無法將LAYOUTManager設置爲RecyclerView。顯示空指針的RecyclerView

Question

我在我的活動onCreate功能如下代碼：

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 

    LinearLayoutManager manager = new LinearLayoutManager(this); 
    manager.setOrientation(LinearLayoutManager.VERTICAL); 
    RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view); 
    recyclerView.setLayoutManager(manager); 
    setContentView(R.layout.my_layout); 
    // some more code 

和my_layout.xml如下所示：

<?xml version="1.0" encoding="utf-8"?> 
<android.support.design.widget.CoordinatorLayout 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    android:id="@+id/annonce.main.coordinator" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    tools:ignore="RtlHardcoded"> 

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

    <android.support.v7.widget.RecyclerView 
     android:id="@+id/recycler_view" 
     android:layout_width="match_parent" 
     android:layout_height="match_parent" 
     android:scrollbars="none" 
     app:behavior_overlapTop="78dp" 
     android:fillViewport="true" 
     android:layout_gravity="fill_vertical" 
     app:layout_behavior="@string/appbar_scrolling_view_behavior"> 

     <android.support.v7.widget.CardView 
      xmlns:card_view="http://schemas.android.com/apk/res-auto" 
      android:id="@+id/card_view" 
      android:layout_width="match_parent" 
      android:layout_height="match_parent" 
      android:layout_marginLeft="8dp" 
      android:layout_marginRight="8dp" 
      android:layout_marginBottom="8dp" 
      card_view:cardCornerRadius="4dp"> 

      <ListView 
       android:id="@+id/listView" 
       android:layout_width="match_parent" 
       android:layout_height="match_parent" 
       android:choiceMode="singleChoice" 
       android:scrollbars="vertical" 
       app:behavior_overlapTop="78dp" 
       app:layout_behavior="@string/appbar_scrolling_view_behavior"> 
      </ListView> 

     </android.support.v7.widget.CardView> 
    </android.support.v7.widget.RecyclerView> 
</android.support.design.widget.CoordinatorLayout> 

我收到錯誤：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method void 
android.support.v7.widget.RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager) on a null object reference` in the line recyclerView.setLayoutManager(manager); 

我確實嘗試過在以前的sof文章中的類似查詢，但沒有一個看起來像d解決我的問題很多。你能告訴我我的代碼在哪裏出錯嗎？謝謝。

Answer 1

1）在 「@Much溢出」 答覆中提到一樣，你onCreate()方法更改爲：

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.my_layout); // setting the layout first 

    LinearLayoutManager manager = new LinearLayoutManager(this); 
    manager.setOrientation(LinearLayoutManager.VERTICAL); 
    RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view); 
    recyclerView.setLayoutManager(manager); 
} 

2）從RecyclerView XML刪除多餘的孩子的。

你my_layout.xml應該看起來像：

<?xml version="1.0" encoding="utf-8"?> 
<android.support.design.widget.CoordinatorLayout 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    android:id="@+id/annonce.main.coordinator" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    tools:ignore="RtlHardcoded"> 

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

    <android.support.v7.widget.RecyclerView 
     android:id="@+id/recycler_view" 
     android:layout_width="match_parent" 
     android:layout_height="match_parent" 
     android:scrollbars="none" 
     app:behavior_overlapTop="78dp" 
     android:fillViewport="true" 
     android:layout_gravity="fill_vertical" 
     app:layout_behavior="@string/appbar_scrolling_view_behavior"> 

    </android.support.v7.widget.RecyclerView> 
</android.support.design.widget.CoordinatorLayout> 

如果你想在你的RecyclerView添加視圖，你應該使用Adapter。

Answer 2

在設置佈局之前，您正在嘗試查找RecyclerView。這樣做

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.my_layout); // setting the layout first 

    LinearLayoutManager manager = new LinearLayoutManager(this); 
    manager.setOrientation(LinearLayoutManager.VERTICAL); 
    RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view); 
    recyclerView.setLayoutManager(manager); 
} 

Answer 3

爲什麼在RecyclerView裏面有CardView和listView？我從來沒有見過這樣的佈局。你的佈局有問題。

嘗試從佈局文件中刪除recyclerView的子項。