項目列表未顯示在 RecyclerView 中

Question

我正在自學 android 應用程序開發（和 Java）。 無法弄清楚為什么我的列表中的項目沒有顯示出來。 彈出窗口工作正常，所以我沒有包含它的代碼。

我試圖通過添加Log.d()消息來查看GroceryItemAdapter是否正常工作，但它沒有顯示。 我猜這是我做的適配器？

更新：修復了一條線，但現在我得到了這個（稍后會更新這個，去研究是什么原因或調試它）：

AutoCrab 的回答幫助解決了空對象問題，我還發現我需要將整數轉換為字符串，然后再將其傳遞給setText() 。

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.TextView.findViewById(int)' on a null object reference
        at com.example.myapplication.GroceryListAdapter$ViewHolder.<init>




package com.example.myapplication;

import android.content.Context;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

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

import java.util.ArrayList;
import java.util.List;

//responsible for displaying data from the model into a row in the recycler view
public class GroceryListAdapter extends RecyclerView.Adapter<GroceryListAdapter.ViewHolder>{
    private List<GroceryList> lists = new ArrayList<>();
    public GroceryListAdapter(List<GroceryList> lists) {
        this.lists = lists;
    }
    

    //onCreateViewHolder is responsible for creating each view
    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        //create a new view and wrap it inside a new viewholder
        //use layout inflator to inflate a view
        View groceryListView = LayoutInflater.from(parent.getContext()).inflate(R.layout.grocery_list_entry,parent,false);
        Log.d("MEJD", "Iamhere?");
        //wrap it inside a ViewHolder and return it
        ViewHolder viewHolder = new ViewHolder(groceryListView);
        return viewHolder;
    }

    //onBindViewHolder is responsible for taking data at a particular position and
    //putting it into a viewholder
    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        //get data model by position
        GroceryList groceryListItem = lists.get(position);

        //set the item views based on the data models
        TextView textView = holder.listTitle;
        textView.setText(groceryListItem.getListName());
        Log.d("MEJD", "Iamhere");
        TextView numItemsView = holder.numItems;
        numItemsView.setText(groceryListItem.getNumItems());

   }

    //get number of items in the data
    @Override
    public int getItemCount() {
        lists.size();
    }

    class ViewHolder extends RecyclerView.ViewHolder{
        //member variables
        public TextView listTitle;
        public TextView numItems;
        public ViewHolder(@NonNull View itemView) {
            super(itemView);

            listTitle.findViewById(R.id.groceryListName);
            numItems.findViewById(R.id.numItemsList);
        }
    }
}

雜貨清單類：

    package com.example.myapplication;
    
    import java.util.List;
    
    public class GroceryList {
        //class to hold the info of each list item
       private String listName;
       private int numItems;
       //will need to refactor later.....
    
        public GroceryList(String listName, int numItems) {
            this.listName = listName;
            this.numItems = numItems;
    
        }
        public String getListName() {
            return listName;
        }
    
        public void setListName(String listName) {
            this.listName = listName;
        }
    
        public int getNumItems() {
            return numItems;
        }
    
        public void setNumItems(int numItems) {
            this.numItems = numItems;
        }
    
    
    }

雜貨清單適配器：

import android.content.Context;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

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

import java.util.ArrayList;
import java.util.List;

//responsible for displaying data from the model into a row in the recycler view
public class GroceryListAdapter extends RecyclerView.Adapter<GroceryListAdapter.ViewHolder>{
    private List<GroceryList> lists = new ArrayList<>();
    public GroceryListAdapter(List<GroceryList> lists) {
        this.lists = lists;
    }
    //create a constructor


    //onCreateViewHolder is responsible for creating each view
    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        //create a new view and wrap it inside a new viewholder
        //use layout inflator to inflate a view
        View groceryListView = LayoutInflater.from(parent.getContext()).inflate(R.layout.grocery_list_entry,parent,false);
        Log.d("MEJD", "Iamhere?");
        //wrap it inside a ViewHolder and return it
        ViewHolder viewHolder = new ViewHolder(groceryListView);
        return viewHolder;
    }

