为什么我使用带有片段的RecyclerView不断收到空引用错误

时间：2019-03-03 23:44:37

标签： java android android-fragments kotlin android-recyclerview

我正在为我的应用程序使用片段。我还使用了一个导航抽屉，该抽屉通过抽屉中的按钮打开片段。

我正在尝试为我的recyclerview显示一些基本图像和文本及其适配器和arraylist。

我遇到的问题是我不知道如何修改代码以对片段使用recyclerview，因为我看到的所有教程都没有使用片段。

下面是我的活动代码：

public class secondActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {

private DrawerLayout drawer;
private RecyclerView mRecyclerView;
private RecyclerView.Adapter mAdapter;
private RecyclerView.LayoutManager mLayoutManager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_second);

    ArrayList<exampleItemBooks> exampleList = new ArrayList<>();
    exampleList.add(new exampleItemBooks(R.drawable.artofwar, "Line 1", "Line 
    2"));
    exampleList.add(new exampleItemBooks(R.drawable.aristotle, "Line 3", 
    "Line 4"));
    exampleList.add(new exampleItemBooks(R.drawable.caesarbook, "Line 5", 
    "Line 6"));
    exampleList.add(new exampleItemBooks(R.drawable.platorepublic, "Line 7", 
    "Line 8"));
    exampleList.add(new exampleItemBooks(R.drawable.senecaletters, "Line 9", 
    "Line 10"));
    exampleList.add(new exampleItemBooks(R.drawable.thehistoryofmypeople, 
    "Line 11", "Line 12"));
    exampleList.add(new exampleItemBooks(R.drawable.theprince, "Line 13", 
    "Line 14"));
    exampleList.add(new exampleItemBooks(R.drawable.thritysixstrategems, 
    "Line 15", "Line 16"));
    exampleList.add(new exampleItemBooks(R.drawable.medidations, "Line 17", 
    "Line 18"));

    mRecyclerView = findViewById(R.id.recyclerViewBooks);
    mRecyclerView.setHasFixedSize(false);
    mLayoutManager = new LinearLayoutManager(this);
    mAdapter = new exampleBooksAdapter(exampleList);

    mRecyclerView.setLayoutManager(mLayoutManager);
    mRecyclerView.setAdapter(mAdapter);

    Toolbar toolbar = findViewById(R.id.toolbarMain);
    setSupportActionBar(toolbar);

    drawer = findViewById(R.id.drawer_layout);
    NavigationView navigationView = findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, 
    toolbar, R.string.navigation_open_drawer, 
    R.string.navigation_close_drawer);
    drawer.addDrawerListener(toggle);
    toggle.syncState();

    if (savedInstanceState == null) {

      getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new HomeFragment()).commit();
        //Set the button as checked.
        navigationView.setCheckedItem(R.id.nav_home);

    }
}

下面是我的片段类代码，该片段代码负责我使用recyclerview的片段：

 import android.os.Bundle;
   import android.support.annotation.NonNull;
   import android.support.annotation.Nullable;
   import android.support.v4.app.Fragment;
   import android.view.LayoutInflater;
   import android.view.View;
   import android.view.ViewGroup;

    public class BooksFragment extends Fragment {

    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_books, container, false);

        View view = inflater.inflate(R.layout.fragment_books, container, false);

        RecyclerView recyclerView = view.findViewById(R.id.recyclerViewBooks);
        //mRecyclerView = findViewById(R.id.recyclerViewBooks);
        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        return view;

    }
}

以下是我的recyclerView适配器：

   public class exampleBooksAdapter extends RecyclerView.Adapter<exampleBooksAdapter.ExampleViewHolder> {

    private ArrayList<exampleItemBooks> mExampleList;

    public static class ExampleViewHolder extends RecyclerView.ViewHolder {

        public ImageView mImageView;
        public TextView mTextView1;
        public TextView mTextView2;

        public ExampleViewHolder(View itemView) {
            super(itemView);

            mImageView = itemView.findViewById(R.id.imageViewCards);
            mTextView1 = itemView.findViewById(R.id.textViewCard1);
            mTextView1 = itemView.findViewById(R.id.textViewCard2);
        }
    }

    public exampleBooksAdapter(ArrayList<exampleItemBooks> exampleItemBooks) {

        mExampleList = exampleItemBooks;

    }

    @Override
    public ExampleViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.example_item_books, parent, false);
        ExampleViewHolder evh = new ExampleViewHolder(v);
        return evh;
    }

    @Override
    public void onBindViewHolder(@NonNull ExampleViewHolder holder, int position) {

        exampleItemBooks currentItem = mExampleList.get(position);

        holder.mImageView.setImageResource(currentItem.getImageResource());
        holder.mTextView1.setText(currentItem.getText1());
        holder.mTextView2.setText(currentItem.getText2());
    }

    @Override
    public int getItemCount() {

        return mExampleList.size();
    }
}

下面是我的arraylist类：

 public class exampleItemBooks {

    private int mImageResource;
    private String mText1;
    private String mText2;

    public exampleItemBooks(int imageResource, String text1, String text2) {

        mImageResource = imageResource;
        mText1 = text1;
        mText2 = text2;

    }

    public int getImageResource() {

        return mImageResource;

    }

    public String getText1() {

        return mText1;
    }

    public String getText2() {

        return mText2;
    }

}

我不断得到的错误是：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.warstories, PID: 5753
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.warstories/com.example.warstories.secondActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
    at android.app.ActivityThread.-wrap11(Unknown Source:0)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
    at android.os.Handler.dispatchMessage(Handler.java:105)
    at android.os.Looper.loop(Looper.java:164)
    at android.app.ActivityThread.main(ActivityThread.java:6541)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference
    at com.example.warstories.secondActivity.onCreate(secondActivity.java:42)
    at android.app.Activity.performCreate(Activity.java:6975)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
    at android.app.ActivityThread.-wrap11(Unknown Source:0) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
    at android.os.Handler.dispatchMessage(Handler.java:105) 
    at android.os.Looper.loop(Looper.java:164) 
    at android.app.ActivityThread.main(ActivityThread.java:6541) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 

关键错误说明：

Unable to start activity ComponentInfo{com.example.warstories/com.example.warstories.secondActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager)' on a null object reference

原因：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ void android.support.v7.widget.RecyclerView.setLayoutManager（android.support.v7.widget.RecyclerView $ LayoutManager）”。

有帮助吗？

1 个答案:

答案 0 :(得分：1)

您的代码失败，因为您的活动正在尝试访问作为Fragment布局一部分的RecyclerView，并且您调用setContentView()之后（甚至在{{ 1}}操作，由于您使用replace()而不是commit()），因此只能异步使用。

片段应该（在尽可能多的程度上）是独立的。这意味着，如果您的片段拥有commitNow()，则它应该负责将数据加载到其中：而不是您的活动。

您应该将活动中与RecyclerView接触的所有代码移到片段上-理想情况下，应移至RecyclerView之类的方法中，该方法可让您访问已拥有的视图夸大了onViewCreated()，是致电onCreateView()并获取findViewById()的合适位置。

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