Android NullPointerException CipherInputStream to Bitmap

I'm tryin to get encrypted files from assets folder,decrypt it in application and show it as a content in list view.I'm using a well known Fedor's Lazy List Loading implementation.As you can see, I changed his getBitmap method with my own method :

package com.custom.lazylist;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.HashMap;
import java.util.Stack;
import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import com.fedorvlasov.lazylist.R;
import android.app.Activity;
import android.content.Context;
import android.content.res.AssetManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.util.Log;
import android.widget.ImageView;

public class ImageLoader extends Activity {

    //the simplest in-memory cache implementation. This should be replaced with something like SoftReference or BitmapOptions.inPurgeable(since 1.6)
    private HashMap<String, Bitmap> cache=new HashMap<String, Bitmap>();

    private File cacheDir;

    public ImageLoader(Context context){
        //Make the background thead low priority. This way it will not affect the UI performance
        photoLoaderThread.setPriority(Thread.NORM_PRIORITY-1);

        //Find the dir to save cached images
        if (android.os.Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED))
            cacheDir=new File(android.os.Environment.getExternalStorageDirectory(),"LazyList");
        else
            cacheDir=context.getCacheDir();
        if(!cacheDir.exists())
            cacheDir.mkdirs();
    }

    final int stub_id=R.drawable.stub;
    public void DisplayImage(String url, Activity activity, ImageView imageView)
    {
        if(cache.containsKey(url))
            imageView.setImageBitmap(cache.get(url));
        else
        {
            queuePhoto(url, activity, imageView);
            imageView.setImageResource(stub_id);
        }    
    }

    private void queuePhoto(String url, Activity activity, ImageView imageView)
    {
        //This ImageView may be used for other images before. So there may be some old tasks in the queue. We need to discard them. 
        photosQueue.Clean(imageView);
        PhotoToLoad p=new PhotoToLoad(url, imageView);
        synchronized(photosQueue.photosToLoad){
            photosQueue.photosToLoad.push(p);
            photosQueue.photosToLoad.notifyAll();
        }

        //start thread if it's not started yet
        if(photoLoaderThread.getState()==Thread.State.NEW)
            photoLoaderThread.start();
    }


    //MY METHOD
private Bitmap getBitmap(String src) {
    Bitmap myBitmap = null;
        //Decryption
        try {
        Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
        SecretKeySpec keySpec = new SecretKeySpec("01234567890abcde".getBytes(), "AES");
        IvParameterSpec ivSpec = new IvParameterSpec("fedcba9876543210".getBytes());
        cipher.init(Cipher.DECRYPT_MODE, keySpec, ivSpec);

        AssetManager is = this.getAssets();        
        InputStream input = is.open(src); //open file in asset manager
        CipherInputStream cis = new CipherInputStream(input, cipher);

        myBitmap = BitmapFactory.decodeStream(cis);

        }
        catch(Exception e){
            e.printStackTrace();
            Log.v("ERROR","Error : "+e);
        }


        return myBitmap;
    }



    //Task for the queue
    private class PhotoToLoad
    {
        public String url;
        public ImageView imageView;
        public PhotoToLoad(String u, ImageView i){
            url=u; 
            imageView=i;
        }
    }

    PhotosQueue photosQueue=new PhotosQueue();

    public void stopThread()
    {
        photoLoaderThread.interrupt();
    }

    //stores list of photos to download
    class PhotosQueue
    {
        private Stack<PhotoToLoad> photosToLoad=new Stack<PhotoToLoad>();

        //removes all instances of this ImageView
        public void Clean(ImageView image)
        {
            for(int j=0 ;j<photosToLoad.size();){
                if(photosToLoad.get(j).imageView==image)
                    photosToLoad.remove(j);
                else
                    ++j;
            }
        }
    }

