我有一个主要活动的应用程序调用3个片段。我在从第一个片段访问getActivity的阶段看到应用程序启动时出现空指针异常。
E/AndroidRuntime: FATAL EXCEPTION: main Process: com.inc.sr, PID: 3019
java.lang.NullPointerException: Attempt to invoke virtual method
'android.support.v7.app.ActionBar
com.inc.sr.MainActivity.getSupportActionBar()' on a null object
reference
at com.inc.sr.fragments.SRFrag.setUserVisibleHint(SRFrag.java:148)
at android.support.v4.app.FragmentPagerAdapter.setPrimaryItem(FragmentPagerAdapter.java:136)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1234)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1084)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1614)
at android.view.View.measure(View.java:18788)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:901)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643)
at android.view.View.measure(View.java:18788)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1452)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
at android.view.Choreographer.doCallbacks(Choreographer.java:670)
at android.view.Choreographer.doFrame(Choreographer.java:606)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 12-04
13:25:33.229 1261-2076/? E/SurfaceFlinger: ro.sf.lcd_density must be
defined as a build property 12-04 13:25:33.237 3019-3042/com.inc.sr
E/FA: Failed to send current screen to service 12-04 13:25:33.237
3019-3042/com.inc.sr E/FA: Discarding data. Failed to send event to
service 12-04 13:25:33.238 3019-3042/com.inc.sr E/FA: Failed to send
current screen to service 12-04 13:26:22.703
2452-2870/com.google.android.gms E/SQLiteCastStore: End saving paired
guest mode devices 12-04 13:30:33.228 1634-3092/system_process
E/Surface: getSlotFromBufferLocked: unknown buffer: 0xb40be100 12-04
13:30:33.239 1634-1645/system_process E/JavaBinder: !!! FAILED BINDER
TRANSACTION !!! (parcel size = 104) 12-04 13:30:33.244
1634-1645/system_process E/JavaBinder: !!! FAILED BINDER TRANSACTION
!!! (parcel size = 104)
主要活动：
public class MainActivity extends AppCompatActivity {
private Toolbar toolbar;
private TabLayout tabLayout;
private CustomViewPager viewPager;
private MyAdapter mAdapter;
TextView mTitle;
String[] title = {"S screen", "Compare", "About US"};
private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//FirebaseDatabase.getInstance().setPersistenceEnabled(true);
if (findViewById(R.id.fragment_container) != null) {
if (savedInstanceState != null) {
return;
}
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(false);
//getSupportActionBar().setDisplayShowHomeEnabled(true);
getSupportActionBar().setDisplayShowTitleEnabled(false);
mTitle = (TextView) toolbar.findViewById(R.id.toolbar_title);
mTitle.setText(title[0]);
mAdapter = new MyAdapter(getSupportFragmentManager());
//viewPager = (ViewPager)findViewById(R.id.viewpager);
viewPager = (CustomViewPager) findViewById(R.id.viewpager);
viewPager.setAdapter(mAdapter);
viewPager.setPagingEnabled(false);
tabLayout = (TabLayout) findViewById(R.id.tabs);
tabLayout.setupWithViewPager(viewPager);
tabLayout.getTabAt(0).setIcon(R.mipmap.searchs);
tabLayout.getTabAt(1).setIcon(R.mipmap.compareu);
tabLayout.getTabAt(2).setIcon(R.mipmap.infou);
tabLayout.setOnTabSelectedListener(
new TabLayout.ViewPagerOnTabSelectedListener(viewPager) {
@Override
public void onTabSelected(TabLayout.Tab tab) {
super.onTabSelected(tab);
int pos = tab.getPosition();
switch (pos) {
case 0:
tabLayout.getTabAt(0).setIcon(R.mipmap.searchs);
break;
case 1:
tabLayout.getTabAt(1).setIcon(R.mipmap.compares);
break;
case 2:
tabLayout.getTabAt(2).setIcon(R.mipmap.infos);
break;
}
}
@Override
public void onTabUnselected(TabLayout.Tab tab) {
super.onTabUnselected(tab);
int pos = tab.getPosition();
switch (pos) {
case 0:
tabLayout.getTabAt(0).setIcon(R.mipmap.searchu);
break;
case 1:
tabLayout.getTabAt(1).setIcon(R.mipmap.compareu);
break;
case 2:
tabLayout.getTabAt(2).setIcon(R.mipmap.infou);
break;
}
}
@Override
public void onTabReselected(TabLayout.Tab tab) {
super.onTabReselected(tab);
}
}
);
viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
}
@Override
