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Question

我跟随这个教程：#127 Android Scrollable Tabs Part 1:Android Tutorial For Beginners 创建了三个具有不同布局的碎片，但是我收到了 java.lang.NullPointerException 的错误信息。我不知道哪里出了问题。

MainActivity.java：

public class MainActivity extends FragmentActivity implements TabListener {

ViewPager viewPager;
ActionBar actionBar;

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

    viewPager = (ViewPager) findViewById(R.id.pager);
    viewPager.setAdapter(new MyAdapter(getSupportFragmentManager()));
    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int position) {
            // TODO Auto-generated method stub
            actionBar.setSelectedNavigationItem(position);
        }
        @Override
        public void onPageScrolled(int position, float arg1, int pixels) {
            // TODO Auto-generated method stub  
        }
        @Override
        public void onPageScrollStateChanged(int position) {
            // TODO Auto-generated method stub
        }
    });

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

    ActionBar.Tab tab1 = actionBar.newTab();
    tab1.setText("Tab 1");
    tab1.setTabListener(this);

    ActionBar.Tab tab2 = actionBar.newTab();
    tab2.setText("Tab 2");
    tab2.setTabListener(this);

    ActionBar.Tab tab3 = actionBar.newTab();
    tab3.setText("Tab 3");
    tab3.setTabListener(this);

    actionBar.addTab(tab1);
    actionBar.addTab(tab2);
    actionBar.addTab(tab3);
}

@Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
    // TODO Auto-generated method stub      
}
@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
    // TODO Auto-generated method stub
    viewPager.setCurrentItem(tab.getPosition());
}
@Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
    // TODO Auto-generated method stub  
}
    }

    class MyAdapter extends FragmentPagerAdapter {

public MyAdapter(FragmentManager fm) {
    super(fm);
    // TODO Auto-generated constructor stub
}
@Override
public Fragment getItem(int position) {
    // TODO Auto-generated method stub
    Fragment fragment = null;

    switch(position)
    {
    case 0:
        fragment = new FragmentA();
        break;      
    case 1:
        fragment = new FragmentB();
        break;  
    case 2:
        fragment = new FragmentC();
        break;
    }
    return fragment;
}
@Override
public int getCount() {
    // TODO Auto-generated method stub
    return 3;
}
   }

这里有所有东西，包括ActionBars、ViewPager和Adapter。请问有谁能告诉我错误在哪里？

LogCat：

05-05 16:38:22.670: E/AndroidRuntime(13118): FATAL EXCEPTION: main
05-05 16:38:22.670: E/AndroidRuntime(13118): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.kiko.swipetabs/com.kiko.swipetabs.MainActivity}: java.lang.NullPointerException
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2099)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.access$600(ActivityThread.java:138)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1205)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.os.Handler.dispatchMessage(Handler.java:99)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.os.Looper.loop(Looper.java:137)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.main(ActivityThread.java:4929)
05-05 16:38:22.670: E/AndroidRuntime(13118): at java.lang.reflect.Method.invokeNative(Native Method)
05-05 16:38:22.670: E/AndroidRuntime(13118): at java.lang.reflect.Method.invoke(Method.java:511)
05-05 16:38:22.670: E/AndroidRuntime(13118): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:798)
05-05 16:38:22.670: E/AndroidRuntime(13118): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:565)
05-05 16:38:22.670: E/AndroidRuntime(13118): at dalvik.system.NativeStart.main(Native Method)
05-05 16:38:22.670: E/AndroidRuntime(13118): Caused by: java.lang.NullPointerException
05-05 16:38:22.670: E/AndroidRuntime(13118): at com.kiko.swipetabs.MainActivity.onCreate(MainActivity.java:44)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.Activity.performCreate(Activity.java:5254)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1082)
05-05 16:38:22.670: E/AndroidRuntime(13118): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2038)
05-05 16:38:22.670: E/AndroidRuntime(13118): ... 11 more

实际上，那个视频中的人将代码发布在了GitHub上。这是代码：

public class MainActivity extends FragmentActivity {

ViewPager viewPager=null;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    viewPager= (ViewPager) findViewById(R.id.pager);
    FragmentManager fragmentManager=getSupportFragmentManager();
    viewPager.setAdapter(new MyAdapter(fragmentManager));
}

}

class MyAdapter extends FragmentStatePagerAdapter
{

public MyAdapter(FragmentManager fm) {
    super(fm);
}

@Override
public Fragment getItem(int i) {
    Fragment fragment=null;
//        Log.d("VIVZ", "get Item is called "+i);
    if(i==0)
    {
        fragment=new FragmentA();
    }
    if(i==1)
    {
        fragment=new FragmentB();
    }
    if(i==2)
    {
        fragment=new FragmentC();
    }
    return fragment;
}

@Override
public int getCount() {
//        Log.d("VIVZ", "get Count is called");
    return 3;
}

@Override
public CharSequence getPageTitle(int position) {
    if(position==0)
    {
        return "Tab 1";
    }
    if(position==1)
    {
        return "Tab 2";
    }
    if(position==2)
    {
        return "Tab 3";
    }
    return null;
}
}

这段代码是可行的，但是这里的标题栏没有ActionBar，只有片段（Fragments）在滑动。

- KiKo

---

请发布logcat输出。 - matiash

请发布输出/错误信息。 - Anubian Noob

发布于.................. - KiKo

这个：actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); - KiKo

Answer 1

1. Your Android SDK version (<uses-sdk android:minSdkVersion="11" />) must be higher than 11.

2. Your theme must have an actionbar like

   android:theme="Theme.AppCompat"
   

3. Don't use this code in your activity:

   requestWindowFeature(Window.FEATURE_NO_TITLE);
   

4. Instead of import android.support.v4.app.ActionBar use import android.support.v7.app.ActionBar in your activity.

5. Change actionBar=getActionBar(); to actionbar=getSupportActionBar();.

- Hojjat Mehri

Answer 2

getActionBar() 似乎返回了 null。这可能是因为主题指定该 Activity 不应该有操作栏。

尝试调用

getWindow().requestFeature(Window.FEATURE_ACTION_BAR);

在 setContentView() 之前。

- matiash

---

嗯，这很奇怪。它是在同一行崩溃吗？ - matiash

实际上是在第46行，但是代码相同：actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); - KiKo

你是否针对API级别>=11，并在AndroidManifest.xml文件中将"Theme.Holo"设置为应用程序主题？ - matiash

哦哦哦哦哦哦哦哦哦哦哦哦哦哦 哈哈 我之前在 MainActivity 中使用了 android:theme="@android:style/Theme.Light.NoTitleBar.Fullscreen"，但是现在我删除了它，现在好使了 哈哈 - KiKo

NoTitleBar = 没有标题栏。 :) - matiash

但是我该如何移除应用程序名称和状态栏，以使我的选项卡置于顶部？ - KiKo