片段和片段列表 Android Studio "Unable to start activity ComponentInfo"

Question

我不知道我做错了什么，但我的应用程序在启动时崩溃了。我想用 FragmentActivity 实现标签栏列表。我真的需要帮助:( 返回的错误是：

Unable to start activity ComponentInfo
Caused by: java.lang.NullPointerException
            at jardelcompany.bundoransurfco.MainActivity.onCreate(MainActivity.java:79)

（第 79 行是 actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);）

MainActivity.java :

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener {
SectionsPagerAdapter mSectionsPagerAdapter;
ViewPager mViewPager;
 protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        mSectionsPagerAdapter = new SectionsPagerAdapter(this,
                getSupportFragmentManager());

        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        mViewPager
                .setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
                    @Override
                    public void onPageSelected(int position) {
                        actionBar.setSelectedNavigationItem(position);
                    }
                });

        for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
            actionBar.addTab(actionBar.newTab()
                    .setText(mSectionsPagerAdapter.getPageTitle(i))
                    .setTabListener(this));
        }

activity_main.xml :

<RelativeLayout 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=".MainActivity"
    android:id="@+id/mainLayout"
    android:background="#FFFFFF"> />
    <view
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        class="android.support.v4.view.ViewPager"
        android:id="@+id/pager"/>
</RelativeLayout>

我将 SectionsPagerAdapter 用于 FragmentPageAdapter

SectionsPagerAdapter.java:

public class SectionsPagerAdapter extends FragmentPagerAdapter {

    protected Context mContext;

    public SectionsPagerAdapter(Context context, FragmentManager fm) {
        super(fm);
        mContext = context;
    }

    @Override
    public Fragment getItem(int position) {
        switch(position) {
            case 0:
                return new FirstFragment();
            case 1:
                return new LoginFragment();
        }

        return null;
    }

    @Override
    public int getCount() {
        return 2;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
            case 0:
                return mContext.getString(R.string.title_section1).toUpperCase(l);
            case 1:
                return mContext.getString(R.string.title_section2).toUpperCase(l);
        }
        return null;
    }
}

FirstFragment.java:

public class FirstFragment extends ListFragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.activity_main,
                container, false);

        return rootView;
    }
}

LoginFragment.java :

    public class LoginFragment extends ListFragment {

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.activity_login,
                    container, false);

            return rootView;
        }
}

这是我的 AndroidManifest.xml :

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

    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.USE_CREDENTIALS" /> <!-- To retrieve the account name (email) as part of sign-in: -->
    <uses-permission android:name="android.permission.GET_ACCOUNTS" /> <!-- To auto-complete the email text field in the login form with the user's emails -->
    <uses-permission android:name="android.permission.READ_PROFILE" />
    <uses-permission android:name="android.permission.READ_CONTACTS" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:screenOrientation="portrait" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".LoginActivity"
            android:label="@string/title_activity_login"
            android:parentActivityName=".MainActivity"
            android:screenOrientation="portrait"
            android:windowSoftInputMode="adjustResize|stateVisible" >
        </activity>
    </application>

</manifest>

还有我的 styles.xml :

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <!-- Customize your theme here. -->
    </style>

</resources>

Answer 1

尝试将这行代码添加到您的清单文件中

  <uses-sdk
    android:minSdkVersion="11"
    android:targetSdkVersion="14" />

Answer 2

将此代码删除到单独的方法中，例如：

private void initUI(){
final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        mSectionsPagerAdapter = new SectionsPagerAdapter(this,
                getSupportFragmentManager());

        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        mViewPager
                .setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
                    @Override
                    public void onPageSelected(int position) {
                        actionBar.setSelectedNavigationItem(position);
                    }
                });

        for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
            actionBar.addTab(actionBar.newTab()
                    .setText(mSectionsPagerAdapter.getPageTitle(i))
                    .setTabListener(this));
        }
}

并从 onResume() 而不是 onCreate() 调用此方法。您还使用 getActionBar()，同时使用 getSupportFragmentManager() 也许您应该使用 getSupportActionBar()？您项目的最低 API 级别是多少？

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