public void onPageSelected(int position) {
FragmentManager fragmentManager = MainActivity.this.getSupportFragmentManager();
List<Fragment> fragments = fragmentManager.getFragments();
if (fragments != null) {
for (Fragment fragment : fragments) {
if (fragment != null && fragment.isVisible()) {
switch (position) {
case 0:
if (fragment.getClass().toString().contains("srFrag")) {
getSupportActionBar().setDisplayHomeAsUpEnabled(false);
mTitle.setText(title[position]);
} else if (fragment.getClass().toString().contains("sresFrag")) {
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
String state = fragment.getArguments().getString("STATES", "");
String grade = fragment.getArguments().getString("GRADES", "");
String title = state + " - " + grade + " Ranking";
mTitle.setText(title);
} else if (fragment.getClass().toString().contains("sdFrag")) {
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
String title = fragment.getArguments().getString("SCNAME", "");
mTitle.setText(title);
}
break;
case 1:
if (fragment.getClass().toString().contains("CompareFrag")) {
getSupportActionBar().setDisplayHomeAsUpEnabled(false);
mTitle.setText(title[position]);
} else if (fragment.getClass().toString().contains("CompareResultsFrag")) {
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
mTitle.setText("Compare Results");
}
break;
case 2:
if (fragment.getClass().toString().contains("InfoFrag")) {
getSupportActionBar().setDisplayHomeAsUpEnabled(false);
mTitle.setText(title[position]);
}
break;
}
}
}
}
}
@Override
public void onPageScrollStateChanged(int state) {
}
});
}
mAuthListener = new FirebaseAuth.AuthStateListener() {
@Override
public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
FirebaseUser user = firebaseAuth.getCurrentUser();
if (user != null) {
// User is signed in
System.out.println("AUTH FirebaseAuth onAuthStateChanged:signed_in:" + user.getUid());
} else {
// User is signed out
System.out.println("AUTH FirebaseAuth onAuthStateChanged:signed_out");
}
}
};
mAuth = FirebaseAuth.getInstance();
mAuth.signInAnonymously()
.addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
System.out.println("AUTH FirebaseAuth signInAnonymously:onComplete:" + task.isSuccessful());
if (!task.isSuccessful()) {
System.out.println("AUTH FirebaseAuth signInAnonymously" + task.getException());
}
}
});
}
@Override
public void onStart() {
super.onStart();
mAuth.addAuthStateListener(mAuthListener);
}
@Override
public void onStop() {
super.onStop();
if (mAuthListener != null) {
mAuth.removeAuthStateListener(mAuthListener);
}
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// handle arrow click here
if (item.getItemId() == android.R.id.home) {
//finish(); // close this activity and return to preview activity (if there is any)
onBackPressed();
}
return super.onOptionsItemSelected(item);
}
public void onBackPressed() {
if (viewPager.getCurrentItem() == 0) {
if (mAdapter.getItem(0) instanceof sresFrag) {
((sresFrag) mAdapter.getItem(0)).backPressed();
} else if (mAdapter.getItem(0) instanceof sdFrag) {
((sdFrag) mAdapter.getItem(0)).backPressed();
} else {
this.moveTaskToBack(true);
}
} else if (viewPager.getCurrentItem() == 1) {
if (mAdapter.getItem(1) instanceof CompareResultsFrag) {
((CompareResultsFrag) mAdapter.getItem(1)).backPressed();
} else if (mAdapter.getItem(1) instanceof CompareFrag) {
viewPager.setCurrentItem(0);
}
} else if (viewPager.getCurrentItem() == 2) {
if (mAdapter.getItem(2) instanceof InfoFrag) {
viewPager.setCurrentItem(1);
}
}
}
private static class MyAdapter extends FragmentPagerAdapter {
private final class FirstPageListener implements
FirstPageFragmentListener {
public void onSwitchToNextFragment(String year, String state, String grade, SchoolItem item, boolean backPressed) {
mFragmentManager.beginTransaction().remove(mFragmentAtPos0)
.commit();
if (mFragmentAtPos0 instanceof srFrag) {
mFragmentAtPos0 = sresFrag.createInstance(FirstListener);
Bundle args = new Bundle();
args.putString("YEARS", year);
args.putString("STATES", state);
args.putString("GRADES", grade);
mFragmentAtPos0.setArguments(args);
