フラグメント内のOnClickListenerでアプリケーションがクラッシュする

Question

3つのタブを持つアプリケーションがあり、そのうちの1つにtodo機能があります。新しいメモを追加したいときは、すべてうまく動作します。今私はノートのための削除機能を実装したいと私はpublic class Tab2 extends Fragment implements View.OnClickListenerとpublic void onClick(View v)としようとしましたが、私はそれを起動するとcompletly私のアプリケーションがクラッシュするボタンをアクセスすることはできません。何が間違っていますか？ここで フラグメント：

public class Tab2 extends Fragment implements View.OnClickListener { 

ImageButton btn_ok; 
EditText edit_task; 
private TaskDbHelper mHelper; 
public ListView mTaskListView; 
private ArrayAdapter<String> mAdapter; 

@Nullable 
@Override 
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.tab2, container, false); 

    setHasOptionsMenu(true); 
    mHelper = new TaskDbHelper(getActivity()); 
    mTaskListView = view.findViewById(R.id.list_todo); 
    updateUI(); 

    Button b = view.findViewById(R.id.btn_delete); 
    b.setOnClickListener(this); 

    return view; 
} 

public void updateUI() { 
    ArrayList<String> taskList = new ArrayList<>(); 
    SQLiteDatabase db = mHelper.getReadableDatabase(); 
    Cursor cursor = db.query(TaskContract.TaskEntry.TABLE, new String[]{TaskContract.TaskEntry._ID, TaskContract.TaskEntry.COL_TASK_TITLE}, 
      null, null, null, null, null, null); 
    while (cursor.moveToNext()) { 
     int idx = cursor.getColumnIndex(TaskContract.TaskEntry.COL_TASK_TITLE); 
     taskList.add(cursor.getString(idx)); 
    } 

    if (mAdapter == null) { 
     mAdapter = new ArrayAdapter<String>(getActivity(), R.layout.item_todo, R.id.task_title, taskList); 
     mTaskListView.setAdapter(mAdapter); 
    } else { 
     mAdapter.clear(); 
     mAdapter.addAll(taskList); 
     mAdapter.notifyDataSetChanged(); 
    } 

    cursor.close(); 
    db.close(); 
} 

public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) { 
    inflater.inflate(R.menu.main_menu, menu); 
    super.onCreateOptionsMenu(menu, inflater); 
} 

@Override 
public boolean onOptionsItemSelected(MenuItem item) { 
    switch (item.getItemId()) { 
     case R.id.action_add_task: 
      final AlertDialog.Builder mBuilder = new AlertDialog.Builder(getActivity()); 
      View mView = getLayoutInflater().inflate(R.layout.dialog_add_task, null); 
      btn_ok = mView.findViewById(R.id.btn_ok5); 
      edit_task = mView.findViewById(R.id.edit_task); 
      mBuilder.setView(mView); 
      final AlertDialog dialog = mBuilder.create(); 
      dialog.show(); 

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

        String task = String.valueOf(edit_task.getText()); 
        SQLiteDatabase db = mHelper.getWritableDatabase(); 
        ContentValues values = new ContentValues(); 
        values.put(TaskContract.TaskEntry.COL_TASK_TITLE, task); 
        db.insertWithOnConflict(TaskContract.TaskEntry.TABLE, null, values, SQLiteDatabase.CONFLICT_REPLACE); 
        db.close(); 

        updateUI(); 

        dialog.cancel(); 
       } 
      }); 

      return true; 
     default: 
      return super.onOptionsItemSelected(item); 
    } 
} 

@Override 
public void onClick(View v) { 
    switch (v.getId()) { 
     case R.id.btn_delete: 
      View parent = (View) v.getParent(); 
      TextView taskTextView = parent.findViewById(R.id.task_title); 
      String task = String.valueOf(taskTextView.getText()); 
      SQLiteDatabase db = mHelper.getWritableDatabase(); 
      db.delete(TaskContract.TaskEntry.TABLE, TaskContract.TaskEntry.COL_TASK_TITLE + " = ?", new String[]{task}); 
      db.close(); 

      updateUI(); 

