无法将Activity界面附加到Fragment。获取NullPointerException

我正在为学校编写一个应用程序，当我在模拟器中运行应用程序时，一切正常。无法将Activity界面附加到Fragment。获取NullPointerException

问题是当我通过USB连接手机并在手机上运行应用程序时。

每当我做一些需要mListener（在整个应用程序的任何片段中）的东西时，我在我的mListener上会得到一个nullpointerexception。

我已经搜索，但还没有找到解决方案或类似的问题。

我在我所有的碎片的问题，但我会只显示1个片段在这里：

片段的代码：

public class QuizFragment extends Fragment { 

private OnFragmentQuizInteractionListener mListener; 

List<Vraag> quesList; 
int qid=0; 
Vraag currentQ; 
TextView txtQuestion; 
RadioButton rda, rdb, rdc; 
Button butNext; 
TextView txtResult; 

Button btnToonAntwoord; 


@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
    getActivity().setTitle("Quiz"); 
    final View view = inflater.inflate(R.layout.quiz_layout, container, false); 

    //btnStart = (Button) getActivity().findViewById(R.id.btnStart); 
    //btnStart.setOnClickListener(new View.OnClickListener() { 
    // @Override 
    // public void onClick(View v) { 
     //  mListener.initQuiz(); 
     // } 
    //}); 

    //reset the score to 0 
    mListener.resetScore(); 


    qid=0; 

    QuizDbHelper db=new QuizDbHelper(view.getContext()); 
    quesList=db.getAllQuestions(); 
    currentQ=quesList.get(qid); 
    txtQuestion=(TextView) view.findViewById(R.id.textView1); 
    rda=(RadioButton)view.findViewById(R.id.radio0); 
    rdb=(RadioButton)view.findViewById(R.id.radio1); 
    rdc=(RadioButton)view.findViewById(R.id.radio2); 

    //zet de radiobuttons klaar om geklikt te worden. 
    rda.setEnabled(true); 
    rdb.setEnabled(true); 
    rdc.setEnabled(true); 


    butNext=(Button)view.findViewById(R.id.btnNext); 
    btnToonAntwoord=(Button)view.findViewById(R.id.btnToonAntwoord); 
    txtResult = (TextView) view.findViewById(R.id.resultTextview); 
    setQuestionView(view); 

    setOnClickListeners(view); 


    return view; 

} 

private void setOnClickListeners(View v){ 
    final View thisView = v; 

    butNext.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 


      if(qid<8){ 

       //zet de radiobuttons klaar om geklikt te worden. 
       rda.setEnabled(true); 
       rdb.setEnabled(true); 
       rdc.setEnabled(true); 

       //zet het resultaat terug op niets 
       txtResult.setText(""); 

       //Wanneer het goed is, voeg 1 bij de score bij 
       RadioGroup grp = (RadioGroup) thisView.findViewById(R.id.radioGroup1); 
       RadioButton answer = (RadioButton) thisView.findViewById(grp.getCheckedRadioButtonId()); 
       if (currentQ.getANSWER().equals(checkChecked(answer.getId()))) { 
        mListener.incrementScore(); 
       } 


       currentQ=quesList.get(qid); 
       setQuestionView(thisView); 
      }else{ 
       //ga naar het eindscherm met het resultaat. 
       mListener.navigeerEindscore(); 
      } 
     } 
    }); 

    btnToonAntwoord.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      RadioGroup grp = (RadioGroup) thisView.findViewById(R.id.radioGroup1); 
      RadioButton answer = (RadioButton) thisView.findViewById(grp.getCheckedRadioButtonId()); 

      //toon de juiste boodschap aan de hand van het antwoord die gegeven is. 
      if (currentQ.getANSWER().equals(checkChecked(answer.getId()))) { 
       txtResult.setText(currentQ.getGoedeFeedback()); 
      }else{ 
       txtResult.setText(currentQ.getSlechteFeedback()); 
      } 
      //zet de radiobuttons klaar om geklikt te worden. 
      rda.setEnabled(false); 
      rdb.setEnabled(false); 
      rdc.setEnabled(false); 
     } 
    }); 


} 

private String checkChecked(int id) { 
    if(id == R.id.radio0){ 
     return "A"; 
    }else if(id == R.id.radio1){ 
     return "B"; 
    }else{ 
     return "C"; 
    } 
} 


private void setQuestionView(View v) 
{ 
    txtQuestion.setText(currentQ.getQUESTION()); 
    rda.setText(currentQ.getOPTA()); 
    rdb.setText(currentQ.getOPTB()); 
    rdc.setText(currentQ.getOPTC()); 
    qid++; 
} 

@Override 
public void onAttach(Context context) { 
    super.onAttach(context); 

    if (context instanceof OnFragmentQuizInteractionListener) { 
     mListener = (OnFragmentQuizInteractionListener) context; 
    } else { 
     throw new RuntimeException(context.toString() 
       + " must implement OnResultFragmentInteractionListener"); 
    } 


