我的申请需要NagivationDrawer以及ViewPager。当用户选择抽屉中的项目时，视图寻呼机将相应地设置片段。用户也可以在页面（片段）之间轻扫。
我的代码如下
public class MainActivity extends AppCompatActivity
implements NavigationDrawerFragment.NavigationDrawerCallbacks {
private NavigationDrawerFragment mNavigationDrawerFragment;
private CharSequence mTitle;
private ViewPager mViewPager;
private LessonPageAdapter lessonPageAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mNavigationDrawerFragment = (NavigationDrawerFragment)
getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
mTitle = getTitle();
// Set up the drawer.
mNavigationDrawerFragment.setUp(
R.id.navigation_drawer,
(DrawerLayout) findViewById(R.id.drawer_layout));
lessonPageAdapter = new LessonPageAdapter(getSupportFragmentManager());
this.mViewPager = (ViewPager) findViewById(R.id.pager);
this.mViewPager.setAdapter(lessonPageAdapter);
}
@Override
public void onNavigationDrawerItemSelected(int position) {
Bundle args = new Bundle();
args.putInt(LessonFragment.ARG_SECTION_NUMBER, position + 1);
//this causes null pointer exception
mViewPager.setCurrentItem(position + 1);
}
........
但mViewPager.setCurrentItem(position + 1)函数中的onNavigationDrawerItemSelected导致空指针异常（如下所示），我发现MainActivity方法中onNavigationDrawerItemSelected的所有实例变量都为空。
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.khaino.drawerandswipedemo/com.example.khaino.drawerandswipedemo.MainActivity}: android.view.InflateException: Binary XML file line #29: Error inflating class fragment
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
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:5257)
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: android.view.InflateException: Binary XML file line #29: Error inflating class fragment
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:763)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:806)
at android.view.LayoutInflater.inflate(LayoutInflater.java:504)
at android.view.LayoutInflater.inflate(LayoutInflater.java:414)
at android.view.LayoutInflater.inflate(LayoutInflater.java:365)
at android.support.v7.app.AppCompatDelegateImplV7.setContentView(AppCompatDelegateImplV7.java:255)
at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:109)
at com.example.khaino.drawerandswipedemo.MainActivity.onCreate(MainActivity.java:37)
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:2390)
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:5257)
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 invoke virtual method 'void android.support.v4.view.ViewPager.setCurrentItem(int)' on a null object reference
at com.example.khaino.drawerandswipedemo.MainActivity.onNavigationDrawerItemSelected(MainActivity.java:66)
at com.example.khaino.drawerandswipedemo.NavigationDrawerFragment.selectItem(NavigationDrawerFragment.java:210)
at com.example.khaino.drawerandswipedemo.NavigationDrawerFragment.onCreate(NavigationDrawerFragment.java:81)
at android.support.v4.app.Fragment.performCreate(Fragment.java:1939)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:988)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1185)
at android.support.v4.app.FragmentManagerImpl.addFragment(FragmentManager.java:1287)
at android.support.v4.app.FragmentManagerImpl.onCreateView(FragmentManager.java:2243)
at android.support.v4.app.FragmentController.onCreateView(FragmentController.java:111)
at android.support.v4.app.FragmentActivity.dispatchFragmentsOnCreateView(FragmentActivity.java:278)
at android.support.v4.app.BaseFragmentActivityHoneycomb.onCreateView(BaseFragmentActivityHoneycomb.java:31)
at android.support.v4.app.FragmentActivity.onCreateView(FragmentActivity.java:78)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:733)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:806)
at android.view.LayoutInflater.inflate(LayoutInflater.java:504)
at android.view.LayoutInflater.inflate(LayoutInflater.java:414)
at android.view.LayoutInflater.inflate(LayoutInflater.java:365)
at android.support.v7.app.AppCompatDelegateImplV7.setContentView(AppCompatDelegateImplV7.java:255)
at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:109)
at com.example.khaino.drawerandswipedemo.MainActivity.onCreate(MainActivity.java:37)
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:2390)
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:5257)
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)
任何人都可以告诉我原因以及获得我的申请要求的解决方案。
答案 0 :(得分：0)
堆栈跟踪显示，在设置onCreate字段之前，会从mViewPager间接调用该方法。
在setContentView调用之后直接移动字段分配应解决该问题：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
this.mViewPager = (ViewPager) findViewById(R.id.pager);
lessonPageAdapter = new LessonPageAdapter(getSupportFragmentManager());
this.mViewPager.setAdapter(lessonPageAdapter);
mNavigationDrawerFragment = (NavigationDrawerFragment)
getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
mTitle = getTitle();
// Set up the drawer.
mNavigationDrawerFragment.setUp(
R.id.navigation_drawer,
(DrawerLayout) findViewById(R.id.drawer_layout));
}
答案 1 :(得分：0)
当我打印执行步骤时，它会显示首先调用onCreate初始化的mViewPager方法，然后调用onNavigationDrawerItemSelected，如下所示
11-01 11:47:07.056 15082-15082/? D/step﹕ onCreate
11-01 11:47:07.153 15082-15082/? D/step﹕ onNavigationDrawerItemSelected
为了避免在运行应用程序时出现空指针异常，我检查mViewPager是否为null，如下所示，并且工作正常
@Override
public void onNavigationDrawerItemSelected(int position) {
Bundle args = new Bundle();
args.putInt(LessonFragment.ARG_SECTION_NUMBER, position + 1);
Log.d("step", "onNavigationDrawerItemSelected");
//to void null pointer exception when I run the app
if(mViewPager != null){
mViewPager.setCurrentItem(position + 1);
}
}