AsyncTask to update adapter UI

Question

im pretty new to Android and AsyncTask and was hoping you could help...Im trying to refresh an ImageView adapter so it updates on the UI... i was told to use notifyDataSetChanged() but i just can get it to work...i have setup an asynctask but the only result i get is a NullPointerException ...i need the update to happen when a new element is added to my nextColorArray ....Please look at my code, i seriously dont even know if im using my asynctask in the right manner or if im even close?!..cheers

public class GameScreen extends Activity{
int yellow = 0xffffff66;
int green = 0xff00EE76;
int red = 0xffff4342;
int blue = 0xff42c3ff;
int purple = 0xff9932CC;
int white = 0xFFFFFFFF;

int total_Count = 0;
int colorPosition = 0;//nextColor position
int colorPickerStart = 0;
ArrayList<Integer>nextColorArray;
ImageView imageView;
ImageAdapter ia;  
private static final String TAG = GameScreen.class.getSimpleName();//log

ArrayList<Integer>colorPicker = new ArrayList<Integer>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.game_screen);
    new upDateNextColor().execute();
}

class upDateNextColor extends AsyncTask<Void, Void, Void> {

    GridView gridview2;

    @Override
    protected Void doInBackground(Void... voids) {
        return null;
    }
    @Override
    protected void onPostExecute(Void result) {
        nextColorArray.add(blue);
        nextColorArray.add(green);
        nextColorArray.add(red);
        nextColorArray.add(yellow);
        nextColorArray.add(purple);
        Collections.shuffle(nextColorArray);
    }

    @Override
    protected void onPreExecute() {
        nextColorArray = new ArrayList<Integer>(10);
        gridview2 = (GridView) findViewById(R.id.colorNext);
        ia = new ImageAdapter(nextColorArray);
        gridview2.setAdapter(ia);
        if(total_count > 10){
        nextColorArray.add(0, white);
        ia.notifyDataSetChanged();
        }           
    }
 }

 public class ImageAdapter extends BaseAdapter {
    private Context mContext;
    private ArrayList aL;

    public ImageAdapter(Context c, ArrayList<Integer>aL) {
        mContext = c;
        this.aL = aL;
    }

    public ImageAdapter(ArrayList<Integer>aL) {
        this.aL = aL;
    }

    public ImageAdapter(Context c){
        mContext = c;
    }

    public int getCount() {

        return 10;
    }

    public Object getItem(int position) {
        return null;
    }

    public long getItemId(int position) {
        return 0;
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        if (convertView == null) {
            imageView = new ImageView(mContext);
            imageView.setLayoutParams(new GridView.LayoutParams(50, 50));
            imageView.setBackgroundColor(nextColorArray.get(colorPosition));
            if(colorPosition < 9) colorPosition++;
        } else {
            imageView = (ImageView) convertView;
        }
        return imageView;
    }
}

Logcat

E/AndroidRuntime: FATAL EXCEPTION: main java.lang.NullPointerException
at android.view.ViewConfiguration.get(ViewConfiguration.java:332)
at android.view.View.<init>(View.java:3254)
at android.widget.ImageView.<init>(ImageView.java:105)

Answer 1

you are getting Null Pointer Exception because :

1. mContext is null in ImageAdapter becuase you are using Single parameter Constructor of ImageAdapter for creating object. so pass Activity Context also as:

ia = new ImageAdapter(GameScreen.this,nextColorArray);

2. use

gridview2 = (GridView)GameScreen.this. findViewById(R.id.colorNext);

for initializing GridView in onPreExecute

Edit: I made these changes in the code.

     return aL.size();   // change this in getCount()
     imageView.setBackgroundColor(nextColorArray.get(position));

FullCode

public class MainActivity extends Activity{
int yellow = 0xffffff66;
int green = 0xff00EE76;
int red = 0xffff4342;
int blue = 0xff42c3ff;
int purple = 0xff9932CC;
int white = 0xFFFFFFFF;

int total_Count = 0;
int colorPosition = 0;//nextColor position
int colorPickerStart = 0;
ArrayList<Integer>nextColorArray;
ImageView imageView;
ImageAdapter ia;  
private static final String TAG = MainActivity.class.getSimpleName();//log

ArrayList<Integer>colorPicker = new ArrayList<Integer>();

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new upDateNextColor().execute();
}

class upDateNextColor extends AsyncTask<Void, Void, Void> {

GridView gridview2;

@Override
protected Void doInBackground(Void... voids) {
    return null;
}
@Override
protected void onPostExecute(Void result) {
    nextColorArray.add(blue);
    nextColorArray.add(green);
    nextColorArray.add(red);
    nextColorArray.add(yellow);
    nextColorArray.add(purple);
    Collections.shuffle(nextColorArray);
}

@Override
protected void onPreExecute() {
    nextColorArray = new ArrayList<Integer>(10);
    gridview2 = (GridView) findViewById(R.id.gridview);
    ia = new ImageAdapter(nextColorArray);
    gridview2.setAdapter(ia);
    nextColorArray.add(0, white);
    ia.notifyDataSetChanged();          
  }
}

public class ImageAdapter extends BaseAdapter {
private Context mContext;
private ArrayList aL;

public ImageAdapter(Context c, ArrayList<Integer>aL) {
    mContext = c;
    this.aL = aL;
}

public ImageAdapter(ArrayList<Integer>aL) {
    this.aL = aL;
}

public ImageAdapter(Context c){
    mContext = c;
}

public int getCount() {
   //int a = 10;
    return aL.size();
}

public Object getItem(int position) {
    return null;
}

public long getItemId(int position) {
    return 0;
}

public View getView(int position, View convertView, ViewGroup parent) {
    if (convertView == null) {
        imageView = new ImageView(MainActivity.this);
        imageView.setLayoutParams(new GridView.LayoutParams(50, 50));
        imageView.setBackgroundColor(nextColorArray.get(position));
        if(colorPosition < 9) colorPosition++;
    } else {
        imageView = (ImageView) convertView;
    }
    return imageView;
 }
}
}

Snap shot