    //onBindViewHolder is responsible for taking data at a particular position and
    //putting it into a viewholder
    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        //get data model by position
        GroceryList groceryListItem = lists.get(position);

        //set the item views based on the data models
        TextView textView = holder.listTitle;
        textView.setText(groceryListItem.getListName());
        Log.d("MEJD", "Iamhere");
        TextView numItemsView = holder.numItems;
        numItemsView.setText(groceryListItem.getNumItems());

   }

    //get number of items in the data
    @Override
    public int getItemCount() {
        return 0;
    }

    class ViewHolder extends RecyclerView.ViewHolder{
        //member variables
        public TextView listTitle;
        public TextView numItems;
        public ViewHolder(@NonNull View itemView) {
            super(itemView);

            listTitle.findViewById(R.id.groceryListName);
            numItems.findViewById(R.id.numItemsList);
        }
    }
}

MainActivity 的 XML 文件：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/homeToolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="#27D3C2"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
        app:layout_constraintTop_toTopOf="parent"
        app:title="Add Grocery List"
        app:titleMarginStart="130dp" />

    <ImageButton
        android:id="@+id/addButton"
        android:layout_width="61dp"
        android:layout_height="56dp"
        android:layout_alignBottom="@+id/homeToolbar"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:layout_alignParentEnd="true"
        android:layout_marginStart="322dp"
        android:layout_marginTop="0dp"
        android:layout_marginEnd="29dp"
        android:layout_marginBottom="0dp"
        android:backgroundTint="#00FFFFFF"
        android:clickable="true"
        android:contentDescription="@+string/add list button"
        app:layout_constraintEnd_toEndOf="@+id/homeToolbar"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="@+id/homeToolbar"
        app:layout_constraintTop_toTopOf="@+id/homeToolbar"
        app:srcCompat="@drawable/add_icon"
        android:focusable="true" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rvView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@+id/homeToolbar"
        android:layout_alignParentStart="true"
        android:layout_alignParentEnd="true"
        android:layout_alignParentBottom="true"
        android:layout_marginStart="0dp"
        android:layout_marginTop="0dp"
        android:layout_marginEnd="0dp"
        android:layout_marginBottom="1dp" />
</RelativeLayout>

我要使用的 listItem 的 XML：

<?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"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/relativeLayout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

<androidx.constraintlayout.widget.Guideline
    android:id="@+id/guideline2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    app:layout_constraintGuide_begin="15dp"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<androidx.constraintlayout.widget.Guideline
    android:id="@+id/guideline4"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    app:layout_constraintGuide_begin="35dp"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<TextView
    android:id="@+id/numItemsList"
    android:layout_width="28dp"
    android:layout_height="23dp"
    android:layout_marginEnd="34dp"
    android:paddingTop="0dp"
    android:paddingBottom="0dp"
    android:text="@string/numberItems"
    android:textSize="12sp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="1.0"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_bias="0.0" />

<androidx.constraintlayout.widget.Guideline
    android:id="@+id/guideline5"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    app:layout_constraintGuide_end="15dp" />

<androidx.constraintlayout.widget.Guideline
    android:id="@+id/guideline6"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    app:layout_constraintGuide_end="35dp" />

<TextView
    android:id="@+id/groceryListName"
    android:layout_width="270dp"
    android:layout_height="22dp"
    android:layout_marginStart="35dp"
    android:layout_marginEnd="44dp"
    android:text="@string/groceryListName"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toStartOf="@+id/numItemsList"
    app:layout_constraintHorizontal_bias="1.0"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

也許這個：

//get number of items in the data
    @Override
    public int getItemCount() {
        return 0;
    }

編輯

//get number of items in the data
    @Override
    public int getItemCount() {
        return lists.size();
    }

Answer 2

添加 itemCount

//
@Override
public int getItemCount() {
   return lists.size();
}

更改 ViewHolder 類

class ViewHolder extends RecyclerView.ViewHolder{
    //member variables
    public TextView listTitle;
    public TextView numItems;
    public ViewHolder(@NonNull View itemView) {
        super(itemView);
        listTitle = itemView.findViewById(R.id.groceryListName);
        numItems = itemView.findViewById(R.id.numItemsList);
    }
}