App doesn't work with ViewHolder

Question

I have this problem with my ImageAdapter in Android Studio. I have huge performance problems when I start the application on a phone. So, people suggested, I should use a ViewHolder. I looked at dozens of examples on how others did it. tried lot's of versions. Still, my app crashes when I try to open that activity. Here's my code:

public class ImageAdap extends BaseAdapter {
private Context mContext;

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

public int getCount() {
    return mThumbIds.length;
}

public Object getItem(int position) {
    return null; //mThumbIds[position];
}

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

static class ViewHolder {
    ImageView image;
}

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

    View vi = convertView;
    ViewHolder holder;

    if(convertView == null) {
        holder = new ViewHolder();
        holder.image = new ImageView(mContext);
        vi.setTag(holder);        //Here is the point the app crashes
    }
    else  {
        holder = (ViewHolder) vi.getTag();
    }

    holder.image.setLayoutParams(new GridView.LayoutParams(800, 600));
    holder.image.setScaleType(ImageView.ScaleType.CENTER_CROP);
    holder.image.setPadding(8, 8, 8, 8);

    holder.image.setImageResource(mThumbIds[position]);

    return vi;
}

// references to my images
private Integer[] mThumbIds = {
        R.drawable.sampleda_01,
        R.drawable.sampleda_02,
        R.drawable.sampleda_03,
        R.drawable.sampleda_04,
        R.drawable.sampleda_05
}; }

The program complies without any problem. But it crashes, when I open the activity. So I used debugger to see, when the app crashes. The app crashes, when

vi.setTag(holder); 

should be performed. I appreciate every kind of help.

logcat:

--------- beginning of crash 07-21 13:42:19.229 3059-3059/com.b.a.test E/AndroidRuntime: FATAL EXCEPTION: main Process: com.b.a.test, PID: 3059 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setTag(java.lang.Object)' on a null object reference at com.b.a.test.ImageAdap.getView(ImageAdap.java:59) at android.widget.AbsListView.obtainView(AbsListView.java:2346) at android.widget.GridView.onMeasure(GridView.java:1065) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465) at android.widget.LinearLayout.measureVertical(LinearLayout.java:748) at android.widget.LinearLayout.onMeasure(LinearLayout.java:630) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465) at android.widget.LinearLayout.measureVertical(LinearLayout.java:748) at android.widget.LinearLayout.onMeasure(LinearLayout.java:630) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643) at android.view.View.measure(View.java:18788) at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100) at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1452) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858) at android.view.Choreographer.doCallbacks(Choreographer.java:670) at android.view.Choreographer.doFrame(Choreographer.java:606) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 07-21 13:42:21.620 3059-3059/com.b.a.test I/Process: Sending signal. PID: 3059 SIG: 9 07-21 13:42:22.939 3315-3315/com.b.a.test W/System: ClassLoader referenced unknown path: /data/app/com.b.a.test-1/lib/x86 07-21 13:42:23.815 3315-3315/com.b.a.test W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable 07-21 13:42:23.971 3315-3348/com.b.a.test D/OpenGLRenderer: Use EGL_SWAP_BEHAVIOR_PRESERVED: true

                                                                       [ 07-21 13:42:24.003  3315: 3315 D/         ]
                                                                       HostConnection::get() New Host Connection established 0xad1b97e0, tid

3315

Answer 1

if(convertView == null) 

You should inflate layout when that assertion is true. Because your view is null. Like that;

LayoutInflater inflater = (LayoutInflater)   getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE); 
view = inflater.inflate(R.layout.mylayout, null);

If you don't want to inflate a layout you can use like that.

View vi = convertView;
ViewHolder holder;

if(convertView == null) {
    vi = new ImageView(mContext);
    holder = new ViewHolder();
    holder.image =(ImageView) vi;
    vi.setTag(holder);        //Here is the point the app crashes
}

Answer 2

your code should look like this

if(convertView == null) {
    holder = new ViewHolder();
    ImageView imageView = new ImageView(mContext);
    vi = imageView;
    holder.image = imageView
    vi.setTag(holder); 
}

You need to set the view first

Answer 3

View vi = convertView;

if(convertView == null) {
    vi.setTag(holder); 
} 

Just check your code carefully. Simply saying you invoke setTag from null, and NullPointerException throws because of that.