Java RecyclerView、CardView和x27；在空白碎片上看不到

我是Android新手。昨天我在BlankFragment上的项目中添加了RecyclerView和CardView。它正在运行，没有错误，但当我打开它时，会显示“程序已停止”。在本教程中，它们将RecyclerView包含在LinearLayout中。但是在我的BlankFragment中，我已经有了FragmentLayout，这就是为什么我认为我不需要它。其他人建议在androidStudio上查看Logcat，之后我得到了AndroidRuntime:FATALEXCEPTION:main

Process: com.example.ganz.afex_with_default_navigation, PID: 10058
                                                   java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ganz.afex_with_default_navigation/com.example.ganz.afex_with_default_navigation.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager)' on a null object reference
                                                       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2957)
                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
                                                       at android.app.ActivityThread.-wrap11(Unknown Source:0)
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
                                                       at android.os.Handler.dispatchMessage(Handler.java:105)
                                                       at android.os.Looper.loop(Looper.java:164)
                                                       at android.app.ActivityThread.main(ActivityThread.java:6944)
                                                       at java.lang.reflect.Method.invoke(Native Method)
                                                       at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
                                                    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager)' on a null object reference
                                                       at com.example.ganz.afex_with_default_navigation.MainActivity.onCreate(MainActivity.java:46)
                                                       at android.app.Activity.performCreate(Activity.java:7174)
                                                       at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220)
                                                       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2910)
                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032) 
                                                       at android.app.ActivityThread.-wrap11(Unknown Source:0) 
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) 
                                                       at android.os.Handler.dispatchMessage(Handler.java:105) 
                                                       at android.os.Looper.loop(Looper.java:164) 
                                                       at android.app.ActivityThread.main(ActivityThread.java:6944) 
                                                       at java.lang.reflect.Method.invoke(Native Method) 
                                                       at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) 
                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374) 

在这个Logcat上，我被链接到我的MainActivity.java:46。第46行保留在OnCreate方法中：

lstTovar = new ArrayList<>();
        lstTovar.add(new Tovar_tavsiya("Modadagi kiyimlar", "Tovar kategoriyasi", "Description", R.drawable.tile_bg_1));
        lstTovar.add(new Tovar_tavsiya("Chet eldan uskunalar", "Tovar kategoriyasi", "Description", R.drawable.tile_bg_2));
        lstTovar.add(new Tovar_tavsiya("Tekstil mahsulotlari", "Tovar kategoriyasi", "Description", R.drawable.tile_bg_3));

        RecyclerView myrv = (RecyclerView) findViewById(R.id.recyclerview_id);
        Recyclerview_Adapter myrv_Adapter = new Recyclerview_Adapter(this, lstTovar);
        myrv.setLayoutManager(new GridLayoutManager(this, 3));    ===IT IS THE 46TH ROW===
        myrv.setAdapter(myrv_Adapter);

lstTovar=newarraylist（）；
lstTovar.add（新增Tovar_tavsiya（“Modadagi kiyimlar”，“Tovar kategoriyasi”，“Description”，R.drawable.tile_bg_1））；
lstTovar.add（新增Tovar_tavsiya（“Chet eldan uskunalar”、“Tovar kategoriyasi”、“Description”、R.drawable.tile_bg_2））；
lstTovar.add（新增Tovar_tavsiya（“Tekstil mahsulotlari”，“Tovar kategoriyasi”，“Description”，R.drawable.tile_bg_3））；
RecyclerView myrv=（RecyclerView）findViewById（R.id.RecyclerView\u id）；
Recyclerview_适配器myrv_适配器=新的Recyclerview_适配器（此，lstTovar）；
myrv.setLayoutManager（新GridLayoutManager（this，3））===这是第46排===
myrv.setAdapter（myrv_适配器）；

这里是fragment_blank.xml：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.ganz.afex_with_default_navigation.BlankFragment">

    <!-- TODO: Update blank fragment layout -->
    <android.support.v7.widget.RecyclerView
        android:id="@+id/recyclerview_id"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

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

</FrameLayout>

---

p.S:我还为它们提供了适配器、活动和java文件。如果我把它们也加在这里，我想问题会变得一团糟。如果你需要他们的部分来解决问题，我会与你分享。提前谢谢你

您的

RecyclerView

为

null

请检查setContentView（…），您似乎没有调用setContentView方法如果

RecyclerView

位于

片段中，您应该在
片段中处理它，而不是在
活动中
@NileshRathod为什么为null？我完成了教程中显示的所有操作。@AnandPhadke我没有使用setContentView（…）。为什么我需要它？