Interface observer pattern: null object reference

Question

Not sure what's wrong with my observer pattern:

I have defined my interface as:

public interface OnBackFilterPressListener {
    public ArrayList<FoodType> filterFoodType ();
}

When I press the back button, I want the listener to be activated - all this code is in my activity class:

        back.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                    onBackFilterPressListener.filterFoodType();
}
}

In my adapter class, I have the following code:

public class RandomRecyclerViewAdapter  extends RecyclerView.Adapter<RecyclerView.ViewHolder> implements SearchActivity.OnBackFilterPressListener {

       @Override
    public ArrayList<FoodType> filterFoodType() {
        return foodTypes;
    }

}

When I press the back button in my activity, I just want the array of data in my adapter to be returned in my activity class.

But I'm getting this error:

java.lang.NullPointerException: Attempt to invoke interface method 'java.util.ArrayList com.example.simon..SearchActivity$OnBackFilterPressListener.filterFoodType()' on a null object reference

foodType definitely exists - the adapter loads the data into activity and I can see it being displayed on the android emulator. I get this error when I click the back button.

Answer 1

I got it to work by following this thread and being a little creative:

How to create our own Listener interface in android?

I wrote a custom object to prevent the null object reference as to use the interface, I would first have to create the object:

public class BackFilterEvent implements Serializable {

    private OnEventListener mOnEventListener;

    public void setOnEventListener(OnEventListener listener) {
        mOnEventListener = listener;
    }

    public ArrayList<FoodType> doEvent() {
        if (mOnEventListener != null) {
            return mOnEventListener.onEvent(); 
        }
        return null;
    }

    public interface OnEventListener {
        ArrayList<FoodType> onEvent();
    }

}

Then in my Activity class, I created the object:

BackFilterEvent backFilterEvent = new BackFilterEvent();

And I pass it as far as it needs to go in my code. In my case, I had an activity that feed a fragment with an recyclerview adapter within the fragment. I passed this object until it got to the recyclerview adapter.

Passing object from Activity to Fragment:

            FilterSearchFragment filterSearchFragment = new FilterSearchFragment();
            Bundle bundle = new Bundle();
            bundle.putSerializable("listener", backFilterEvent);
            filterSearchFragment.setArguments(bundle);
            getSupportFragmentManager().beginTransaction()
                    .replace(R.id.fragment_container, filterSearchFragment)
                    .commit();

Passing from Fragment to Adapter:

    Bundle bundle = getArguments();
    BackFilterEvent backFilterEvent = (BackFilterEvent) bundle.getSerializable("listener");
    RandomRecyclerViewAdapter randomRecyclerViewAdapter  = new RandomRecyclerViewAdapter(getContext(), backFilterEvent);

In my adapter constructor, I set it to do the work once I call it from the activity class:

    this.backFilterEvent = backFilterEvent;
    backFilterEvent.setOnEventListener(new BackFilterEvent.OnEventListener() {
        @Override
        public ArrayList<FoodType> onEvent() {
            return foodTypes;
        }
    });

And in my back button in my activity class, I call the listener, which will trigger the work in the adapter class:

    back.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

                ArrayList<FoodType> foodTypes = backFilterEvent.doEvent(); }