将变量从Custom类传递到Fragment（Android）

Question

I'm currently working with android for my project and I can't figure out the way to passing variable from Custom class to Fragment. I can exchange variable between MainActivity and Fragment but I can't use the same way to do it in Custom class and Fragment.

The way that I use to send variable between MainActivity and Fragment is something like this

In Fragment file:

 public class FourthFragment extends Fragment { MainActivity activity; @Override public void onAttach(Context context) { super.onAttach(context); activity = (MainActivity) getActivity(); } 

and after that I can use any methods from MainActivity in my Fragment.It's work fine! So I try to do the same way to passing variable from Custom class to Fragment but it's didn't work out.

The code that I tried. This is my Custom Class (MyAdapter.java)

 public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> { MainActivity activity; FourthFragment fourthfragment = new FourthFragment(); private ArrayList<CreateList> galleryList; private Context context; public MyAdapter(Context context, ArrayList<CreateList> galleryList) { this.galleryList = galleryList; this.context = context; } @Override public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) { View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.cell_layout, viewGroup, false); return new ViewHolder(view); } @Override public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, final int i) { viewHolder.title.setText(galleryList.get(i).getImage_title()); viewHolder.img.setScaleType(ImageView.ScaleType.CENTER_CROP); viewHolder.img.setImageResource((galleryList.get(i).getImage_ID())); viewHolder.img.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { AlertDialog.Builder builder1 = new AlertDialog.Builder(context); builder1.setMessage("Are you sure you want to send this "+ galleryList.get(i).getImage_title()); builder1.setPositiveButton("Yes", new DialogInterface.OnClickListener() { @Override public void onClick(DialogInterface dialog, int which) { Toast.makeText(context,"Yes",Toast.LENGTH_SHORT).show(); //TODO: Problem here NULL pointer exception fourthfragment.imgSend(); } }); builder1.setNegativeButton("No", new DialogInterface.OnClickListener() { @Override public void onClick(DialogInterface dialog, int which) { Toast.makeText(context,"No",Toast.LENGTH_SHORT).show(); } }); builder1.show(); Toast.makeText(context,"Image", Toast.LENGTH_SHORT).show(); } }); } @Override public int getItemCount() { return galleryList.size(); } public class ViewHolder extends RecyclerView.ViewHolder{ private TextView title; private ImageView img; public ViewHolder(View view) { super(view); title = (TextView)view.findViewById(R.id.title); img = (ImageView) view.findViewById(R.id.img); } } } 

And This one is my Fragment (FourthFragment.java) I only cut some parts of it that important.

 public class FourthFragment extends Fragment { MainActivity activity; MyAdapter myAdapter; RecyclerView recyclerView; byte[] dataOut2 = {(byte)255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ,0}; @Override public void onAttach(Context context) { super.onAttach(context); activity = (MainActivity) getActivity(); } @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); init(savedInstanceState); if (savedInstanceState != null) onRestoreInstanceState(savedInstanceState); } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View rootView = inflater.inflate(R.layout.fragment_fourth, container, false); initInstances(rootView, savedInstanceState); return rootView; } private void init(Bundle savedInstanceState) { // Init Fragment level's variable(s) here } @SuppressWarnings("UnusedParameters") private void initInstances(View rootView, Bundle savedInstanceState) { // Init 'View' instance(s) with rootView.findViewById here recyclerView = (RecyclerView) rootView.findViewById(R.id.imageGallery); recyclerView.setHasFixedSize(true); RecyclerView.LayoutManager layoutManager = new GridLayoutManager(getContext(),2); recyclerView.setLayoutManager(layoutManager); ArrayList<CreateList> createLists = prepareData(); MyAdapter adapter = new MyAdapter(getContext(), createLists); recyclerView.setAdapter(adapter); } //Below this is a method that I create to recieve variable(i) from Custom Class (MyAdapter.java) public void imgSend(int i) { if( i< 28) { dataOut2[8] = (byte)1; int j = 0; j = i + 1; dataOut2[9] = (byte)j; activity.sendMessage(dataOut2); } else if(i>=28) { dataOut2[8] = (byte)2; int k = 0; k = i - 27; dataOut2[9]= (byte)k; activity.sendMessage(dataOut2); } Toast.makeText(getContext(),""+dataOut2,Toast.LENGTH_SHORT).show(); } 

And when I run all the program its caused a problem like this when I start to click 'Yes' in Dialog that start to do a method fourthFragment.imgSend(i);

 FATAL EXCEPTION: main Process: myapplication.example.com.botcontrol, PID: 22397 java.lang.NullPointerException: Attempt to invoke virtual method 'void myapplication.example.com.botcontrol.MainActivity.sendMessage(byte[])' on a null object reference at myapplication.example.com.botcontrol.Fragment.FourthFragment.imgSend(FourthFragment.java:156) at myapplication.example.com.botcontrol.Fragment.MyAdapter$1$1.onClick(MyAdapter.java:70) at android.support.v7.app.AlertController$ButtonHandler.handleMessage(AlertController.java:157) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:145) at android.app.ActivityThread.main(ActivityThread.java:6939) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199) 

So what can I do or any other way to send the variable? If anything else is needed, I will post it! Thank you

Answer 1

It's because you cast the context to MainActivity, you need to be more general in this case. Use just Activity for the variable. And check the type before casting it.

Activity activity;

...

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    if(getActivity() instanceOf MainActivity){
        activity = (MainActivity) getActivity();
    } else if(getActivity() instanceOf CustomActivity){
        activity = (CustomActivity) getActivity();
    }
}

Answer 2

Your activity variable is null. Have you committed the fragment? I don't see a place in your code that you do that.

A fragment is committed and attached to its activity using the FragmentManager . Typically, you will commit a fragment in Activity.onCreate() , but you can also commit elsewhere depending on your use case.

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if(savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction().add(R.id.my_fragment_container, new FourthFragment()).commitNow();
    }

}

Once your Fragment is committed and attached, you shouldn't get a null pointer exception anymore.

For reference, here's the documentation related to attaching fragments