为什么 RecyclerView 没有显示在实际应用程序中？

问：

我一直在开发一个 android studios 应用程序，其中我有一个片段，我想在其中显示一个 ImageView，并在其下方显示一个 RecyclerView。问题是 RecyclewView 没有显示在实际应用程序中。如果 XML 文件中只有 RecyclerView，而不在 ImageView 中，则可以看到 RecyclerView。

下面是 XML 文件：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/deliveryMapImageView"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:src="@drawable/baseline_map_24"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"/>

    <androidx.recyclerview.widget.RecyclerView
        xmlns:tools="http://schemas.android.com/tools"
        android:name="com.example.hci_shop.DeliveryFragment"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="16dp"
        android:layout_marginRight="16dp"
        app:layoutManager="LinearLayoutManager"
        tools:context=".ui.deliveries.DeliveriesFragment"
        tools:listitem="@layout/fragment_delivery"
        app:layout_constraintTop_toBottomOf="@+id/deliveryMapImageView" />
</androidx.constraintlayout.widget.ConstraintLayout>

```[enter image description here](https://i.stack.imgur.com/dsnYC.png)
this is what it looks like in the editor:


Here is the java file for the fragment:

软件包 com.example.hci_shop.ui.deliveries;

导入android.content.Context; 导入 android.os.Bundle; 导入 android.view.LayoutInflater; 导入android.view.View; 导入 android.view.ViewGroup;

导入 androidx.fragment.app.Fragment; 导入androidx.recyclerview.widget.GridLayoutManager; 导入androidx.recyclerview.widget.LinearLayoutManager; 导入androidx.recyclerview.widget.RecyclerView;

导入com.example.hci_shop。R; 导入 com.example.hci_shop.ui.placeholder.PlaceholderContent;

/**

• 表示项列表的片段。 */ public class DeliveriesFragment extends Fragment {

  TODO：自定义参数参数名称 private static final String ARG_COLUMN_COUNT = “column-count”; TODO：自定义参数 私有 int mColumnCount = 1;

  /**

  • 片段管理器的必需空构造函数，用于实例化
  • 片段（例如，在屏幕方向发生变化时）。 */

  TODO：自定义参数初始化 @SuppressWarnings（“未使用”） public static DeliveriesFragment newInstance（int columnCount） { DeliveriesFragment fragment = new DeliveriesFragment（）; Bundle args = new Bundle（）; args.putInt（ARG_COLUMN_COUNT， columnCount）; fragment.setArguments（参数）; 返回片段; }

  @Override public void onCreate（Bundle savedInstanceState） { super.onCreate（savedInstanceState）;

   if (getArguments() != null) {
       mColumnCount = getArguments().getInt(ARG_COLUMN_COUNT);
   }
  

  }

  @Override public View onCreateView（LayoutInflater inflater， ViewGroup 容器， 捆绑包 savedInstanceState） { 视图视图 = inflater.inflate（R.layout.fragment_delivery_list， container， false）;

   // Set the adapter
   if (view instanceof RecyclerView) {
       Context context = view.getContext();
       RecyclerView recyclerView = (RecyclerView) view;
       if (mColumnCount <= 1) {
           recyclerView.setLayoutManager(new LinearLayoutManager(context));
       } else {
           recyclerView.setLayoutManager(new GridLayoutManager(context, mColumnCount));
       }
       recyclerView.setAdapter(new MyDeliveriesRecyclerViewAdapter(PlaceholderContent.ITEMS));
   }
   return view;
  

  } }

Here is the RecyclerViewAdapter:

软件包 com.example.hci_shop.ui.deliveries;

导入 android.view.LayoutInflater; 导入 android.view.ViewGroup; 导入android.widget.TextView;

导入 androidx.annotation.NonNull; 导入androidx.recyclerview.widget.RecyclerView;

导入 com.example.hci_shop.databinding.FragmentDeliveryBinding; 导入 com.example.hci_shop.models.Order; 导入 com.example.hci_shop.ui.interfaces.OnItemClickListener; 导入 com.example.hci_shop.ui.placeholder.PlaceholderContent.PlaceholderItem;

导入 java.util.List;

/**

• {@link RecyclerView.Adapter}，可以显示 {@link PlaceholderItem}。

• TODO：将实现替换为数据类型的代码。 */ 公共类 MyDeliveriesRecyclerViewAdapter 扩展 RecyclerView.Adapter<MyDeliveriesRecyclerViewAdapter.ViewHolder> {

  private final list mValues;

  public MyDeliveriesRecyclerViewAdapter（列表项） { this.mValues = 项目; }

