在片段中的recyclerView上使用findViewById时获取NullPointerException

Question

Trying to build an app with 5 fragments. The first one contains the recyclerView, whenever I switch tabs the Views get discarded and every time I come back when the inflator is inflating the view, at the same time the app tries to get the id of the recyclerView which returns a null pointer exception. And this problem is only getting more complicated, I don't think in using the correct way to do this as I am getting errors will context as well.

If you have any simple examples of apps with fragments having recyclerViews please drop the link below, thank you in advance

MainActivity.java

class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";

private FragmentHome fragmentHome = new FragmentHome(this);

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    fragmentHome.initGetData();
    BottomNavigationView bottomNavigationView = findViewById(R.id.bottomNavBar);
    getSupportFragmentManager().beginTransaction().replace(R.id.container, fragmentHome).commit();
    fragmentHome.initRecyclerView();
    bottomNavigationView.setOnNavigationItemSelectedListener(navListener);
    bottomNavigationView.setOnNavigationItemReselectedListener(new BottomNavigationView.OnNavigationItemReselectedListener() {
        @Override
        public void onNavigationItemReselected(@NonNull MenuItem menuItem) {

        }
    });

}

private BottomNavigationView.OnNavigationItemSelectedListener navListener = new BottomNavigationView.OnNavigationItemSelectedListener() {
    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {

        switch (menuItem.getItemId()) {
            case R.id.home: {
                getSupportFragmentManager().beginTransaction().replace(R.id.container, fragmentHome).commit();
                fragmentHome.initRecyclerView();
                break;
            }
        }
        return true;
    }
};
}

FragmentHome.java

public class FragmentHome extends Fragment {

private String BASE_URL = "https://www.reddit.com";

// INIT RETROFIT AND CALL ONCE AND FOR ALL
Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(BASE_URL)
        .addConverterFactory(GsonConverterFactory.create())
        .build();

RedditAPI redditAPI = retrofit.create(RedditAPI.class);


private static final String TAG = "FragmentHome";
private RecyclerView feedRecyclerView;
private List<ChildrenData> childrenData;
private String after;
private String previousAfter = "";
private Boolean isScrolling = false;
private int currentItems;
private int totalItems;
private int scrolledOutItems;
private Context context;
private View inflatingView;


public FragmentHome() {
}

public FragmentHome(Context context) {
    this.context = context; //I also get some attribute warning 
                               here not important right now though.
}

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    inflatingView =  inflater.inflate(R.layout.fragment_home, container, false);
    initRecyclerView();
    return inflatingView;
}


public void initGetData() {
    Call<FeedData> feedDataCall = redditAPI.getFeed();
    feedDataCall.enqueue(new Callback<FeedData>() {
        @Override
        public void onResponse(Call<FeedData> call, final Response<FeedData> response) {
         /*   Log.d(TAG, "onResponse: " + response.body().getData().getChildren().get(0).getDataModel().getTitle());
            Log.d(TAG, "onResponse: " + response.body().getData().getChildren());*/
            childrenData = response.body().getData().getChildren();
            Log.d(TAG, "onResponse:  " + childrenData);
            after = response.body().getData().getAfter();
            Log.d(TAG, "onResponse: " + after);
        }

        @Override
        public void onFailure(Call<FeedData> call, Throwable t) {
            Log.d(TAG, "onFailure: " + t.getMessage());
        }
    });
}

public void initRecyclerView() {

    feedRecyclerView = inflatingView.findViewById(R.id.recyclerView);

    final FeedRecyclerViewAdapter feedRecyclerViewAdapter = new FeedRecyclerViewAdapter(childrenData, context);
    feedRecyclerView.setAdapter(feedRecyclerViewAdapter);
    final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(context);
    feedRecyclerView.setLayoutManager(linearLayoutManager);

    feedRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
        @Override
        public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);
            isScrolling = true;
        }

        @Override
        public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);
            currentItems = linearLayoutManager.getChildCount();
            totalItems = linearLayoutManager.getItemCount();
            scrolledOutItems = linearLayoutManager.findFirstVisibleItemPosition();

            if (isScrolling && (totalItems - 1 == currentItems + scrolledOutItems) && (!previousAfter.equals(after))) {
                isScrolling = false;
                fetchNewData(after, feedRecyclerViewAdapter);
            }
        }
    });

    }


public void fetchNewData(final String after_use, final FeedRecyclerViewAdapter feedRecyclerViewAdapter) {
    Call<FeedData> feedDataCall_withAfter = redditAPI.getFeed_withAfter(after_use);
    previousAfter = after_use;
    feedDataCall_withAfter.clone().enqueue(new Callback<FeedData>() {
        @Override
        public void onResponse(Call<FeedData> call, Response<FeedData> response) {
            Log.d(TAG, "onResponse: " + response.body().toString());
            childrenData.addAll(response.body().getData().getChildren());
            after = response.body().getData().getAfter();
            feedRecyclerViewAdapter.notifyDataSetChanged();
        }

        @Override
        public void onFailure(Call<FeedData> call, Throwable t) {

        }
    });

    Log.d(TAG, "fetchNewData: " + childrenData.size());
}
}

Error I am getting I am getting the following error:

2019-04-15 13:00:28.449 18359-18359/com.example.recyclerview2 E/AndroidRuntime: FATAL EXCEPTION: main

Process: com.example.recyclerview2, PID: 18359

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.recyclerview2/com.example.recyclerview2.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method android.view.View android.view.View.findViewById(int) on a null object reference

StackTrace 2019-04-15 13:57:00.234 5508-5508/com.example.recyclerview2 E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.recyclerview2, PID: 5508 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.recyclerview2/com.example.recyclerview2.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:193) at android.app.ActivityThread.main(ActivityThread.java:6669) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference at com.example.recyclerview2.Fragments.FragmentHome.initRecyclerView(FragmentHome.java:92) at com.example.recyclerview2.MainActivity.onCreate(MainActivity.java:44) at android.app.Activity.performCreate(Activity.java:7136) at android.app.Activity.performCreate(Activity.java:7127) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) at android.app.servertr ansaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:193) at android.app.ActivityThread.main(ActivityThread.java:6669)

Sorry, first post on stack, I don't know how to format code correctly.

Answer 1

Problem in your code is that you are calling fragmentHome.initRecyclerView(); in onCreate() method in MainActivity . Fragment view at that point is not created yet. That's why you get NullPointerException Remove this line from MainActivity.

Answer 2

It seems there is problem in below line of initRecyclerView

feedRecyclerView = inflatingView.findViewById(R.id.recyclerView);

While the line is executing it seems inflatingView is null. But I can't understand why. Please try moving the above line inside onCreateView() . Hope this might solve the problem.