请教多线程问题！

 //MultiMedianImage.java
/* 程序说明：用多线程实现对图像的中值滤波调用MedianThread.java
作 者：lai
*/import java.awt.*;
import java.awt.image.*;
import java.awt.event.*;
import javax.swing.*;
 class  MultiMedianImage extends Frame 
{
Image im,tmp;
int iw,ih;
int[] pixels;
boolean flag=false;

//构造方法
public MultiMedianImage(){
super("中值滤波");
Panel pdown;
Button load,run,save,quit;
//添加窗口监听事件
addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent e){
System.exit(0);
}
});

pdown = new Panel();
pdown.setBackground(Color.lightGray);

load=new Button("装载图像");
run = new Button("中值滤波");
save=new Button("保存");
quit=new Button("退出");

this.add(pdown,BorderLayout.SOUTH);

pdown.add(load);
pdown.add(run);
pdown.add(save);
pdown.add(quit);

load.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
jLoad_ActionPerformed(e);
}
});

run.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
jRun_ActionPerformed(e);
}
});

quit.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
jQuit_ActionPerformed(e);
}
});
save.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
jSave_ActionPerformed(e);
}
});
}

public void jLoad_ActionPerformed(ActionEvent e){
//利用MediaTracker跟踪图像的加载
MediaTracker tracker = new MediaTracker(this);
im=Toolkit.getDefaultToolkit().getImage("001.jpg");
tracker.addImage(im,0);

//等待图像的完全加载
try{
tracker.waitForID(0);
}catch(InterruptedException e2){ e2.printStackTrace();}

//获取图像的宽度iw和高度ih
iw=im.getWidth(this);
ih=im.getHeight(this);
pixels=new int[iw*ih];

try{
PixelGrabber pg=new PixelGrabber(im,0,0,iw,ih,pixels,0,iw);
pg.grabPixels();
}catch (InterruptedException e3) {
e3.printStackTrace();
}

//将数组中的象素产生一个图像
ImageProducer ip=new MemoryImageSource(iw,ih,pixels,0,iw);
tmp=createImage(ip);
flag=true;
repaint();
}

public  void jRun_ActionPerformed(ActionEvent e){
if(flag){
   //调用MedianThread.java进行多线程中值滤波
    MedianThread mt=new MedianThread(iw,ih,im,pixels);    
    for(int i=1;i<ih;i++)
{
Thread thread=new Thread(mt);
thread.start();
}
pixels=mt.GetPixels();
//System.out.println(pixels.length);       
    //将数组中的象素产生一个图像
ImageProducer ip=new MemoryImageSource(iw,ih,pixels,0,iw);
tmp=createImage(ip);
flag=true;
repaint();
   }else
   {
   JOptionPane.showMessageDialog(null,"请先打开一幅图片!",
                         "Alert",JOptionPane.WARNING_MESSAGE);
}
  
 }
  
public void jSave_ActionPerformed(ActionEvent e){
//Save Image
if(flag){ }
else{
 JOptionPane.showMessageDialog(null,"请先打开一幅图片!",
                         "Alert",JOptionPane.WARNING_MESSAGE);
}
}

public void jQuit_ActionPerformed(ActionEvent e){
//System.exit(0);
JOptionPane op =new JOptionPane();
int exit=op.showConfirmDialog(this,"你要退出吗? ? ?","退出",JOptionPane.YES_NO_OPTION);

if(exit==JOptionPane.YES_OPTION)
{
System.exit(0);

}else{ }
} //调用paint()方法，显示图像信息。
public void paint(Graphics g){
if(flag){
g.drawImage(tmp,10,20,this);
}else { }
} public static void main(String[] args)
{
MultiMedianImage mi = new MultiMedianImage();
mi.setLocation(50,50);
mi.setSize(500,400);
mi.setVisible(true);
}
}  class MedianThread implements Runnable
{


int  pixels[];
int iw,ih,i;
Image im;

public MedianThread(int iw,int ih,Image im,int[] pixels)//final int i
{
//this.i=i;
this.iw=iw;
this.ih=ih;
this.im=im;
this.pixels=pixels;
}

public void run()
{


try{
PixelGrabber pg=new PixelGrabber(im,0,0,iw,ih,pixels,0,iw);
pg.grabPixels();
}catch (InterruptedException e3) {
e3.printStackTrace();
}
//对图像进行中值滤波，Alpha值保持不变
ColorModel cm=ColorModel.getRGBdefault();
                    

//++i;
synchronized(this) 
{
if(i<ih)
                    {
                   for(int j=1;j<iw-1;j++)
                   {
                     int red,green,blue;
                     int alpha=cm.getAlpha(pixels[i*iw+j]);

                     //int red2=cm.getRed(pixels[(i-1)*iw+j]);
                     int red4=cm.getRed(pixels[i*iw+j-1]);
                     int red5=cm.getRed(pixels[i*iw+j]);
                     int red6=cm.getRed(pixels[i*iw+j+1]);
                     //int red8=cm.getRed(pixels[(i+1)*iw+j]);

                     //水平方向进行中值滤波
                     if(red4>=red5){
                     if(red5>=red6) {red=red5;}
                     else{
                     if(red4>=red6) {red=red6;}
                     else{red=red4;}
                      }}
                     else{
                        if(red4>red6) {red=red4;}
                    else{
                    if(red5>red6) {red=red6;}
                    else{red=red5;}
                     }}

                     //int green2=cm.getGreen(pixels[(i-1)*iw+j]);
                     int green4=cm.getGreen(pixels[i*iw+j-1]);
                     int green5=cm.getGreen(pixels[i*iw+j]);
                     int green6=cm.getGreen(pixels[i*iw+j+1]);
                     //int green8=cm.getGreen(pixels[(i+1)*iw+j]);

                     //水平方向进行中值滤波
if(green4>=green5){
                    if(green5>=green6) {green=green5;}
                    else{
                    if(green4>=green6) {green=green6;}
                    else{green=green4;}
                    }}
                    else{
                        if(green4>green6) {green=green4;}
                    else{
                    if(green5>green6) {green=green6;}
                    else{green=green5;}
                    }}


                    //int blue2=cm.getBlue(pixels[(i-1)*iw+j]);
                    int blue4=cm.getBlue(pixels[i*iw+j-1]);
                    int blue5=cm.getBlue(pixels[i*iw+j]);
                    int blue6=cm.getBlue(pixels[i*iw+j+1]);
                   // int blue8=cm.getBlue(pixels[(i+1)*iw+j]);

                    //水平方向进行中值滤波
                    if(blue4>=blue5){
                    if(blue5>=blue6) {blue=blue5;}
                    else{
                    if(blue4>=blue6) {blue=blue6;}
                    else{blue=blue4;}
                    }}
                    else{
                        if(blue4>blue6) {blue=blue4;}
                    else{
                    if(blue5>blue6) {blue=blue6;}
                    else{blue=blue5;}
                    }}
                  pixels[i*iw+j]=alpha<<24|red<<16|green<<8|blue;
                  //System.out.println(pixels[i*iw+j]);
               }
                }
     }

 }
public int[] GetPixels() 
{
return pixels;
}
}
这是代码，我想用多线程来实现图像的中值滤波，即一个线程来处理一行像素，并行计算。
请各位大侠给帮忙改改这个程序！