尝试在空对象引用上调用虚拟方法'java.lang.String android.content.Context.getPackageName（）'

Question

我有一个Activity，它本身有三个Fragment s。

在其中一个片段中，有一个RecyclerView带有自定义适配器，并且单击其中一个项目将转到另一个页面，该页面是相同Activity的新实例。但是，某些行为会在我的应用程序中导致错误。

从我的活动中，点击其中一个项目可以调出相同Activity的新实例，这很好。然后我按下后退按钮，然后回到第一个活动。但是，再次点击这些项目之一（启动同一活动的新实例）导致以下错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference

同样重要的是要考虑到我打电话了活动的新实例（即其中这三个项目是），我在我的活动中的一个片段。所以，当我打电话吧，我有这样的：

public class MyActivity extends AppCompatActivity { 

    ... 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     ... 
     ViewPager viewPager = (ViewPager) findViewById(R.id.detail_viewpager); 
     viewPager.setAdapter(new ViewPagerAdapter(getSupportFragmentManager())); 
     TabLayout tabLayout = (TabLayout) findViewById(R.id.detail_tabs); 
     tabLayout.setTabTextColors(
       ContextCompat.getColor(this, R.color.text_white_secondary), 
       ContextCompat.getColor(this, R.color.text_white)); 
     tabLayout.setSelectedTabIndicatorColor(ContextCompat.getColor(this, R.color.white)); 
     tabLayout.setupWithViewPager(viewPager); 
     viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout)); 
    } 

    ... 

    public class ViewPagerAdapter extends FragmentPagerAdapter { 

     public ViewPagerAdapter(FragmentManager fm) { 
      super(fm); 
     } 

     @Override 
     public int getCount() { 
      return 3; 
     } 

     @Override 
     public Fragment getItem(int position) { 
      switch (position) { 
       case 0: return new MainFragment(); 
       case 1: return new MyFragment(); 
       case 2: return new MyOtherFragment(); 
      } 
      return null; 
     } 

     @Override 
     public CharSequence getPageTitle(int position) { 
      Locale l = Locale.getDefault(); 
      switch (position) { 
       case 0: 
        return getString(R.string.tab_main_frag).toUpperCase(l); 
       case 1: 
        return getString(R.string.tab_my_frag).toUpperCase(l); 
       case 2: 
        return getString(R.string.tab_my_other_frag).toUpperCase(l); 
      } 
      return null; 
     } 
    } 

    ... 

    public static class MyFragment extends Fragment implements MyRVAdapter.OnEntryClickListener { 

     ... 

     private ArrayList<ItemObj> mArrayList; 

     @Override 
     public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
      ... 
      doStuff(); 
      ... 
     } 

     private void doStuff() { 
      ... 
      mArrayList = ...; 
      MyRVAdapter adapter = new MyRVAdapter(getActivity(), mArrayList); 
      adapter.setOnEntryClickListener(new MyRVAdapter.OnEntryClickListener() { 
       @Override 
       public void onEntryClick(View view, int position) { 
        Intent intent = new Intent(getActivity(), MyActivity.class); 
        intent.putExtra("INFORMATION", mArrayList.get(position)); 
        startActivity(intent); 
       } 
      }); 
     } 

     ... 

    } 

    ... 
} 

这里是我的自定义接口的一部分：

public class MyRVAdapter extends RecyclerView.Adapter<MyRVAdapter.MyViewHolder> { 

    public static class MyViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener { 

     ... 

     MyViewHolder(View itemView) { 
      super(itemView); 
      itemView.setOnClickListener(this); 
      ... 
     } 

     @Override 
     public void onClick(View v) { 
      // The user may not set a click listener for list items, in which case our listener 
      // will be null, so we need to check for this 
      if (mOnEntryClickListener != null) { 
       mOnEntryClickListener.onEntryClick(v, getLayoutPosition()); 
      } 
     } 
    } 

    private Context mContext; 
    private ArrayList<ItemObj> mArray; 

    public MyRVAdapter(Context context, ArrayList<ItemObj> array) { 
     mContext = context; 
     mArray = array; 
    } 

    @Override 
    public int getItemCount() { 
     return mArray.size(); 
    } 

    @Override 
    public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
     View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.tile_simple, parent, false); 
     return new MyViewHolder(view); 
    } 

    @Override 
    public void onBindViewHolder(MyViewHolder holder, int position) { 
     ItemObj anItem = mArray.get(position); 

     ... 
    } 

    @Override 
    public void onAttachedToRecyclerView(RecyclerView recyclerView) { 
     super.onAttachedToRecyclerView(recyclerView); 
    } 


    private static OnEntryClickListener mOnEntryClickListener; 

    public interface OnEntryClickListener { 
     void onEntryClick(View view, int position); 
    } 

    public void setOnEntryClickListener(OnEntryClickListener onEntryClickListener) { 
     mOnEntryClickListener = onEntryClickListener; 
    } 