} 

public interface OnFragmentQuizInteractionListener { 


    void incrementScore(); 

    void resetScore(); 

    void navigeerEindscore(); 
} 

@Override 
public void onDetach() { 
    super.onDetach(); 
    mListener = null; 
}

mainActivity：

{ 
public class MainActivity extends AppCompatActivity 
    implements  NavigationView.OnNavigationItemSelectedListener,DashboardFragment.OnFragmentLoginInteractionListener,LoginFragment.OnFragmentLoginInteractionListener, RegistratieFragment.OnFragmentRegistratieInteractionListener,QuizFragment.OnFragmentQuizInteractionListener, ResultFragment.OnResultFragmentInteractionListener{ 


private FragmentManager fragmentManager; 

public int quizScore; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 

    Utils.onActivityCreateSetTheme(this); 
    setContentView(R.layout.activity_main); 

    quizScore = 0; 


    SharedPreferences pref = PreferenceManager 
      .getDefaultSharedPreferences(this); 
    String themeName = pref.getString("Theme1", "Blauw"); 


    //toolbar ophalen 
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    Log.i("huidige preferences", themeName); 

    //toolbar instellen 
    setSupportActionBar(toolbar); 

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
      this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close); 
    drawer.addDrawerListener(toggle); 
    toggle.syncState(); 




    /**if (themeName.equals("Blauw")) { 
     setTheme(R.style.AppTheme_blauw); 

     Log.i("MyActivity", "in blauw"); 
    } else if (themeName.equals("Groen")) { 

     this.setTheme(R.style.AppTheme_groen); 
     Log.i("MyActivity", "in groen"); 
    } else if (themeName.equals("Roze")) { 
     this.setTheme(R.style.AppTheme_roze); 
     Log.i("MyActivity", "in roze"); 
    }**/ 

    fragmentManager = getFragmentManager(); 

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view); 
    navigationView.setNavigationItemSelectedListener(this); 

    fragmentManager.beginTransaction().replace(R.id.content_frame, new DashboardFragment()).commit(); 

} 


//backknop werkt hoe hij moet werken 
@Override 
public void onBackPressed() { 
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 

    int count = getFragmentManager().getBackStackEntryCount(); 


    if (drawer.isDrawerOpen(GravityCompat.START)) { 
     drawer.closeDrawer(GravityCompat.START); 
    } else { 

     if (count == 0) { 
      super.onBackPressed(); 
      //additional code 
     } else { 
      getFragmentManager().popBackStack(); 
     } 

    } 
} 

@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    // Inflate the menu; this adds items to the action bar if it is present. 
    getMenuInflater().inflate(R.menu.main, menu); 
    return true; 
} 

@Override 
public boolean onOptionsItemSelected(MenuItem item) { 
    // Handle action bar item clicks here. The action bar will 
    // automatically handle clicks on the Home/Up button, so long 
    // as you specify a parent activity in AndroidManifest.xml. 


    switch (item.getItemId()) { 
     case R.id.action_settings: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new InstellingenFragment()).commit(); 
      return true; 
    } 

    return super.onOptionsItemSelected(item); 
} 

//region dashboard 
public void MijnProfielClicked() { 
    getFragmentManager().beginTransaction().replace(R.id.content_frame, new MijnProfielFragment()) 
      .addToBackStack("MijnProfiel") 
      .commit(); 
} 

public void AlarmClicked() { 
    getFragmentManager().beginTransaction().replace(R.id.content_frame, new PilAlarmFragment()) 
      .addToBackStack("Pilalarm") 
      .commit(); 
} 

public void KalenderClicked() { 
    getFragmentManager().beginTransaction().replace(R.id.content_frame, new MenstruatieFragment()) 
      .addToBackStack("Kalender") 
      .commit(); 
} 
//endregion 


//Navigatiemethodes LOGIN 

public void Login() { 
    getFragmentManager().beginTransaction().replace(R.id.content_frame, new DashboardFragment()) 
      .addToBackStack("Login") 
      .commit(); 

} 

//Navigatiemethodes REGISTREREN 
public void Registreer() { 
    getFragmentManager().beginTransaction().replace(R.id.content_frame, new DashboardFragment()) 
      .addToBackStack("Registreer") 
      .commit(); 
} 
public void Annuleren() { 
    getFragmentManager().beginTransaction().replace(R.id.content_frame, new LoginFragment()) 
      .addToBackStack("Registreer") 
      .commit(); 
} 

@SuppressWarnings("StatementWithEmptyBody") 
@Override 
public boolean onNavigationItemSelected(MenuItem item) { 
    // Handle navigation view item clicks here. 