} else if (mFragmentAtPos0 instanceof sresFrag && !(backPressed)) {
mFragmentAtPos0 = sdFrag.createInstance(FirstListener);
Bundle args = new Bundle();
args.putString("YEARS", year);
args.putString("STATES", state);
args.putString("GRADES", grade);
args.putSerializable("ITEM", item);
args.putString("SCNAME", item.Name);
mFragmentAtPos0.setArguments(args);
} else if (mFragmentAtPos0 instanceof sdFrag && (backPressed)) {
mFragmentAtPos0 = sresFrag.createInstance(FirstListener);
Bundle args = new Bundle();
System.out.println(year + state + grade);
args.putString("YEARS", year);
args.putString("STATES", state);
args.putString("GRADES", grade);
mFragmentAtPos0.setArguments(args);
} else { // Instance of NextFragment
mFragmentAtPos0 = srFrag.createInstance(FirstListener);
System.out.println("UNI00 A");
}
notifyDataSetChanged();
}
}
private final class SecondPageListener implements
SecondPageFragmentListener {
public void onSwitchToNextFragment(ArrayList<SchoolItem> schoolList, SchoolItem[] compareList, boolean backPressed) {
mFragmentManager.beginTransaction().remove(mFragmentAtPos1)
.commit();
if (mFragmentAtPos1 instanceof CompareFrag) {
mFragmentAtPos1 = CompareResultsFrag.createInstance(SecondListener);
} else { // Instance of NextFragment
mFragmentAtPos1 = CompareFrag.createInstance(SecondListener);
}
Bundle args = new Bundle();
args.putSerializable("SCLIST", schoolList);
args.putSerializable("CMLIST", compareList);
mFragmentAtPos1.setArguments(args);
notifyDataSetChanged();
}
}
private final class ThirdPageListener implements
ThirdPageFragmentListener {
public void onSwitchToNextFragment() {
mFragmentManager.beginTransaction().remove(mFragmentAtPos2)
.commit();
mFragmentAtPos2 = InfoFrag.createInstance(ThirdListener);
notifyDataSetChanged();
}
}
private String[] titles = {"Search", "Compare", " Info"};
private final FragmentManager mFragmentManager;
public Fragment mFragmentAtPos0;
public Fragment mFragmentAtPos1;
public Fragment mFragmentAtPos2;
private Context context;
FirstPageListener FirstListener = new FirstPageListener();
SecondPageListener SecondListener = new SecondPageListener();
ThirdPageListener ThirdListener = new ThirdPageListener();
public MyAdapter(FragmentManager fragmentManager) {
super(fragmentManager);
mFragmentManager = fragmentManager;
System.out.println("UNI00 B");
}
@Override
public CharSequence getPageTitle(int position) {
return titles[position];
}
@Override
public Fragment getItem(int position) {
switch (position) {
case 0: // Fragment # 0
if (mFragmentAtPos0 == null) {
//mFragmentAtPos0 = new OneFragment(listener);
mFragmentAtPos0 = srFrag.createInstance(FirstListener);
System.out.println("UNI00 1");
}
System.out.println(mFragmentAtPos0.toString());
return mFragmentAtPos0;
case 1: // Fragment # 1
if (mFragmentAtPos1 == null) {
//mFragmentAtPos0 = new OneFragment(listener);
mFragmentAtPos1 = CompareFrag.createInstance(SecondListener);
System.out.println("UNI00 2");
}
//return new BuyMainFrag();
return mFragmentAtPos1;
case 2:// Fragment # 2
if (mFragmentAtPos2 == null) {
//mFragmentAtPos0 = new OneFragment(listener);
mFragmentAtPos2 = InfoFrag.createInstance(ThirdListener);
System.out.println("UNI00 3");
}
return mFragmentAtPos2;
//return new SellMainFrag();
}
System.out.println("UNI00 4");
return null;
}
@Override
public int getCount() {
return titles.length;
}
@Override
public int getItemPosition(Object object) {
if (object instanceof srFrag &&
mFragmentAtPos0 instanceof sresFrag) {
return POSITION_NONE;
}
if (object instanceof sresFrag &&
mFragmentAtPos0 instanceof srFrag) {
return POSITION_NONE;
}
if (object instanceof sresFrag &&
mFragmentAtPos0 instanceof sdFrag) {
return POSITION_NONE;
}
if (object instanceof sdFrag &&
mFragmentAtPos0 instanceof srFrag) {
return POSITION_NONE;
}
if (object instanceof CompareFrag &&
mFragmentAtPos1 instanceof CompareResultsFrag) {
return POSITION_NONE;
}
if (object instanceof CompareResultsFrag &&
mFragmentAtPos1 instanceof CompareFrag) {
return POSITION_NONE;
}
if (object instanceof InfoFrag) {
return POSITION_NONE;
}
return POSITION_UNCHANGED;