} 

这里是完全错误：

01-23 14:07:59.083 388-388/com.mycompany.myapp E/AndroidRuntime: FATAL EXCEPTION: main 
Process: com.mycompany.myapp, PID: 388 
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference 
    at android.content.ComponentName.<init>(ComponentName.java:77) 
    at android.content.Intent.<init>(Intent.java:4570) 
    at com.mycompany.myapp.MyActivity$MyFragment$1.onEntryClick(MyActivity.java:783) 
    at com.mycompany.myapp.adapter.MyRVAdapter$MyViewHolder.onClick(MyRVAdapter.java:42) 
    at android.view.View.performClick(View.java:5197) 
    at android.view.View$PerformClick.run(View.java:20926) 
    at android.os.Handler.handleCallback(Handler.java:739) 
    at android.os.Handler.dispatchMessage(Handler.java:95) 
    at android.os.Looper.loop(Looper.java:145) 
    at android.app.ActivityThread.main(ActivityThread.java:5951) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195) 

错误指向第一行：Intent intent = new Intent(getActivity(), MyActivity.class);（来自片段）首先，线后（在错误中）指向mOnEntryClickListener.onEntryClick(v, getLayoutPosition());来自自定义适配器中的覆盖onClick方法河

我也读过类似的答案，但他们还没有解决我的问题。

编辑：

通过使用：

if (getActivity() == null) { 
    Log.d(LOG_TAG, "Activity context is null"); 
} else { 
    Intent intent = new Intent(getActivity(), MyActivity.class); 
    intent.putExtra("INFORMATION", mArrayList.get(position)); 
    startActivity(intent); 
} 

在片段内部类（ onEntryClick）

，我发现主叫getActivity()返回null。

Answer 1

所以，问题是这条线

private static OnEntryClickListener mOnEntryClickListener; 

由于它是静态的，你在运行时拥有类的一个实例。当您单击某个项目时，会创建同一活动的第二个实例，并创建另一个实例mOnEntryClickListener，覆盖前一个实例。因此，当您按回以返回到活动的第一个实例时，您正在使用第二个活动的mOnEntryClickListener实例，该实例已被销毁。

Answer 2

该问题很可能与您的匿名OnClickListener从原始片段捕获getActivity()方法有关。你可以尝试实现您MyFragment的OnClickListener，看看行为在所有的变化（可能不是那么简单）：

public static class MyFragment extends Fragment implements View.OnClickListener { 

    ... 

    private void doStuff() { 
     button.setOnClickListener(this); 
    } 

    @Override 
    public void onClick(View v) { 
     Intent intent = new Intent(getActivity(), MyActivity.class); 
     intent.putExtra("INFORMATION", value); 
     startActivity(intent); 

    } 

    ... 

} 

---

编辑：这是一个黑客，而不是东西，我一般会建议但可以让你周围的问题，如果你在紧要关头是......

延长Application提供静态应用程序实例：

public class MyApplication extends Application { 
    private static MyApplication _instance; 

    @Override 
    public void onCreate() { 
     super.onCreate(); 

     _instance = this; 
    } 

    public static MyApplication getInstance() { 
     return _instance; 
    } 
} 

修改您的AndroidManifest。XML使您的应用程序运行MyApplication：

<application 
    android:name="com.package.MyApplication" 
<!-- Rest of your manifest --> 

然后用调用替换getActivity()到MyApplication.getInstance()：

private void doStuff() { 
    ... 
    mArrayList = ...; 
    MyRVAdapter adapter = new MyRVAdapter(getActivity(), mArrayList); 
    adapter.setOnEntryClickListener(new MyRVAdapter.OnEntryClickListener() { 
     @Override 
     public void onEntryClick(View view, int position) { 
      Intent intent = new Intent(MyApplication.getInstance(), MyActivity.class); 
      intent.putExtra("INFORMATION", mArrayList.get(position)); 
      startActivity(intent); 
     } 
    }); 
} 

Answer 3

尝试使用我的解决方案： 在MyActivity，创建一个新的功能：

public void openAnotherActivity(ObjectItem item) { 
     Intent intent = new Intent(MyApplication.getInstance(), MyActivity.class); 
     intent.putExtra("INFORMATION", item); 
     startActivity(intent); 
} 

然后修改doStuff（）为：

private void doStuff() { 
    ... 
    mArrayList = ...; 
    MyRVAdapter adapter = new MyRVAdapter(getActivity(), mArrayList); 
    adapter.setOnEntryClickListener(new MyRVAdapter.OnEntryClickListener() { 
     @Override 
     public void onEntryClick(View view, int position) { 
      openAnotherActivity(mArrayList.get(position)) 
     } 
    }); 
} 

我不知道它可以帮助你或没有，但我认为它至少可以解决的问题context

Answer 4

在你片段的OnCreate，请将活动的参考和检查，如果你的活动是在在开展其他活动之前恢复了状态。

public static class MyFragment extends Fragment implements MyRVAdapter.OnEntryClickListener { 

    ... 

    private ArrayList<ItemObj> mArrayList; 
    private MyActivity mActivity; 
    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     mActivity = getActivity(); 
    } 
    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
     ... 
     doStuff(); 
     ... 
    } 

    private void doStuff() { 
     ... 
     mArrayList = ...; 
     MyRVAdapter adapter = new MyRVAdapter(getActivity(), mArrayList); 
     adapter.setOnEntryClickListener(new MyRVAdapter.OnEntryClickListener() { 
      @Override 
      public void onEntryClick(View view, int position) { 
       if (!mActivity.isFinishing() { 
        Intent intent = new Intent(mActivity, MyActivity.class); 
        intent.putExtra("INFORMATION", mArrayList.get(position)); 
        startActivity(intent); 
       } 
      } 
     }); 
    } 

    ... 

}