来自Activity的Fragment中的调用方法返回java.lang.NullPointerException错误

我在下面调用Progress_Limits方法Fragment时遇到此错误，如何正确初始化Fragment？来自Activity的Fragment中的调用方法返回java.lang.NullPointerException错误

logcat的

09-05 06:37:30.075: E/AndroidRuntime(8676): FATAL EXCEPTION: main 
09-05 06:37:30.075: E/AndroidRuntime(8676): Process: developer.mohab.gymee, PID: 8676 
09-05 06:37:30.075: E/AndroidRuntime(8676): java.lang.RuntimeException: Unable to start activity ComponentInfo{developer.mohab.gymee/developer.mohab.gymee.Cardio.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.ContentResolver android.support.v4.app.FragmentActivity.getContentResolver()' on a null object reference 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2345) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2405) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.app.ActivityThread.access$800(ActivityThread.java:149) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1324) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.os.Handler.dispatchMessage(Handler.java:102) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.os.Looper.loop(Looper.java:211) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.app.ActivityThread.main(ActivityThread.java:5333) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at java.lang.reflect.Method.invoke(Native Method) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at java.lang.reflect.Method.invoke(Method.java:372) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1016) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811) 
09-05 06:37:30.075: E/AndroidRuntime(8676): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.ContentResolver android.support.v4.app.FragmentActivity.getContentResolver()' on a null object reference 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at developer.mohab.gymee.Cardio.Progress.getCursor(Progress.java:70) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at developer.mohab.gymee.Cardio.Progress.Prgress_Limits(Progress.java:43) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at developer.mohab.gymee.Cardio.MainActivity.onCreate(MainActivity.java:55) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.app.Activity.performCreate(Activity.java:5933) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298) 
09-05 06:37:30.075: E/AndroidRuntime(8676):  ... 10 more

片段包含方法，我想打电话

public class Progress extends Fragment { 
TextView Abs ; 
TextView Per ; 
MainActivity mainActivity ; 
Database database ; 



@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    mainActivity = (MainActivity) getActivity(); 
    database = new Database(getActivity()); 
} 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.progress,container,false); 
    Abs = (TextView) view.findViewById(R.id.above); 
    Per = (TextView) view.findViewById(R.id.below); 
    Prgress_Limits(); 
    return view ; 
} 
public void Prgress_Limits(){ 
    Cursor cursor ; 
    cursor = getCursor(); 
    int LAST_INPUT ; 
    int FIRST_INPUT ; 
    if(cursor!=null&&cursor.moveToFirst()) { 
     Log.e("mohab", "cursor not null"); 
     int VALUE_COLUMN = cursor.getColumnIndex(Contract.Cardio_chart.Values.VALUE); 
     FIRST_INPUT = cursor.getInt(VALUE_COLUMN); 
     cursor.moveToLast(); 
     LAST_INPUT = cursor.getInt(VALUE_COLUMN); 
     int abs = LAST_INPUT-FIRST_INPUT ; 
     Abs.setText(""+abs); 
     int per = (abs*100)/FIRST_INPUT ; 
     Per.setText(""+per+" %"); 
    } 
    else { 

     Abs.setText("null"); 

     Per.setText("null"); 
    } 


} 
public Cursor getCursor() { 
    Cursor c; 
    String Selection; 
    Cursor check; 
    ContentResolver hesolver = getActivity().getContentResolver(); 
    check = hesolver.query(DatabaseFilter.ValueTable(), null, null, null, null); 
    Cursor check2; 
    ContentResolver resolver = getActivity().getContentResolver(); 
    check2 = resolver.query(DatabaseFilter.LimitTable(), null, null, null, null); 
    if (check != null && check.moveToFirst() && check2 != null && check2.moveToFirst()) { 
     int Offset = offset(); 
     int Limit = limit(); 
     Selection = "SELECT * FROM " + DatabaseFilter.valueTable_Name() + " LIMIT " + Limit + " OFFSET " + Offset + ";"; 
     SQLiteDatabase db = database.getReadableDatabase(); 
     c = db.rawQuery(Selection, null); 
    } else { 
     ContentResolver sesolver = getActivity().getContentResolver(); 
     c = sesolver.query(DatabaseFilter.ValueTable(), null, null, null, null); 
    } 
    return c; 
} 
public int offset() { 
    Cursor save; 
    ContentResolver resolver = getActivity().getContentResolver(); 
    save = resolver.query(DatabaseFilter.LimitTable(), null, null, null, null); 
    save.moveToLast(); 
    int high = save.getColumnIndex(Contract.Cardio_chart.Limits.HIGH); 
    int Offset = save.getInt(high); 
    save.close(); 
    return Offset; 
} 

public int limit() { 

    Cursor h; 
    ContentResolver resolver = getActivity().getContentResolver(); 
    h = resolver.query(DatabaseFilter.ValueTable(), null, null, null, null); 
    h.moveToLast(); 
    int id = h.getColumnIndex(Contract.Cardio_chart.Values._ID); 
    int last_chart_input = h.getInt(id); 
    int Offset = offset(); 
    int Limit = last_chart_input - Offset; 
    h.close(); 
    return Limit; 
}

}

我如何实例化片段，并呼吁Progress_Limits了Methode从活动

protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    p = new Progress(); 
    getSupportFragmentManager().beginTransaction() 
      .add(R.id.group, p) 
      .commit(); 
    p.Prgress_Limits(); 
    database = new Database(this) ; 
    setSpinner(); 
    addEntries(0, 0, true); 
    addPlus(); 


}

来源

2015-09-05 Error

所以，你必须使用onattach活动和使用活动，你可以参考数据库中的某些延迟。 –

尝试调用'p.Prgress_Limits（）;''中的问题onResume'方法发生可能是你之前的片段使用getActivity（）访问上下文准备 –

，你也尝试它获得创建 –

这里：

p.Prgress_Limits(); //<<<< 
database = new Database(this) ;

p.Prgress_Limits()导致问题监守，此方法只调用commit()方法，这意味着可能片段是没有准备好之后调用。所以致电getActivity()返回null。

要解决此问题，调用p.Prgress_Limits();在活动的onResume或使用Handler使从现场片段访问方法

来源

2015-09-05 05:06:44

与片段进行通信的推荐的和最简单的方法是使用接口。

定义一个接口在您的活动，让你的片段实现该接口。

例子：

要声明一个接口

public interface ActivityToFragment{ void executeMethodInFragment(); }

而在你的活动，声明一个变量为这个接口，

private ActivityToFragment mListener;

现在扎听者片段

p = mListener

现在实现该接口在你的片段

public class Progress extends Fragment implements ActivityToFragment

来源

2015-09-05 05:01:40 stallianz

来自Activity的Fragment中的调用方法返回java.lang.NullPointerException错误

回答

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