在片段内使用回收器视图时出错

Question

我正在尝试在我的仪表板片段中使用回收器视图，但是当我尝试运行该应用程序并在 logcat 窗口中显示此错误时该应用程序崩溃 ->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，您之前正在使用它。 表示布局创建未完成，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。

您将代码放在错误的方法（）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。

Answer 2

您应该先膨胀视图以获取其 RecyclerView。

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) {
        View view = inflater.inflate(R.layout.fragment_dashboard, container, false);

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

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

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

        return view;
    }
}

建议：您应该以驼峰格式命名您的类（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;

对此

RecyclerView recycler;

还有这条线

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

对此

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