.Arrays.fill(pointsOccupied[i], false);    
  pointsOccupied[size/2][size/2] = true;   
 }   
 public void draw() {    
  thread.start();   
 }
 public void run() {    
  while(true) {   
   if (!randomChooseNext())
    break; 
   repaint();    
   try {    
    Thread.sleep(1*100); 
    } catch(Exception e) { } 
  }
  if (JOptionPane.showConfirmDialog(this, "Path ends. Still want to see another one?") == JOptionPane.YES_OPTION) {
   getGraphics().clearRect(0, 0, size*squarePixels, size*squarePixels);   
   reset();     
   run();    
  }    
  System.exit(0);   
 }      
 public void paint(Graphics g) {    
  g.drawLine(currentPosition[0]*squarePixels, currentPosition[1]*squarePixels, nextPosition[0]*squarePixels, nextPosition[1]*squarePixels);
  currentPosition[0] = nextPosition[0];   
  currentPosition[1] = nextPosition[1];   
  }    
 private boolean randomChooseNext() { 
  int dirsAvailable = 4;   
  int newX, newY;    
  int dir;   
  while (dirsAvailable>0) {     
   newX = currentPosition[0];    
   newY = currentPosition[1];    
   dir = random.nextInt(4);    
   switch(dir) {    
   case 0: // left     
    newX--;break;    
   case 1: // top    
    newY--; break;    
   case 2: // right     
    newX++; break;    
   case 3: // bottom    
    newY++; break;    
   }     
   if (newX <0 || newY <0 || newX>size-1 || newY>size-1 || pointsOccupied[newX][newY])   
    dirsAvailable--;     
   else {      
    nextPosition[0] = newX;    
    nextPosition[1] = newY;    
    pointsOccupied[newX][newY] = true;   
    return true;     
    }    
   }    
  return false; 
  }      
 public static void main(String[] args) {   
  String input;    
  int size, squarePixels;   
  while (true) {    
   input = JOptionPane.showInputDialog("Please tell me the size of the grid you want to create (between 10 to 100):");    
   try {     
    size = Integer.parseInt(input); 
    if (size  < 10 || size > 100)
     throw new Exception();    
    break;     
   } catch (Exception e) {   
    JOptionPane.showMessageDialog(null, "Sorry, input unacceptable. Please try again.");   
   }   
  }    
  while (true) {   
   input = JOptionPane.showInputDialog("Now please tell me the number of pixels of each grid square (between 5 to 10):");    
   try {     
    squarePixels = Integer.parseInt(input);     
    if (squarePixels  < 5 || squarePixels > 10)
     throw new Exception();   
    break;
    } catch (Exception e) {
     JOptionPane.showMessageDialog(null, "Sorry, input unacceptable. Please try again.");    
    }
    }
  Testsync frame = new Testsync(size, squarePixels); 
  frame.draw();
  }
 }