Question

尝试构建包含5个片段的应用。第一个包含recyclerView，每当我切换选项卡时，视图都会被丢弃，并且每次当充气机对视图充气时，我都会回来，同时应用程序尝试获取recyclerView的ID，该ID返回空指针异常。而且这个问题只会变得越来越复杂，我不认为使用正确的方法来执行此操作，因为我也会遇到错误。

如果您有带有recyclerViews片段的简单应用示例，请删除以下链接，在此先谢谢您

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());
}
}

我遇到错误 我收到以下错误：

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

进程：com.example.recyclerview2，PID：18359

java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.recyclerview2 / com.example.recyclerview2.MainActivity}：java.lang.NullPointerException：尝试调用虚拟方法android.view.View android.view。空对象引用上的View.findViewById（int）

StackTrace 2019-04-15 13：57：00.234 5508-5508 / com.example.recyclerview2 E / AndroidRuntime：FATAL EXCEPTION：main 流程：com.example.recyclerview2，PID：5508 java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.recyclerview2 / com.example.recyclerview2.MainActivity}：java.lang.NullPointerException：尝试调用虚拟方法'android.view.View android.view.View。空对象引用上的findViewById（int）' 在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.servertransaction.TransactionExecutor.executeCallbacks（TransactionExecutor.java:108）在android.app.servertransaction.TransactionExecutor.execute（TransactionExecutor.java:68）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1808）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java.lang.reflect.Method.invoke（本机方法）在com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run（RuntimeInit.java:493）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:858）原因：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'android.view.View android.view.View.findViewById（int）' 在com.example.recyclerview2.Fragments.FragmentHome.initRecyclerView（FragmentHome.java:92）在com.example.recyclerview2.MainActivity.onCreate（MainActivity.java:44）在android.app.Activity.performCreate（Activity.java:7136）在android.app.Activity.performCreate（Activity.java:7127）在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1271）在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2893）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3048）在android.app.servertransaction.LaunchActivityItem.execute（LaunchActivityItem.java:78）在android.app.servertransaction.TransactionExecutor.executeCallbacks（TransactionExecutor.java:108）在android.app.servertransaction.TransactionExecutor.execute（TransactionExecutor.java:68）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1808）在android.os.Handler.dispatchMessage（Handler.java:106）在android.os.Looper.loop（Looper.java:193）在android.app.ActivityThread.main（ActivityThread.java:6669）

对不起，第一篇文章是关于堆栈的，我不知道如何正确设置代码格式。

Answer 1

代码中的问题是您正在 MainActivity 中的fragmentHome.initRecyclerView();方法中调用onCreate()。此时尚未创建片段视图。这就是为什么您得到 NullPointerException 从MainActivity中删除此行的原因。

Answer 2

在initRecyclerView的下一行似乎有问题

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

该行正在执行时，似乎inflatingView为空。但是我不明白为什么。请尝试将以上行移动到onCreateView()中。希望这可以解决问题。

在片段中的recyclerView上使用findViewById时获取NullPointerException

2 个答案: