在片段中的recyclerView上使用findViewById時獲取NullPointerException

Question

嘗試用5個片段構建一個應用程序。 第一個包含recyclerView，每當我切換選項卡時，視圖都會被丟棄，並且每次當充氣機對視圖充氣時，我都會回來，同時應用程序嘗試獲取recyclerView的ID，該ID返回空指針異常。 而且這個問題只會變得越來越復雜，我不認為使用正確的方法來執行此操作，因為我也會遇到錯誤。

如果您有片段帶有recyclerViews的應用程序的任何簡單示例，請刪除以下鏈接，在此先感謝您

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());
}
}

錯誤，我得到以下錯誤：

2019-04-15 13：00：28.449 18359-18359 / com.example.recyclerview2 E / Android運行時：致命異常：主要

流程：com.example.recyclerview2，PID：18359

java.lang.RuntimeException：無法啟動活動ComponentInfo {com.example.recyclerview2 / com.example.recyclerview2.MainActivity}：java.lang.NullPointerException：嘗試調用虛擬方法android.view.View android.view.View.findViewById空對象引用上的（int）

StackTrace 2019-04-15 13：57：00.234 5508-5508 / com.example.recyclerview2 E / AndroidRuntime：致命例外：主進程：com.example.recyclerview2，PID：5508 java.lang.RuntimeException：無法啟動活動ComponentInfo {com.example.recyclerview2 / com.example.recyclerview2.MainActivity}：java.lang.NullPointerException：嘗試在null對象引用上調用虛擬方法“ android.view.View android.view.View.findViewById（int）” android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2913）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3048）在android.app.servertransaction.LaunchActivityItem.execute（LaunchActivityItem.java:78）在android.app android.app.servertransaction.TransactionExecutor.execute（TransactionExecutor.java:68：android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1808）上的.servertransaction.TransactionExecutor.executeCallbacks（TransactionExecutor.java:108）在android。 os.Handler.dispatchMessage（Handler.java:106） 在android.os.Looper.loop（Looper.java:193）在android.app.ActivityThread.main（ActivityThread.java:6669）在com.android.internal的java.lang.reflect.Method.invoke（本機方法） com.android.internal.os.ZygoteInit.main（ZygoteInit.java:858）上的.os.RuntimeInit $ MethodAndArgsCaller.run（RuntimeInit.java:493）原因：java.lang.NullPointerException：嘗試調用虛擬方法'android .view.View android.view.View.findViewById（int）'，位於com.example.recyclerview2.Fragments.FragmentHome.initRecyclerView（FragmentHome.java:92）上的com.example.recyclerview2.MainActivity.onCreate（ android.app.Activity.performCreate（Activity.java:7136）上的MainActivity.java:44）android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1271）上android.app.Activity.performCreate（Activity.java:7127）上的）在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2893）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3048）在android.app.servertr android.app.servertransaction.TransactionExecutor.executeCallbacks（TransactionExecutor.java:108）位於android.app.servertransaction.TransactionExecutor.execute（TransactionExecutor.java:68）具有ansaction.LaunchActivityItem.execute（LaunchActivityItem.java:78） android.os.Handler.dispatchMessage（Handler.java:106）上的.ActivityThread $ H.handleMessage（ActivityThread.java:1808）android.os.Looper.loop（Looper.java:193）上的android.app.ActivityThread。主要（ActivityThread.java:6669）

抱歉，第一篇文章是關於堆棧的，我不知道如何正確格式化代碼。

Answer 1

代碼中的問題是您正在調用fragmentHome.initRecyclerView(); 在MainActivity的 onCreate()方法中。 此時尚未創建片段視圖。 這就是為什么您會得到NullPointerException從MainActivity刪除此行的原因。

Answer 2

看來initRecyclerView行有問題

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

在執行該行時，似乎inflatingView為null。 但是我不明白為什么。 請嘗試將上述行移動到onCreateView() 。 希望這可以解決問題。