程序运行之后出错

小弟我最近在学习编写拖动图标的程序，现在遇到的问题时：下面的代码可以在模拟器上面跑起来，但是当我向左拖动到第四幅图片是，就会出现如下错误： ERROR/AndroidRuntime(600): FATAL EXCEPTION: main， ERROR/AndroidRuntime(600): java.lang.NullPointerException，希望哪位高手能够指点一下小弟
代码如下：package com.sky;import android.app.Activity;
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Matrix;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.BaseAdapter;
import android.widget.Gallery;
import android.widget.ImageView;
import android.widget.AdapterView.OnItemSelectedListener;public class RecialGalleryActivity extends Activity implements OnItemSelectedListener{
 private Gallery gallery;
  //  private ImageSwitcher imageSwitcher;
    private ImageAdapter imageAdapter;     private int[] resIds = new int[]
    { R.drawable.img1, R.drawable.img2, R.drawable.img3,
    
    R.drawable.img4,
            R.drawable.img5, R.drawable.img6, R.drawable.img7,
            R.drawable.img8 };     public class ImageAdapter extends BaseAdapter
    {
        int mGalleryItemBackground;
        private Context mContext;         public ImageAdapter(Context context)
        {
            mContext = context;
            TypedArray typedArray = obtainStyledAttributes(R.styleable.Gallery);
            mGalleryItemBackground = typedArray.getResourceId(
                    R.styleable.Gallery_android_galleryItemBackground, 0);                        
        }
        // 第1点改进，返回一个很大的值，例如，Integer.MAX_VALUE
        public int getCount()
        {
            return resIds.length;
        }         public Object getItem(int position)
        {
            return position;
        }         public long getItemId(int position)
        {
            return position;
        }
 
        public View getView(int position, View convertView, ViewGroup parent)
        {
            ImageView imageView = new ImageView(mContext);
                // 第2点改进，通过取余来循环取得resIds数组中的图像资源ID
            imageView.setImageResource(resIds[position]);
            imageView.setLayoutParams(new Gallery.LayoutParams(200, 200));
            // 设置显示比例类型
            imageView.setScaleType(ImageView.ScaleType.FIT_CENTER);
            imageView.setBackgroundResource(mGalleryItemBackground);
            return imageView;
        }
    }
    public void onItemSelected(AdapterView<?> parent, View view, int position,long id)
    {
        // 选中Gallery中某个图像时，在ImageSwitcher组件中放大显示该图像
                //我估计问题出现在这个函数中，但是自己试了一些改动仍然不成功，哪位高手能够指点一下
       //imageSwitcher.setImageResource(resIds[position]);
/*   view.setLayoutParams(new Gallery.LayoutParams(300,300));
   int mCounts = gallery.getCount() - 1;
   
   if((position>0) && (position < mCounts)){
   gallery.getChildAt(position - 1).setLayoutParams(new Gallery.LayoutParams(200, 200));
   gallery.getChildAt(position + 1).setLayoutParams(new Gallery.LayoutParams(200, 200));
                                   }
   if(position == 0){
   gallery.getChildAt(position + 1).setLayoutParams(new Gallery.LayoutParams(200, 200));
                                   }
   if(position == mCounts){
   gallery.getChildAt(position - 1).setLayoutParams(new Gallery.LayoutParams(200, 200));
                                   }**/
     gallery.getChildAt(position).setLayoutParams(new Gallery.LayoutParams(300,300));
        int mCounts = 2*gallery.getCount() - 1;
   
   if((position>0) && (position < mCounts)){
   gallery.getChildAt(position - 1).setLayoutParams(new Gallery.LayoutParams(200, 200));
   gallery.getChildAt(position + 1).setLayoutParams(new Gallery.LayoutParams(200, 200));
                                   }
   if(position == 0){
   gallery.getChildAt(position + 1).setLayoutParams(new Gallery.LayoutParams(200, 200));
                                   }
   if(position == mCounts){
   gallery.getChildAt(position - 1).setLayoutParams(new Gallery.LayoutParams(200, 200));
                                   }
    
    
    }
    public void onNothingSelected(AdapterView<?> parent)
    {
    
    }   /**  @Override
    // ImageSwitcher组件需要这个方法来创建一个View对象（一般为ImageView对象）
    //  来显示图像
  public View makeView()
    {
        ImageView imageView = new ImageView(this);
        imageView.setBackgroundColor(0xFF000000);
        imageView.setScaleType(ImageView.ScaleType.FIT_CENTER);
        imageView.setLayoutParams(new ImageSwitcher.LayoutParams(
                LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT));
        return imageView;
    }
   **/
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        gallery = (Gallery) findViewById(R.id.gallery);
        imageAdapter = new ImageAdapter(this);
        gallery.setAdapter(imageAdapter);
        gallery.setOnItemSelectedListener(this);
       // imageSwitcher = (ImageSwitcher) findViewById(R.id.imageswitcher);
        // 设置ImageSwitcher组件的工厂对象
       // imageSwitcher.setFactory(this);
        // 设置ImageSwitcher组件显示图像的动画效果
   // imageSwitcher.setInAnimation(AnimationUtils.loadAnimation(this,
            //    android.R.anim.fade_in));    
        //  imageSwitcher.setOutAnimation(AnimationUtils.loadAnimation(this,
              //  android.R.anim.fade_out));     }
}