NullPointerException when downloading images for ImageView in android

Question

I am attempting to brush up on my android for a project at work, and it seams that im more out of touch than i first thought.

I am creating an app that uploads pictures to a remote server and then shows these uploads as thumbnails.

The section i am struggling is with downloading the image and applying it to the image view within a list view.

Im receiving a Null Pointer Exception which is never nice.

Im not sure if this is due to me starting a number of ASync tasks (one for each image) or if its something more obvious

Stack Trace

Process: com.example.alex.documentupload, PID: 5788
java.lang.NullPointerException
        at com.example.alex.documentupload.DownloadImageTask.onPostExecute(DownloadImage.java:35)
        at com.example.alex.documentupload.DownloadImageTask.onPostExecute(DownloadImage.java:14)
        at android.os.AsyncTask.finish(AsyncTask.java:632)
        at android.os.AsyncTask.access$600(AsyncTask.java:177)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:146)
        at android.app.ActivityThread.main(ActivityThread.java:5748)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
        at dalvik.system.NativeStart.main(Native Method)

Android Code Show images Class

package com.example.alex.documentupload;

import java.util.ArrayList;
import java.util.HashMap;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;
import android.widget.Toast;

import com.example.alex.documentupload.JSONParser;
import com.example.alex.documentupload.DownloadImageTask;

public class ShowImages extends Activity {
    ListView list;
    TextView ver;
    TextView name;
    TextView api;
    ImageView img;

    Button Btngetdata;
    ArrayList<HashMap<String, String>> oslist = new ArrayList<HashMap<String, String>>();

    //URL to get JSON Array
    private static String url = "http://www.500kgiveaway.co.uk/getimagesmob.php";

    //JSON Node Names
    private static final String TAG_PATH = "path";

    JSONArray android = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_show_images);
        oslist = new ArrayList<HashMap<String, String>>();

        new JSONParse().execute();

        Btngetdata = (Button)findViewById(R.id.getdata);
        Btngetdata.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                // clear the list before adding more

                //update the list
                new JSONParse().execute();

            }
        });

    }

    private class JSONParse extends AsyncTask<String, String, JSONArray> {
        private ProgressDialog pDialog;
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            ver = (TextView)findViewById(R.id.vers);
            name = (TextView)findViewById(R.id.name);
            api = (TextView)findViewById(R.id.api);
            img = (ImageView)findViewById(R.id.img);


            pDialog = new ProgressDialog(ShowImages.this);
            pDialog.setMessage("Getting Data ...");
            pDialog.setIndeterminate(false);
            pDialog.setCancelable(true);
            pDialog.show();

        }

        @Override
        protected JSONArray doInBackground(String... args) {

            JSONParser jParser = new JSONParser();

            // Getting JSON from URL
            JSONArray json = jParser.getJSONFromUrl(url);
            return json;
        }
        @Override
        protected void onPostExecute(JSONArray json) {
            pDialog.dismiss();
            try {
                // Getting JSON Array from URL


               android = json;


                for(int i = 0 ; i < android.length(); i++){

                    JSONObject c = android.getJSONObject(i);

                    // Storing  JSON item in a Variable
                    String path = c.getString(TAG_PATH);

                    // Adding value HashMap key => value

                    HashMap<String, String> map = new HashMap<String, String>();

                    map.put(TAG_PATH, path);

                    oslist.add(map);
                    list=(ListView)findViewById(R.id.list);



                    ListAdapter adapter = new SimpleAdapter(ShowImages.this, oslist,
                            R.layout.list_v,
                            new String[] { TAG_PATH }, new int[] {
                            R.id.vers});

                    list.setAdapter(adapter);

                    new DownloadImageTask((ImageView) list.findViewById(R.id.img))
                            .execute("http://www.500kgiveaway.co.uk/" + path);

//                    list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
//
//                        @Override
//                        public void onItemClick(AdapterView<?> parent, View view,
//                                                int position, long id) {
//                            Toast.makeText(ShowImages.this, "You Clicked at "+oslist.get(+position).get("name"), Toast.LENGTH_SHORT).show();


//                        }
//                    });

                }
            } catch (JSONException e) {
                e.printStackTrace();
            }

        }
    }

    public void loadcamera(View view) {
        // Do something in response to button

        Intent myIntent = new Intent(ShowImages.this, MainActivity.class);
        myIntent.putExtra("dir", "BS"); //Optional parameters
        ShowImages.this.startActivity(myIntent);

    }

}

DownloadImages Class

package com.example.alex.documentupload;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.util.Log;
import android.widget.ImageView;

import java.io.InputStream;

/**
 * Created by Alex on 03/05/2015.
 */
 class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
    ImageView bmImage;

    public DownloadImageTask(ImageView bmImage) {
        this.bmImage = bmImage;
    }

    protected Bitmap doInBackground(String... urls) {
        String urldisplay = urls[0];
        Bitmap mIcon11 = null;
        try {
            InputStream in = new java.net.URL(urldisplay).openStream();
            mIcon11 = BitmapFactory.decodeStream(in);
        } catch (Exception e) {
            Log.e("Error", e.getMessage());
            e.printStackTrace();
        }
        return mIcon11;
    }

    protected void onPostExecute(Bitmap result) {
        bmImage.setImageBitmap(result);
    }
}

Answer 1

This turned out to be a bit of a tricky fix, but got there in the end.

The issue was that i was unable to reference the imageview object inside the listview object whilst using a SimpleAdaptor.

The answer is that you need to create a custom class that extends SimpleAdaptor

I ended up using the picasso library, not because it helped fix the problem, more that it offered some image processing features, like resize etc which i had not previously allowed for.

I followed this answer Android - How can diplay pictures in a simpleApapter list view

However i run in to additional issues with context.

Here is my custom class that extends SimpleAdaptor

package com.example.alex.documentupload;

import android.content.Context;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.SimpleAdapter;

import com.squareup.picasso.Picasso;

import java.util.List;
import java.util.Map;

public class ExtendedAdaptor extends SimpleAdapter {

    public static Context NewContext;

    public ExtendedAdaptor(Context context, List<? extends Map<String, ?>> data, int     resource, String[] from, int[] to){
        super(context, data, resource, from, to);
        NewContext = context;
    }

    public View getView(int position, View convertView, ViewGroup parent){


        // here you let SimpleAdapter built the view normally.
        View v = super.getView(position, convertView, parent);

        // Then we get reference for Picasso
        ImageView img = (ImageView) v.getTag();
        if(img == null){
            img = (ImageView) v.findViewById(R.id.img);
            v.setTag(img); // <<< THIS LINE !!!!
        }
        // get the url from the data you passed to the `Map`
        String url = ((Map)getItem(position)).get("path").toString();
        // do Picasso
        // maybe you could do that by using many ways to start



        Picasso.with(NewContext).load(url)
                .resize(100, 100).into(img);

        // return the view
        return v;
    }
}

and here is how i called it

            ListView list = (ListView) findViewById(R.id.list);

            ListAdapter adapter =
                    new ExtendedAdaptor(
                            context,
                            oslist,
                            R.layout.list_v,
                            new String[]{TAG_PATH},
                            new int[]{R.id.vers});

            list.setAdapter(adapter);

Its all works pretty well, the image cache and image only download if they change.

I hope this helps someone else in the future

Thanks to all that contributed