Activity cannot be cast to Interface (ClassCastException)

Question

I'm currently getting a ClassCastException in my android app caused by "activity cannot be cast to interface". Here is my code:

Logcat says the Exception is thrown in the onAttach part in MovieGridFragment on the line "this.clickListener = (clickInterfaceHelper) context;".

My Interface:

public interface clickInterfaceHelper {
void clickOnItem(int id);
void favoriteMovieItem(int movieId); }

The Fragment Class:

public class MovieGridFragment extends Fragment {

public clickInterfaceHelper clickListener;
private int index;
private GridView movieGridView;
public List<movieData> movieDataList = new ArrayList<>();

public MovieGridFragment() {} //empty constructor

@Override
public void onAttach(Context context) {
    this.clickListener = (clickInterfaceHelper) context;
    super.onAttach(context);
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    if(savedInstanceState != null) {
        if (!movieDataList.isEmpty()) {
            movieDataList = Arrays.asList((movieData[]) savedInstanceState.getSerializable("OLDMOVIEDATA"));
        }
    }
    super.onCreate(savedInstanceState);
}

The MainActivity:

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    setHasOptionsMenu(true);


    View rootView = inflater.inflate(R.layout.movie_display_fragment, container, false);

    movieGridView = (GridView) rootView.findViewById(R.id.gv_movie_display);
    movieAdapter adapter = new movieAdapter(getActivity(),movieDataList);
    adapter.notifyDataSetChanged();
    movieGridView.setAdapter(adapter);
    movieGridView.setSelection(index);
    movieGridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            if(clickListener != null)
                    clickListener.clickOnItem(position);

        }
    });
    return rootView;
}

@Override
public void onSaveInstanceState(Bundle outState) {
    index = movieGridView.getFirstVisiblePosition();
    outState.putSerializable("OLDMOVIEDATA",movieData.movieDataArray);
    super.onSaveInstanceState(outState);
}}
and mainactivity:

public class MainActivity extends AppCompatActivity implements clickInterfaceHelper {

public static String sorterString = null;
public static String urlBase = "https://api.themoviedb.org/3/movie/";
public static String urlFinal = null;

RequestQueue requestQueue;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if(savedInstanceState == null) {
                getSupportFragmentManager().beginTransaction()
                .add(R.id.activity_container, new MovieGridFragment())
                .commit();
        movieData.movieDataPosition = 0;
    }
    if(savedInstanceState != null) {
        sorterString = savedInstanceState.getString("SORTER");
    }

    if(savedInstanceState == null)
        movieData.movieDataPosition = 0;

    if(sorterString==null)
        sorterString="popular?";
    if(sorterString!="favorite" && sorterString!=null) {
        if(networkChecker.isNetworkAvailableChecker(this)) {
            movieRequest();
        }
    }



}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main_menu_act, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();

    if(id == R.id.m_popularity_action) {
        if(sorterString != "popular?") {
            sorterString = "popular?";
            if(networkChecker.isNetworkAvailableChecker(this))
            movieRequest();
        }
        return true;
    }

    if(id == R.id.m_action_voter) {
        if(sorterString != "top_rated?") {
            sorterString = "top_rated?";
            if(networkChecker.isNetworkAvailableChecker(this))
            movieRequest();
        }
        return true;
    }

    if(id == R.id.m_favorite_btn) {
        if(sorterString != "favorite") {
            SQLiteOpenHelper helper = new movieDataDbHelper(this);
            SQLiteDatabase database = helper.getReadableDatabase();
            Cursor cursor= database.query(movieDataContract.contractEntry.TABLE_NAME,
                    new String[] {
                            movieDataContract.contractEntry.ID,
                            movieDataContract.contractEntry.IMG_PATH},null,null,null,null,null);
            if(cursor.getCount() == 0) {
                Toast.makeText(this, "there are no favorite movies yet!",Toast.LENGTH_SHORT).show();
            } else {
                sorterString = "favorite";
                showFavoriteFragment();
            }
            database.close();
            helper.close();
            cursor.close();
        }
        return true;
    }
    return super.onOptionsItemSelected(item);
}

public void showFavoriteFragment() {
    favoriteMoviesDetailsFragment fragment = new favoriteMoviesDetailsFragment();
    try {
        getFragmentManager().beginTransaction().replace(R.id.activity_container,fragment).commit();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