    class PhotosLoader extends Thread {
        public void run() {
            try {
                while(true)
                {
                    //thread waits until there are any images to load in the queue
                    if(photosQueue.photosToLoad.size()==0)
                        synchronized(photosQueue.photosToLoad){
                            photosQueue.photosToLoad.wait();
                        }
                    if(photosQueue.photosToLoad.size()!=0)
                    {
                        PhotoToLoad photoToLoad;
                        synchronized(photosQueue.photosToLoad){
                            photoToLoad=photosQueue.photosToLoad.pop();
                        }
                        Bitmap bmp=getBitmap(photoToLoad.url);
                        cache.put(photoToLoad.url, bmp);
                        Object tag=photoToLoad.imageView.getTag();
                        if(tag!=null && ((String)tag).equals(photoToLoad.url)){
                            BitmapDisplayer bd=new BitmapDisplayer(bmp, photoToLoad.imageView);
                            Activity a=(Activity)photoToLoad.imageView.getContext();
                            a.runOnUiThread(bd);
                        }
                    }
                    if(Thread.interrupted())
                        break;
                }
            } catch (InterruptedException e) {
                //allow thread to exit
            }
        开发者_运维技巧}
    }

    PhotosLoader photoLoaderThread=new PhotosLoader();

    //Used to display bitmap in the UI thread
    class BitmapDisplayer implements Runnable
    {
        Bitmap bitmap;
        ImageView imageView;
        public BitmapDisplayer(Bitmap b, ImageView i){bitmap=b;imageView=i;}
        public void run()
        {
            if(bitmap!=null)
                imageView.setImageBitmap(bitmap);
            else
                imageView.setImageResource(stub_id);
        }
    }

    public void clearCache() {
        //clear memory cache
        cache.clear();

        //clear SD cache
        File[] files=cacheDir.listFiles();
        for(File f:files)
            f.delete();
    }  

}

.....but the problem is that I'm getting this Exception :

08-11 11:11:45.293: WARN/System.err(1073): java.lang.NullPointerException
08-11 11:11:45.293: WARN/System.err(1073):     at android.content.ContextWrapper.getAssets(ContextWrapper.java:74)
08-11 11:11:45.293: WARN/System.err(1073):     at com.custom.lazylist.ImageLoader.getBitmap(ImageLoader.java:79)
08-11 11:11:45.293: WARN/System.err(1073):     at com.custom.lazylist.ImageLoader.access$0(ImageLoader.java:70)
08-11 11:11:45.293: WARN/System.err(1073):     at com.custom.lazylist.ImageLoader$PhotosLoader.run(ImageLoader.java:200)

this is how I use LazyAdapter in my main class :

 public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        list=(ListView)findViewById(R.id.list);
        adapter=new LazyAdapter(this, mStrings);
        list.setAdapter(adapter);

        list.setOnItemClickListener(new OnItemClickListener() {
            public void onItemClick(AdapterView<?> a, View v, int position, long id) 
            {
                Intent i = new Intent(v.getContext(), Cards.class);
                startActivity(i);

            }
        });

        Button btn = (Button) findViewById(R.id.btn);
        btn.setOnClickListener(listener);

/*          adapter.imageLoader.clearCache();
            adapter.notifyDataSetChanged();*/

        }

    @Override
    public void onDestroy()
    {
        adapter.imageLoader.stopThread();
        list.setAdapter(null);
        super.onDestroy();
    }



    public OnClickListener listener=new OnClickListener(){
        @Override
        public void onClick(View arg0) {
            adapter.imageLoader.clearCache();
            adapter.notifyDataSetChanged();
        }
    };

    private String[] mStrings={
            "card1_encrypted.jpg",
            "card1_encrypted.jpg",
            "card2_encrypted.jpg",
            "card1_encrypted.jpg",
            "card3_encrypted.jpg",
            "card1_encrypted.jpg",
            "card1_encrypted.jpg",
            "card3_encrypted.jpg",
            "card3_encrypted.jpg",
            "card2_encrypted.jpg",
            "card4_encrypted.jpg",
            "card5_encrypted.jpg",
            "card3_encrypted.jpg",
            "card2_encrypted.jpg",
            "card5_encrypted.jpg",
            "card1_encrypted.jpg"

    };

So any ideas how to fix that?An last thing: at line 79 I have this:

AssetManager is = this.getAssets();   

Thanks in advance!

If the first class is not really an activity, doen't extend activity just to use this.getAssets(), pass the context (i.e. your activity) when instantiating the class, and use this instance to call getAssets()

You need an onCreate() method.

  @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        AssetManager am = getAssets();//this should not get a nullpointerexception
  }

Or do the following:

In your Activity (whichever one calls image loader) write this:

ImageLoader il = new ImageLoader(this);//this being the context.

Then in your ImageLoader class you can use

AssetManager am = context.getAssets();//context being the activity which called this class.