}
}
public void setToolBarTitle(String title) {
mTitle.setText(title);
}
}
First Fragment ：
public class srFrag extends Fragment {
Spinner spinnerYear, spinnerState, spinnerGrade;
static FirstPageFragmentListener firstPageListener;
private String year = "", state = "", grade = "";
String FragTitle = "s screen";
private SchoolItem notUsed;
public static srFrag createInstance(FirstPageFragmentListener listener) {
srFrag frag = new srFrag();
firstPageListener = listener;
return frag;
}
public srFrag() {
// Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setRetainInstance(true);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.s_r, container, false);
spinnerYear = (Spinner) view.findViewById(R.id.spinnerYear);
ArrayAdapter<CharSequence> adapterYear = ArrayAdapter
.createFromResource(view.getContext(), R.array.year, R.layout.spinner_row);
spinnerYear.setAdapter(adapterYear);
spinnerState = (Spinner) view.findViewById(R.id.spinnerState);
ArrayAdapter<CharSequence> adapterState = ArrayAdapter
.createFromResource(view.getContext(), R.array.state, R.layout.spinner_row);
spinnerState.setAdapter(adapterState);
spinnerGrade = (Spinner) view.findViewById(R.id.spinnerGrade);
ArrayAdapter<CharSequence> adapterGrade = ArrayAdapter
.createFromResource(view.getContext(), R.array.grade, R.layout.spinner_row);
spinnerGrade.setAdapter(adapterGrade);
spinnerYear.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos,
long arg3) {
year = parent.getItemAtPosition(pos).toString();
SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPref.edit();
editor.putString("YEARS", year);
editor.commit();
}
@Override
public void onNothingSelected(AdapterView<?> arg0) {
}
});
spinnerState.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos,
long arg3) {
state = parent.getItemAtPosition(pos).toString();
SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPref.edit();
editor.putString("STATES", state);
editor.commit();
}
@Override
public void onNothingSelected(AdapterView<?> arg0) {
}
});
spinnerGrade.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos,
long arg3) {
grade = parent.getItemAtPosition(pos).toString();
SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPref.edit();
editor.putString("GRADES", grade);
editor.commit();
}
@Override
public void onNothingSelected(AdapterView<?> arg0) {
}
});
Button button = (Button) view.findViewById(R.id.search_button);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
firstPageListener.onSwitchToNextFragment(year, state, grade, notUsed, false);
}
});
return view;
}
@Override
public void onResume() {
super.onResume();
}
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser) {
((MainActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(false); //Null pointer exception thrown here
((MainActivity) getActivity()).setToolBarTitle(FragTitle);
}
}
}
答案 0 :(得分：1)
如果片段未附加到Activity，则getActivity将返回null。因此，您需要检查此值是否为null或检查附加到活动的片段或onAttach（）之后。
boolean isAttached = false;
@Override
public void onAttach(Context context) {
super.onAttach(context);
isAttached=true;
// save the context here if needed
}
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if(getActivity()!=null){// Or use the context saved in onAttach()
// Do your stuff
}
}
我遇到了同样的问题，因为当getActivity（）返回上下文时，setUserVisibleHint不会在第二个实例上被触发。 因此，管理是使用Listener更好的方式。使用 ViewPager.OnPageChangeListener 通知您当前的片段。
答案 1 :(得分：0)
是的，它会抛出错误，因为它会在你的布局没有被加载时调用它所以你必须等待一段时间，
这个技巧可能对你有帮助
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser) {
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
((MainActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(false); //Null pointer exception thrown here
((MainActivity) getActivity()).setToolBarTitle(FragTitle);
}
}, 500);
}
}
答案 2 :(得分：0)
这样做
申报 私人活动活动;
在上面。然后写下面的函数
@Override
public void onAttach(Context context)
{
super.onAttach(context);
if (context instanceof Activity)
{
activity = (Activity) context;
}
}
然后像这样使用
if (isVisibleToUser) {
(activity).getSupportActionBar().setDisplayHomeAsUpEnabled(false); //Null pointer exception thrown here
(activity).setToolBarTitle(FragTitle);
}