Question

我正在尝试在我创建的3个标签之一中添加回收站视图。我希望卡片可以在其中一个标签中查看。

但是，我一直收到以下错误：

02-04 01:27:59.447 31760-31760/app.com.example.android.prototype E/AndroidRuntime: FATAL EXCEPTION: main
Process: app.com.example.android.prototype, PID: 31760
     java.lang.RuntimeException: Unable to start activity ComponentInfo{app.com.example.android.prototype/app.com.example.android.prototype.MainActivity}: 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:2416)
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
         at android.app.ActivityThread.-wrap11(ActivityThread.java)
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
         at android.os.Handler.dispatchMessage(Handler.java:102)
         at android.os.Looper.loop(Looper.java:148)
         at android.app.ActivityThread.main(ActivityThread.java:5417)
         at java.lang.reflect.Method.invoke(Native Method)
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
      Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference
         at app.com.example.android.prototype.MainActivity.onCreate(MainActivity.java:34)
         at android.app.Activity.performCreate(Activity.java:6251)
         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
         at android.app.ActivityThread.-wrap11(ActivityThread.java) 
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
         at android.os.Handler.dispatchMessage(Handler.java:102) 
         at android.os.Looper.loop(Looper.java:148) 
         at android.app.ActivityThread.main(ActivityThread.java:5417) 
         at java.lang.reflect.Method.invoke(Native Method) 
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

XML文件：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="40dp"
        android:textSize="30sp"
        android:gravity="center"
        android:id="@+id/textView"
        android:layout_centerHorizontal="true"
        android:textColor="@android:color/holo_blue_dark"
        android:text="Primary\nFragment"/>
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:autoLink="web"
        android:textSize="15sp"
        android:layout_marginTop="10dp"
        android:layout_centerHorizontal="true"
        android:text="androidbelieve.com"
        android:textColor="#000"
        android:layout_below="@+id/textView"
        android:textStyle="italic"/>
    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
</RelativeLayout>

MainActivity.java：

public class MainActivity extends AppCompatActivity {

RecyclerView mRecyclerView;
RecyclerView.LayoutManager mLayoutManager;
RecyclerView.Adapter mAdapter;
DrawerLayout mDrawerLayout;
NavigationView mNavigationView;
FragmentManager mFragmentManager;
FragmentTransaction mFragmentTransaction;

//private Toolbar toolbar;

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

    mRecyclerView = (RecyclerView)findViewById(R.id.recycler_view);
    mRecyclerView.setHasFixedSize(false);

    mLayoutManager = new LinearLayoutManager(this);
    mRecyclerView.setLayoutManager(mLayoutManager);


    mAdapter = new CardAdapter();
    mRecyclerView.setAdapter(mAdapter);

    //toolbar = (Toolbar) findViewById(R.id.tool_bar); // Attaching the layout to the toolbar object
   // setSupportActionBar(toolbar);


    /**
     *Setup the DrawerLayout and NavigationView
     */

    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawerLayout);
    mNavigationView = (NavigationView) findViewById(R.id.shitstuff) ;

    /**
     * Lets inflate the very first fragment
     * Here , we are inflating the TabFragment as the first Fragment
     */

    mFragmentManager = getSupportFragmentManager();
    mFragmentTransaction = mFragmentManager.beginTransaction();
    mFragmentTransaction.replace(R.id.containerView,new TabFragment()).commit();
    /**
     * Setup click events on the Navigation View Items.
     */

    mNavigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(MenuItem menuItem) {
            mDrawerLayout.closeDrawers();



            if (menuItem.getItemId() == R.id.nav_item_sent) {
                FragmentTransaction fragmentTransaction = mFragmentManager.beginTransaction();
                fragmentTransaction.replace(R.id.containerView,new SentFragment()).commit();

            }

            if (menuItem.getItemId() == R.id.nav_item_inbox) {
                FragmentTransaction xfragmentTransaction = mFragmentManager.beginTransaction();
                xfragmentTransaction.replace(R.id.containerView,new TabFragment()).commit();
            }

            return false;
        }

    });

    /**
     * Setup Drawer Toggle of the Toolbar
     */

    android.support.v7.widget.Toolbar toolbar = (android.support.v7.widget.Toolbar) findViewById(R.id.toolbar);
    ActionBarDrawerToggle mDrawerToggle = new ActionBarDrawerToggle(this,mDrawerLayout, toolbar,R.string.app_name,
            R.string.app_name);

    mDrawerLayout.setDrawerListener(mDrawerToggle);

    mDrawerToggle.syncState();



}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
//        if (id == R.id.action_settings) {
//            return true;
//        }

        return super.onOptionsItemSelected(item);
    }
}

有人可以帮我吗？

谢谢！

Answer 1

您在Recycler视图上的findViewById返回null。您正尝试在其中一个选项卡中添加recyclerview，但引用活动中的recyclerview，这将抛出NullPointerException。如果要从活动代码中引用回收器视图，请确保将回收器视图添加到活动布局中。

如果要在其中一个选项卡中使用回收站视图，则应在相应的选项卡片段中调用findViewById，而不是活动。

Recycler View - 空指针异常

1 个答案: