왜 RecyclerView가 null입니까?

Question

정보가 담긴 카드 목록을 만들려고합니다.

public boolean onNavigationItemSelected(MenuItem item) { 
    // Handle navigation view item clicks here. 
    int id = item.getItemId(); 

    if (id == R.id.add_new_user) {    
     Intent intent = new Intent(this, AddNewUserActivity.class); 
     startActivity(intent);   
    } else if (id == R.id.view_rating) { 
     Intent intent=new Intent(this,ViewAllPlayersActivity.class); 
     startActivity(intent); 
    }   
    return true; 
} 

ViewAllPlayersActivity :

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_view_all_players); 
    final RecyclerView _recyclerView = (RecyclerView) findViewById(R.id.activity_view_all_players); 
// use a linear layout manager 
final LinearLayoutManager _layoutManager = new LinearLayoutManager(this); 
    _recyclerView.setLayoutManager(_layoutManager); //null here 


    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 

    // use this setting to improve performance if you know that changes 
    // in content do not change the layout size of the RecyclerView 
    _recyclerView.setHasFixedSize(true); 
    PlayerManager playerManager=new PlayerManager(); 
    // specify an adapter (see also next example) 
    _adapter = new ShowAllPlayersAdapter(playerManager.getAllPlayers()); 
    _recyclerView.setAdapter(_adapter);  
     } 
    }); 
} 

레이아웃 :

<?xml version="1.0" encoding="utf-8"?> 

<android.support.design.widget.AppBarLayout 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:theme="@style/AppTheme.AppBarOverlay"> 
    <android.support.v7.widget.Toolbar 
     android:id="@+id/toolbar" 
     android:layout_width="match_parent" 
     android:layout_height="?attr/actionBarSize" 
     android:background="?attr/colorPrimary" 
     app:popupTheme="@style/AppTheme.PopupOverlay" /> 

</android.support.design.widget.AppBarLayout> 

<include layout="@layout/content_view_all_players" /> 

<android.support.design.widget.FloatingActionButton 
    android:id="@+id/fab" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_gravity="bottom|end" 
    android:layout_margin="@dimen/fab_margin" 
    app:srcCompat="@android:drawable/ic_dialog_email" /> 

0 그래서 , 난, 느릅 나무 시작 카드 actovity 주요 활동을

그리고 content_view_all_players 레이아웃 파일 :

<?xml version="1.0" encoding="utf-8"?> 

<!-- A CardView that contains a TextView --> 
<android.support.v7.widget.CardView 
    xmlns:card_view="http://schemas.android.com/apk/res-auto" 
    android:id="@+id/card_view" 
    android:layout_gravity="center" 
    android:layout_width="200dp" 
    android:layout_height="200dp" 
    card_view:cardCornerRadius="4dp"> 
    <TextView 
     android:id="@+id/player_name" 
     android:layout_width="match_parent" 
     android:layout_height="match_parent" /> 
</android.support.v7.widget.CardView> 

문제는 그 라인에 있습니다

_recyclerView.setLayoutManager(_layoutManager); //null here 

왜 내가 널거야? 도와 주시겠습니까?

P. 당신은 findViewById 전에 setContentView를 호출 할 필요가

RecyclerView has no LayoutManager

Answer 1

: 내가 findViewById를하기 전에 setContentView 쓰기, 봇 지금은 오류가 있습니다.