在片段内使用回收器视图时出错

Question

我试图在我的仪表板片段中使用回收器视图，但是当我尝试运行应用程序并在 logcat 窗口中显示此错误时应用程序崩溃 -> 尝试调用虚拟方法 'void androidx.recyclerview.widget .RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' 在空对象引用上

这是我的片段代码

package com.example.bookhub;
import android.content.Context;
import android.os.Bundle;

import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import java.util.Arrays;
import java.util.List;

public class Dashboard_Fragment extends Fragment {

    RecyclerView Recycler;
    RecyclerView.LayoutManager layoutManager;
    Dashboard_Recycler_Adapter recyclerAdapter;
    List<String> bookList = Arrays.asList(
            "P.S. I love You",
            "The Great Gatsby",
            "Anna Karenina",
            "Madame Bovary",
            "War & Peace",
            "Middlemarch",
            "The Adventures of Huckleberry Finn",
            "Moby-Dick",
            "The Lord of the Rings");

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        Recycler = getActivity().findViewById(R.id.Recycler);

        layoutManager = new LinearLayoutManager(getActivity());

        recyclerAdapter = new Dashboard_Recycler_Adapter((Context) getActivity(),bookList);

        Recycler.setLayoutManager(layoutManager);
        Recycler.setAdapter(recyclerAdapter);

        return inflater.inflate(R.layout.fragment_dashboard, container, false);
    }
}

这是适配器的代码

package com.example.bookhub;

import android.content.Context;
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;

public class Dashboard_Recycler_Adapter extends 
RecyclerView.Adapter<Dashboard_Recycler_Adapter.DashboardViewHolder>{

List<String> bookList;
public Dashboard_Recycler_Adapter(Context context, List<String> list)
{
    this.bookList = list;
}


@NonNull
@Override
public DashboardViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType)
{
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.recycler_view_single_row, parent, false);
    return new DashboardViewHolder(view);
}

@Override
public void onBindViewHolder(@NonNull DashboardViewHolder holder, int position)
{
    holder.textView.setText(bookList.get(position));
}

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

public static class DashboardViewHolder extends RecyclerView.ViewHolder
{
    TextView textView;
    public DashboardViewHolder(@NonNull View itemView) {
        super(itemView);

        textView = itemView.findViewById(R.id.txtRecyclerRowItem);
    }
}

}

这是仪表板片段中回收站视图的 XML 代码

<?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:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".Dashboard_Fragment">

    <!-- TODO: Update blank fragment layout -->
    <TextView
        android:id="@+id/textView2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:padding="10dp"
        android:text="@string/dashboard_fragment"
        android:textSize="20sp" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/Recycler"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="35dp"
        android:padding="10dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.088"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.075" />

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

您的布局尚未膨胀，并且您之前正在使用它。表示布局创建尚未完成，您在此之前正在使用它。在 onViewCreated() 方法中编写代码以获取布局的所有组件。

你把你的代码放在了错误的method()中。从 onCreatview() 中删除以下代码。

Recycler = getActivity().findViewById(R.id.Recycler);

layoutManager = new LinearLayoutManager(getActivity());

recyclerAdapter = new Dashboard_Recycler_Adapter((Context) 
getActivity(),bookList);

Recycler.setLayoutManager(layoutManager);
Recycler.setAdapter(recyclerAdapter);

覆盖片段中的 onViewCreated() 方法并将上面的代码复制并粘贴到其中。

更改后，您的片段类将如下面的代码所示。

public class Dashboard_Fragment extends Fragment {

public Dashboard_Fragment() {
    // Required empty public constructor
}

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

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup 
container,
                         Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_dashboard, 
container, false);
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle 
savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);

    Recycler = getActivity().findViewById(R.id.Recycler);

    layoutManager = new LinearLayoutManager(getActivity());

    recyclerAdapter = new Dashboard_Recycler_Adapter((Context) 
    getActivity(),bookList);

    Recycler.setLayoutManager(layoutManager);
    Recycler.setAdapter(recyclerAdapter);

   }
}

Answer 2

你应该在获取它的 RecyclerView 之前对视图进行膨胀。

import java.util.Arrays;
import java.util.List;

public class Dashboard_Fragment extends Fragment {

    RecyclerView recycler;
    
    Dashboard_Recycler_Adapter recyclerAdapter;
    List<String> books= Arrays.asList(
        "P.S. I love You",
        "The Great Gatsby",
        "Anna Karenina",
        "Madame Bovary",
        "War & Peace",
        "Middlemarch",
        "The Adventures of Huckleberry Finn",
        "Moby-Dick",
        "The Lord of the Rings");

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_dashboard, container, false);
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle 
        savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        recycler = view.findViewById(R.id.Recycler);

        recyclerAdapter = new Dashboard_Recycler_Adapter(requiredContext(), books);

        recycler.setLayoutManager(new LinearLayoutManager(requiredActivity()));
        recycler.setAdapter(recyclerAdapter);
    }
}

建议：您应该以驼峰式格式命名您的类（DashboardFragment、DashboardRecyclerAdapter 等）。 当您需要上下文或活动时，在片段中使用 resquiredContext 和 requiredActivity。

Answer 3

改变这一行

RecyclerView Recycler;

到这里

RecyclerView recycler;

还有这一行

Recycler = getActivity().findViewById(R.id.Recycler);

到这里

recycler= getActivity().findViewById(R.id.Recycler);