    switch (item.getItemId()) { 
     case R.id.nav_dashboard_layout: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new DashboardFragment()).addToBackStack("dashboard").commit(); 
      break; 
     case R.id.nav_mijnprofiel_layout: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new MijnProfielFragment()).addToBackStack("mijnprofiel").commit(); 
      break; 
     case R.id.nav_categorieen_layout: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new CategorieenFragment()).addToBackStack("categorieen").commit(); 
      break; 
     case R.id.nav_pilalarm_layout: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new PilAlarmFragment()).addToBackStack("pilalarm").commit(); 
      break; 
     case R.id.nav_menstruatie_layout: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new MenstruatieFragment()).addToBackStack("menstruatie").commit(); 
      break; 
     case R.id.nav_quiz_layout: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new QuizFragment()).addToBackStack("quiz").commit(); 
      break; 
     case R.id.nav_instellingen_layout: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new InstellingenFragment()).addToBackStack("instellingen").commit(); 
      break; 
     case R.id.nav_partners_layout: 
      fragmentManager.beginTransaction().replace(R.id.content_frame, new PartnersFragment()).addToBackStack("partners").commit(); 
      break; 


    } 

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 
    drawer.closeDrawer(GravityCompat.START); 
    return true; 
} 


//region Partner onclick listeners 
public void onVivesPartner_clicked(View view) { 
    // Perform action on click 

    goToUrl("http://www.vives.be/"); 

} 

public void onSensoaPartner_clicked(View view) { 
    // Perform action on click 

    goToUrl("http://www.sensoa.be/"); 
} 

public void onUgentPartner_clicked(View view) { 
    // Perform action on click 

    goToUrl("http://www.ugent.be/"); 
} 

public void onUCCVPartner_clicked(View view) { 
    // Perform action on click 

    goToUrl("http://www.ucvvgent.be/nl/"); 
} 

public void onOnePartner_clicked(View view) { 
    // Perform action on click 

    goToUrl("http://www.one.be//"); 
} 


private void goToUrl(String url) { 
    Uri uriUrl = Uri.parse(url); 
    Intent launchBrowser = new Intent(Intent.ACTION_VIEW, uriUrl); 
    startActivity(launchBrowser); 
} 


//endregion 


//region quizfragment 
@Override 
public void incrementScore() { 
    quizScore++; 
} 

@Override 
public void resetScore() { 
    quizScore = 0; 
} 

@Override 
public void navigeerEindscore() { 
    fragmentManager.beginTransaction().replace(R.id.content_frame, new ResultFragment()).commit(); 
} 


@Override 
public int getScore() { 
    return quizScore; 
} 

//endregion

错误我得到了（无论什么片段，它在mlistener上始终为nullpointerexception）

java.lang.NullPointerException: Attempt to invoke interface method 'void be.vives.pwo_kinderwens.Quiz.QuizFragment$OnFragmentQuizInteractionListener.resetScore()' on a null object reference 
at be.vives.pwo_kinderwens.Quiz.QuizFragment.onCreateView(QuizFragment.java:55) 
at android.app.Fragment.performCreateView(Fragment.java:2053) 
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:894) 
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067) 
at android.app.BackStackRecord.run(BackStackRecord.java:834) 
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1452) 
at android.app.FragmentManagerImpl$1.run(FragmentManager.java:447) 
at android.os.Handler.handleCallback(Handler.java:739) 
at android.os.Handler.dispatchMessage(Handler.java:95) 
at android.os.Looper.loop(Looper.java:135) 
at android.app.ActivityThread.main(ActivityThread.java:5294) 
at java.lang.reflect.Method.invoke(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:372) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)

来源

2016-04-29 Denzel van Belle

如果您在'onAttach'内'mListener.resetScore（）;'会发生什么？ –

这工作重置分数，但只要我需要mListener onclicklistener（这是一个测验，所以在几个问题后，我需要mlistener增加分数），它再次失败。那么我应该在OnAttach中设置onclicklistener来解决这个问题吗？ –

mListener为null，因为onAttach方法未被调用。请参阅下面的答案。 – Luis

问题是您的mListener未实现。

我会尝试以下方法：

我想补充的方法来QuizFragment像：

public void setFragmentQuizInteractionListener(OnFragmentQuizInteractionListener mListener) 
{ 
this.mListener = mListener;  
}

然后

case R.id.nav_quiz_layout:  
    QuizFragment quizFragment = new QuizFragment(); 
    quizFragment.setFragmentQuizInteractionListener(this); 
    fragmentManager.beginTransaction().replace(R.id.content_frame, quizFragment).addToBackStack("quiz").commit(); 
    break;

，丢弃onAttach()方法。
可能不是最好的方式，但值得一试。

来源

2016-04-29 22:01:44 aborocz

谢谢！这解决了问题！（不能upvote你的评论，没有足够的声誉） –

你需要重写的

public void onAttach (Activity activity)

代替

public void onAttach (Context context)

后者包括API等级23，因此Android的早期版本不会调用它。

见片段onAttach (Activity activity)

来源

2016-04-29 22:34:40 Luis

使用onAttach方法。检查片段生命周期

Context context;         
     @Override 
      public void onAttach(Context context) { 
      super.onAttach(context); 
      this.context=context; 
     }

来源

2017-10-28 12:29:38

无法将Activity界面附加到Fragment。获取NullPointerException

回答

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