滑动选项卡 - NullPointerException - Android

Question

我遵循本教程：#127 Android Scrollable Tabs Part 1:Android Tutorial For Beginners 并创建了三个具有三种布局的片段，但我得到了 java.lang.NullPointerException。我不知道哪里错了。

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;
}
   }

这就是一切，ActionBars、ViewPager 和 Adapter。有人可以告诉我错误在哪里吗？

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，视频中的人将代码发布在 GutHub 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;
}
}

此代码有效，但此处标题上没有 ActionBar，只有片段在滑动。

Answer 1

1. 您的 Android SDK 版本 (<uses-sdk android:minSdkVersion="11" />) 必须高于 11。

2. 您的主题必须具有类似的操作栏

   android:theme="Theme.AppCompat"
   

3. 不要在您的活动中使用此代码：

   requestWindowFeature(Window.FEATURE_NO_TITLE);
   

4. 在您的活动中使用import android.support.v7.app.ActionBar 而不是import android.support.v4.app.ActionBar。

5. 将actionBar=getActionBar(); 更改为actionbar=getSupportActionBar();。

Answer 2

getActionBar() 似乎返回 null。如果主题指定 Activity 不应具有操作栏，则可能会发生这种情况。

尝试调用

getWindow().requestFeature(Window.FEATURE_ACTION_BAR);

在setContentView()之前。