尝试在空对象引用上调用虚拟方法“void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapt

Posted 2023-02-19

【中文标题】尝试在空对象引用上调用虚拟方法“void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)”

【英文标题】：Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference

【发布时间】：2015-10-27 14:55:04

【问题描述】：

我正在尝试使用 android 材料设计实现滑动标签布局。但它给了我NullPointerException。到目前为止，这是我的代码：

MainActivity.java

public class MainActivity extends ActionBarActivity 
    private Toolbar toolbar;
    private ViewPager pager;
    private SlidingTabLayout nLayout;


    @Override
    protected void onCreate(Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main_appbar);
        toolbar = (Toolbar) findViewById(R.id.app_bar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayShowHomeEnabled(true);

        NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment)
                getSupportFragmentManager().findFragmentById(R.id.navigation_drawer_fragment);
        drawerFragment.setUp(R.id.navigation_drawer_fragment, (DrawerLayout) findViewById(R.id.drawer_layout), toolbar);
        pager = (ViewPager) findViewById(R.id.pager);
        pager.setAdapter(new MyPagerAdapter(getSupportFragmentManager()));

        nLayout = (SlidingTabLayout) findViewById(R.id.tabs);
        nLayout.setViewPager(pager);
    

    @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;
        
        if (id == R.id.navigate) 
            startActivity(new Intent(this, SubActivity.class));

        

        return super.onOptionsItemSelected(item);
    

    class MyPagerAdapter extends FragmentPagerAdapter 

        String tabs[];

        public MyPagerAdapter(FragmentManager fm) 
            super(fm);
            tabs = getResources().getStringArray(R.array.tabs);
        

        @Override
        public Fragment getItem(int position) 
            MyFragment myFragment = MyFragment.getInstance(position);
            return myFragment;
        

        @Override
        public CharSequence getPageTitle(int position) 
            return tabs[position];
        

        @Override
        public int getCount() 
            return 3;
        
    

    public static class MyFragment extends Fragment 
        private TextView textView;

        public static MyFragment getInstance(int position) 
            MyFragment myFragment = new MyFragment();
            Bundle args = new Bundle();
            args.putInt("position", position);
            myFragment.setArguments(args);
            return myFragment;
        

        @Nullable
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) 
            View layout = inflater.inflate(R.layout.fragment_my, container, false);
            textView = (TextView) layout.findViewById(R.id.position);
            Bundle bundle = getArguments();
            if (bundle != null) 
                textView.setText("The page selected is " + bundle.getInt("position"));
            
            return layout;
        
    

activity_main.xml

<LinearLayout
    android:layout_
    android:layout_
    android:orientation="vertical"
    tools:context="carsaleapp.shanaka.com.carsaleapp.MainActivity">

    <include
        android:id="@+id/app_bar"
        layout="@layout/app_bar" />

    <app.shanaka.tabs.SlidingTabLayout
        android:id="@+id/tabs"
        android:layout_
        android:layout_ />


    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_
        android:layout_
        android:layout_weight="1" />


</LinearLayout>

这是我运行应用程序后遇到的错误：

java.lang.RuntimeException: Unable to start activity ComponentInfocarsaleapp.shanaka.com.carsaleapp/carsaleapp.shanaka.com.carsaleapp.MainActivity: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            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:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference
            at carsaleapp.shanaka.com.carsaleapp.MainActivity.onCreate(MainActivity.java:42)
            at android.app.Activity.performCreate(Activity.java:5933)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            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:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

他们一直说pager 对象为空。但对我来说，我已经正确设置了它。

pager = (ViewPager) findViewById(R.id.pager);

【参考方案1】：

您的视图寻呼机位于activity_main.xml，而您的活动布局是 setContentView(R.layout.activity_main_appbar)。您在activity_main_appbar.xml 中没有查看寻呼机。

findViewById 在当前视图层次结构中查找视图。我想应该是setContentView(R.layout.activity_main);，但请检查您在包含的布局中拥有哪些视图app_bar.xml

【讨论】：

当然是。不知道我是怎么错过这一点的。无论如何它在setContentView(R.layout.activity_main); 之后完美运行谢谢。

我有同样的错误，但因为我错过了 pager = (ViewPager) findViewById(R.id.pager);部分

【参考方案2】：

可能是您传递了错误的 viewPager id。请从 XML 文件中检查您的 id

【讨论】：

这对问题没有任何价值。

解决了我的问题。谢谢。