      break; 
    } 
} } 

Logcat：

12-06 19:35:58.710 20306-20306/com.example.rike1.final_logbuch E/AndroidRuntime: FATAL EXCEPTION: main 
                       Process: com.example.rike1.final_logbuch, PID: 20306 
                       java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
                        at com.example.rike1.final_logbuch.Tab2.onCreateView(Tab2.java:49) 
                        at android.support.v4.app.Fragment.performCreateView(Fragment.java:2354) 
                        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1419) 
                        at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1740) 
                        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1809) 
                        at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:799) 
                        at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2580) 
                        at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367) 
                        at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322) 
                        at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:2199) 
                        at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:651) 
                        at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:145) 
                        at android.support.v4.view.ViewPager.populate(ViewPager.java:1236) 
                        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:19917) 
                        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139) 
                        at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:714) 
                        at android.support.design.widget.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:91) 
                        at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1361) 
                        at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:784) 
                        at android.view.View.measure(View.java:19917) 
                        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139) 
                        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185) 
                        at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139) 
                        at android.view.View.measure(View.java:19917) 
                        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139) 
                        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1485) 
                        at android.widget.LinearLayout.measureVertical(LinearLayout.java:775) 
                        at android.widget.LinearLayout.onMeasure(LinearLayout.java:657) 
                        at android.view.View.measure(View.java:19917) 
                        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139) 
                        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185) 
                        at android.view.View.measure(View.java:19917) 
                        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139) 
                        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1485) 
                        at android.widget.LinearLayout.measureVertical(LinearLayout.java:775) 
                        at android.widget.LinearLayout.onMeasure(LinearLayout.java:657) 
                        at android.view.View.measure(View.java:19917) 
                        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139) 
                        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185) 
                        at com.android.internal.policy.DecorView.onMeasure(DecorView.java:729) 
                        at android.view.View.measure(View.java:19917) 
                        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2436) 
                        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1482) 
                        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1741) 
                        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1366) 
                        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6768) 
                        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:926) 
                        at android.view.Choreographer.doCallbacks(Choreographer.java:735) 
12-06 19:35:58.710 20306-20306/com.example.rike1.final_logbuch E/AndroidRuntime:  at android.view.Choreographer.doFrame(Choreographer.java:667) 
                        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:912) 
                        at android.os.Handler.handleCallback(Handler.java:761) 
                        at android.os.Handler.dispatchMessage(Handler.java:98) 
                        at android.os.Looper.loop(Looper.java:156) 
                        at android.app.ActivityThread.main(ActivityThread.java:6523) 
                        at java.lang.reflect.Method.invoke(Native Method) 
                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942) 
                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832) 

ボタンは、私がアクセスしたいですXMLのitem_todo：

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:layout_gravity="center_vertical"> 
    <TextView 
     android:id="@+id/task_title" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignParentLeft="true" 
     android:layout_alignParentStart="true" 
     android:text="Hello" 
     android:textSize="20sp" /> 
    <Button 
     android:id="@+id/btn_delete" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignParentEnd="true" 
     android:layout_alignParentRight="true" 
     android:text="Done" 
     android:backgroundTint="@color/colorAccent"/> 
</RelativeLayout> 

TAB2のXML：

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin"> 
    <ListView 
     android:id="@+id/list_todo" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content"> 
    </ListView> 
</RelativeLayout> 

Answer 1

ますアイテムclを追加する必要がありますListViewのListenerを使用してください。 ListViewsアイテムが複数存在するので、ListViewアイテムを見つけることはできません。

mTaskListView.setOnItemClickListener(this); // And remember to implement the interface 

https://developer.android.com/reference/android/widget/AdapterView.OnItemClickListener.html

次に、あなたがonItemClickあなたがnullポインタ例外の原因となっている行を削除する必要が

@Override 
public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
    TextView taskTextView = view.findViewById(R.id.task_title); 
    String task = String.valueOf(taskTextView.getText()); 
    SQLiteDatabase db = mHelper.getWritableDatabase(); 
    // and so on... 
} 

とofcourseのに方法onClickから、あなたのコードを移動することができます

Button b = view.findViewById(R.id.btn_delete); 
b.setOnClickListener(this); 

を

Answer 2

あなたは私ですこの

View view = inflater.inflate(R.layout.tab2, container, false); 

ようtab2.xmlをnflatingしかし、あなたは、あなたが膨張しbtn_deleteを含まないビューので

Button b = view.findViewById(R.id.btn_delete); 

からあなたのボタンを取得することはできません。そのため、上記の行からNull例外が発生しています。