  @NonNull @Override public ViewHolder onCreateViewHolder（ViewGroup parent， int viewType） {

   return new ViewHolder(FragmentDeliveryBinding.inflate(LayoutInflater.from(parent.getContext()), parent, false));
  

  }

  @Override public void onBindViewHolder（final ViewHolder holder， int position） { holder.mItem = mValues.get（位置）; holder.textViewOrderID.setText（mValues.get（position）.id）; holder.textViewOrderAddress.setText（mValues.get（position）.content）; }

  @Override 公共 int getItemCount（） { 返回 mValues.size（）; }

  公共静态类 ViewHolder extends RecyclerView.ViewHolder { public final TextView textViewOrderID; public final TextView textViewOrderAddress; public final TextView textViewBoughtAt; public PlaceholderItem mItem;

   public ViewHolder(FragmentDeliveryBinding binding) {
       super(binding.getRoot());
       textViewOrderID = binding.textViewOrderID;
       textViewOrderAddress = binding.textViewOrderAddress;
       textViewBoughtAt = binding.textViewOrderBoughtAt;
   }
  
   @Override
   public String toString() {
       return super.toString() + " '" + textViewOrderAddress.getText() + "'";
   }
  

  } }

and here is the other xml file that is displayed above the fragment:

<androidx.constraintlayout.widget.ConstraintLayout xmlns：android=“http://schemas.android.com/apk/res/android” xmlns：app=“http://schemas.android.com/apk/res-auto” android：id=“@+id/container” android：layout_width=“match_parent” android：layout_height=“match_parent”>

<androidx.constraintlayout.widget.ConstraintLayout
    android:id="@+id/profileBar"
    android:layout_width="match_parent"
    android:layout_height="60dp"
    android:background="@color/blueBlack"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent" >

    <de.hdodenhof.circleimageview.CircleImageView
        android:id="@+id/profilePictureImageView"
        android:layout_width="50dp"
        android:layout_height="50dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:backgroundTint="@color/tanAccent"
        android:tint="@color/tanAccent"
        android:src="@drawable/profile_picture_placeholder"
        app:civ_border_width="3dp"
        app:civ_border_color="@color/tanAccent"/>

</androidx.constraintlayout.widget.ConstraintLayout>

<SearchView
    android:id="@+id/searchbarShopActivity"
    android:layout_width="match_parent"
    android:layout_height="70dp"
    android:queryHint="Suche ..."
    android:iconifiedByDefault="false"
    app:layout_constraintTop_toBottomOf="@id/profileBar"
    app:layout_constraintStart_toStartOf="parent" />

<fragment
    android:id="@+id/nav_host_fragment_activity_shop"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    app:defaultNavHost="true"
    app:layout_constraintBottom_toTopOf="@id/nav_view"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/searchbarShopActivity"
    app:layout_constraintVertical_bias="1.0"
    app:navGraph="@navigation/mobile_navigation" />

<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/nav_view"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_marginStart="0dp"
    android:layout_marginEnd="0dp"
    android:background="?android:attr/windowBackground"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:menu="@menu/bottom_nav_menu" />

</androidx.constraintlayout.widget.ConstraintLayout>

and here is what the final result looks like:[enter image description here](https://i.stack.imgur.com/uCILd.png)



If somebody here could please tell me what my error is it would be very helpful. Thank you in advance





I have tried to see if I can find a solution in the documentation, on stackoverflow (if somebody my have had the same problem) and I have even stooped so low as to ask chatgpt. None of which could give me a solution that worked for me. I have come here in the hopes that maybe someone here could spot my error.