@Override
public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
    outState.putString("SORTER", sorterString);
    outState.putInt("POSITION",movieData.movieDataPosition);
    super.onSaveInstanceState(outState, outPersistentState);
}

public void movieRequest() {
        urlFinal = urlBase + sorterString + movieData.apiKey;
        urlFinal.trim();
        requestQueue = Volley.newRequestQueue(this);
        JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.GET, urlFinal, null, new Response.Listener<JSONObject>() {
            @Override
            public void onResponse(JSONObject response) {
                try {
                    JSONArray array = response.getJSONArray("results");
                    movieData.movieDataArray = new movieData[array.length()];

                    for (int i = 0; i < array.length(); i++) {
                        movieData movie = new movieData();
                        JSONObject jsonObject = array.getJSONObject(i);
                        //movie.setPosition(i);
                        movie.setMovieId(jsonObject.getString("id"));
                        movie.setMovieImagePath(jsonObject.getString("poster_path"));
                        movie.setMovieTitle(jsonObject.getString("original_title"));
                        movie.setMoviePlot(jsonObject.getString("overview"));
                        movie.setMovieVoting(jsonObject.getString("vote_average"));
                        movie.setMovieReleaseDate(jsonObject.getString("release_date"));
                        movieData.movieDataArray[i] = movie;
                    }

                    MovieGridFragment gridFragment = new MovieGridFragment();
                    gridFragment.movieDataList = Arrays.asList(movieData.movieDataArray); //hier wird datalist eigentlich zugewiesen
                    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
                    transaction.replace(R.id.activity_container, gridFragment);
                    try {
                        transaction.commitAllowingStateLoss();
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                Log.e("volley", String.valueOf(error));
            }
        }
        );
        requestQueue.add(jsonObjectRequest);

}

@Override
public void clickOnItem(int id) {
    movieData.movieDataPosition = id;
    if(movieData.movieDataArray == null) {
        movieRequest();
    } else {
        Intent intent = new Intent(this, detailsActivity.class);
        intent.putExtra("FRAGMENT","MOVIE");
        startActivity(intent);
    }

}

@Override
public void favoriteMovieItem(int movieId) {
    movieData.dbPosition = movieId;

    Intent intent = new Intent(this,detailsActivity.class);
    intent.putExtra("FRAGMENT","favorite");
    startActivity(intent);
} }

Answer 1

You can try this

this.clickListener = (MainActivity) getActivity();

You get the FragmentActivity and cast it into your MainActivity

EDIT:

I suggest you to add a function in your fragment like :

public void setListener(clickInterfaceHelper listener) {
     this.clickListener = listener;
}

And in your activity :

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if(savedInstanceState == null) {
    MovieGridFragment fragment = new MovieGridFragment();
    fragment.setListener(this);   // some change here
                getSupportFragmentManager().beginTransaction()
                .add(R.id.activity_container, fragment)
                .commit();
        movieData.movieDataPosition = 0;
    }
    if(savedInstanceState != null) {
        sorterString = savedInstanceState.getString("SORTER");
    }

    if(savedInstanceState == null)
        movieData.movieDataPosition = 0;

    if(sorterString==null)
        sorterString="popular?";
    if(sorterString!="favorite" && sorterString!=null) {
        if(networkChecker.isNetworkAvailableChecker(this)) {
            movieRequest();
        }
    }

}

.... no relevant functions 


public void movieRequest() {
        urlFinal = urlBase + sorterString + movieData.apiKey;
        urlFinal.trim();
        requestQueue = Volley.newRequestQueue(this);
        JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.GET, urlFinal, null, new Response.Listener<JSONObject>() {
            @Override
            public void onResponse(JSONObject response) {
                try {
                    JSONArray array = response.getJSONArray("results");
                    movieData.movieDataArray = new movieData[array.length()];

                    for (int i = 0; i < array.length(); i++) {
                        movieData movie = new movieData();
                        JSONObject jsonObject = array.getJSONObject(i);
                        //movie.setPosition(i);
                        movie.setMovieId(jsonObject.getString("id"));
                        movie.setMovieImagePath(jsonObject.getString("poster_path"));
                        movie.setMovieTitle(jsonObject.getString("original_title"));
                        movie.setMoviePlot(jsonObject.getString("overview"));
                        movie.setMovieVoting(jsonObject.getString("vote_average"));
                        movie.setMovieReleaseDate(jsonObject.getString("release_date"));
                        movieData.movieDataArray[i] = movie;
                    }

                    MovieGridFragment gridFragment = new MovieGridFragment();
                    gridfragment.setListener(this);  // some change here
                    gridFragment.movieDataList = Arrays.asList(movieData.movieDataArray); //hier wird datalist eigentlich zugewiesen
                    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
                    transaction.replace(R.id.activity_container, gridFragment);
                    try {
                        transaction.commitAllowingStateLoss();
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                Log.e("volley", String.valueOf(error));
            }
        }
        );
        requestQueue.add(jsonObjectRequest);

}

Hope this helps.

Answer 2

The context passed in is not necessarily your activity. It could be a wrapper around it with various theming and other overrides. You can never assume that when passed a Context, you're passed an Activity. If you need a reference to a click handler, write a setClickHandler function and call it explicitly.

Even if it is the Activity, the app doesn't know that- at that point its a Context. You'd need to explicitly cast it, which may not work (and may work on some versions of the OS and not others) due to paragraph 1.