我有一个带有操作栏的主要活动。在运行时，尽管actionbar对象初始化为getActionBar（），我收到带有logcat的NPE 输出
主要活动：
public class MainActivity extends FragmentActivity implements ActionBar.TabListener {
private ViewPager mViewPager;
private MyTabsPagerAdapter mPagerAdapter;
private ActionBar mActionBar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
List<Fragment> mFragList = new ArrayList<Fragment>();
mFragList.add(new Aufgabe_1());
mFragList.add(new Fragment02());
mFragList.add(new Fragment03());
mViewPager = (ViewPager) findViewById(R.id.pager);
mActionBar = getActionBar();
mActionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
mPagerAdapter = new MyTabsPagerAdapter(getSupportFragmentManager(), mFragList);
mViewPager.setAdapter(mPagerAdapter);
for(int i=0; i<mFragList.size(); i++) {
mActionBar.addTab(mActionBar.newTab().setText("Aufgabe_"+(i+1)).setTabListener(this));
}
mViewPager.setOnPageChangeListener(new OnPageChangeListener() {
@Override
public void onPageSelected(int arg0) {
// TODO Auto-generated method stub
mActionBar.setSelectedNavigationItem(arg0);
}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
// TODO Auto-generated method stub
}
@Override
public void onPageScrollStateChanged(int arg0) {
// TODO Auto-generated method stub
}
});
}
@Override
public void onTabReselected(Tab arg0, FragmentTransaction arg1) {
// TODO Auto-generated method stub
}
@Override
public void onTabSelected(Tab arg0, FragmentTransaction arg1) {
// TODO Auto-generated method stub
mViewPager.setCurrentItem(arg0.getPosition());
//mActionBar.setSelectedNavigationItem(arg0.getPosition());
}
@Override
public void onTabUnselected(Tab arg0, FragmentTransaction arg1) {
// TODO Auto-generated method stub
}
}
logcat的：
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: FATAL EXCEPTION: main
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: Process: com.example.com.myapplication, PID: 26079
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.com.myapplication/com.example.com.vpager_02.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setNavigationMode(int)' on a null object reference
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2689)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2754)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread.access$900(ActivityThread.java:177)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.os.Looper.loop(Looper.java:145)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5938)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:372)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setNavigationMode(int)' on a null object reference
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at com.example.com.vpager_02.MainActivity.onCreate(MainActivity.java:33)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.Activity.performCreate(Activity.java:6288)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2642)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2754)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread.access$900(ActivityThread.java:177)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.os.Looper.loop(Looper.java:145)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5938)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:372)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
02-09 15:34:35.080 26079-26079/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)
02-09 15:34:35.080 795-1150/? V/ApplicationPolicy: isApplicationStateBlocked userId 0 pkgname com.example.com.myapplication
答案 0 :(得分：1)
而不是FragmentActivity的延伸，只需从AppCompatActivity延伸，然后拨打getSupportActionBar()而不是getActionBar()
答案 1 :(得分：0)
使用support:appcompat库来使用ToolBar / ActionBar。
AppCompat（又名ActionBarCompat）开始作为运行在Gingerbread上的设备的Android 4.0 ActionBar API的后端，在后端实现和框架实现之上提供了一个通用的API层。
使用AppCompatActivity代替FragmentActivity
getSupportActionBar()代替getActionBar()
答案 2 :(得分：0)
导入ActionBar import android.support.v7.app.ActionBar;
从AppCompatActivity
然后：
mActionBar = getSupportActionBar();
mActionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); //but this is deprecated