无法启动活动：java.lang.NullPointerException：

Question

我对安卓很陌生。我正在使用 Material Design 制作滑动标签布局，遵循特定的教程。现在我收到此错误，如我的标题中所述。我正在从 MainActivity 调用活动 Main3Activity 。如果有人能指出错误，更重要的是帮助我纠正它。

PS。这是我在这里的第一篇文章，所以提前道歉。

MainActivity.java

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_appbar);
        toolbar = (Toolbar) findViewById(R.id.app_bar);
        setSupportActionBar(toolbar);




        //getSupprtActionBar().setDisplayShowHomeEnabled(true);


        NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment) getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);

        drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById(R.id.drawer_layout), toolbar);


    }



    @Override
    public boolean onCreateOptionsMenu(Menu menu)
    {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu, menu);
        SearchManager searchManager =
                (SearchManager) getSystemService(Context.SEARCH_SERVICE);
        SearchView searchView =
                (SearchView) menu.findItem(R.id.action_search).getActionView();
        searchView.setSearchableInfo(
                searchManager.getSearchableInfo(getComponentName()));

        return true;

    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        int id=item.getItemId();
        switch(id)
        {
            case R.id.menu_all_reminder:
                if(item.isChecked())
                    item.setChecked(false);
                else
                    item.setChecked(true);
                getActionBar().setBackgroundDrawable(new ColorDrawable(Color.parseColor("#F44336")));
                return true;

            case R.id.menu_ToDo:
                if(item.isChecked())
                    item.setChecked(false);
                else
                    item.setChecked(true);
                getActionBar().setBackgroundDrawable(new ColorDrawable(Color.parseColor("#03A9F4")));
                return true;

            case R.id.menu_bills:
                if(item.isChecked())
                    item.setChecked(false);
                else
                    item.setChecked(true);
                getActionBar().setBackgroundDrawable(new ColorDrawable(Color.parseColor("#4CAF50")));
                return true;

            default:
                return super.onOptionsItemSelected(item);

        }
    }


}

这是Main3Activity，基本上就是滑动布局的Activity。这是在 Main Activity 中导航抽屉的 Recycler View 的一个点击

Main3Activity.java

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main3);

        Toolbar toolbar= (Toolbar) findViewById(R.id.app_bar);
        setSupportActionBar(toolbar);
       getSupportActionBar().setHomeButtonEnabled(true);
       getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        NavigationDrawerFragment drawerFragment=(NavigationDrawerFragment)getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);

        drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById(R.id.drawer_layout), toolbar);


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

        mPager.setAdapter(new MyPagerAdapter(getSupportFragmentManager()));
        mTabs = (SlidingTabLayout) findViewById(R.id.tabs);
    mTabs.setViewPager(mPager);
    }


    class MyPagerAdapter extends FragmentPagerAdapter {
        String[] tabs;
        public MyPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @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 6;
        }
    }

    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;

        }

        @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;
        }
    }

}

这是清单

    <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="demo.demo2">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="Remind Me"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>

            <meta-data
                android:name="android.app.searchable"
                android:resource="@xml/searchable" />
        </activity>
        <activity android:name=".SearchResultsActivity">
            <intent-filter>
                <action android:name="android.intent.action.SEARCH" />
            </intent-filter>
        </activity>
        <activity android:name=".Main2Activity">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="Demo.Demo2.MainActivity" />
        </activity>
        <activity android:name=".Main3Activity">

        </activity>
    </application>

</manifest>

现在，这是 activity_main3.xml

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"

    tools:context="demo.demo2.Main3Activity"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical">

      <android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/drawer_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <LinearLayout

            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_below="@id/app_bar"
            android:orientation="vertical"
            tools:context="demo.demo2.Main3Activity">
            <include
                android:id="@+id/app_bar"
                layout="@layout/app_bar" />

            <tabs.SlidingTabLayout
                android:id="@+id/tabs"
                android:layout_width="match_parent"
                android:layout_height="wrap_content" />

            <android.support.v4.view.ViewPager
                android:id="@+id/pager"
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_weight="1"/>





        </LinearLayout>
        <fragment
            android:id="@+id/fragment_navigation_drawer"
            android:layout_width="@dimen/nav_drawer_width"
            android:layout_height="match_parent"
            app:layout="@layout/fragment_navigation_drawer"
            android:layout_gravity="start"
            android:name="demo.demo2.NavigationDrawerFragment"
            tools:layout="@layout/fragment_navigation_drawer" />
    </android.support.v4.widget.DrawerLayout>







</LinearLayout>

还有logcat中的错误

    5366-5366/demo.demo2 E/AndroidRuntime: FATAL EXCEPTION: main
                                                          Process: demo.demo2, PID: 5366
                                                          java.lang.RuntimeException: Unable to start activity ComponentInfo{demo.demo2/demo.demo2.Main3Activity}: java.lang.NullPointerException: Attempt to read from null array
                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
                                                              at android.app.ActivityThread.access$800(ActivityThread.java:151)
                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
                                                              at android.os.Handler.dispatchMessage(Handler.java:102)
                                                              at android.os.Looper.loop(Looper.java:135)
                                                              at android.app.ActivityThread.main(ActivityThread.java:5254)
                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                              at java.lang.reflect.Method.invoke(Method.java:372)
                                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
                                                           Caused by: java.lang.NullPointerException: Attempt to read from null array
                                                              at demo.demo2.Main3Activity$MyPagerAdapter.getPageTitle(Main3Activity.java:63)
                                                              at tabs.SlidingTabLayout.populateTabStrip(SlidingTabLayout.java:198)
                                                              at tabs.SlidingTabLayout.setViewPager(SlidingTabLayout.java:142)
                                                              at demo.demo2.Main3Activity.onCreate(Main3Activity.java:45)
                                                              at android.app.Activity.performCreate(Activity.java:5990)
                                                              at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
                                                              at android.app.ActivityThread.access$800(ActivityThread.java:151) 
                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
                                                              at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                              at android.os.Looper.loop(Looper.java:135) 
                                                              at android.app.ActivityThread.main(ActivityThread.java:5254) 
                                                              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:903) 
                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 
01-19 02:03:52.050 5366-5366/demo.demo2 I/Process: Sending signal. PID: 5366 SIG: 9

提前谢谢你。

Answer 1

问题是：at demo.demo2.Main3Activity$MyPagerAdapter.getPageTitle(Main3Activity.java:63)

您从未初始化或填充 tabs 数组，但您尝试在 getPageTitle(int position) 方法中使用它：

String[] tabs;

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

---

我已经在我的 strings.xml 中初始化了一个同名的 "tabs" 。它不能在这里引用吗？

将字段命名为与资源同名的字段不会自动加载该资源。

如果不是，那么我应该在代码中将这里初始化为 String[] tabs={"Tab1", "Tab2" ,"Tab3"}

你可以，但你也可以从资源中加载你的字符串数组。您通过Resources 类加载资源。在这种情况下，您可能会使用getStringArray (int id) 方法：

class MyPagerAdapter extends FragmentPagerAdapter {
    String[] tabs;
    public MyPagerAdapter(FragmentManager fm) {
        super(fm);

        tabs = getResources().getStringArray(R.array.tabs);
    }
    // ...
}