回收站查看空指针异常 [重复]答案

【问题标题】：Recycler View Null Pointer Exception [duplicate]回收站查看空指针异常 [重复]
【发布时间】：2021-05-27 05:09:31
【问题描述】：

**我是android studio和java的新手

我正在尝试将按钮绑定到包含回收器视图和随附适配器的活动。问题是当我单击按钮时活动崩溃。我知道按钮不是问题，因为我有一个不同的活动，它使用一个按钮并转到活动。

问题出在名为 EdwardActivity 的回收器视图的活动或适配器中，我似乎无法弄清楚。

错误：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.edu.pfrfitness, PID: 10520
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.edu.pfrfitness/com.edu.pfrfitness.EdwardActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference
        at com.edu.pfrfitness.EdwardActivity.onCreate(EdwardActivity.java:31)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
I/Process: Sending signal. PID: 10520 SIG: 9

回收者的活动（名为 Edward Activity）：

package com.edu.pfrfitness;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

public class EdwardActivity extends AppCompatActivity {

    RecyclerView recyclerView;

    String s1[], s2[];
    int images [] = {R.drawable.black, R.drawable.black, R.drawable.black, R.drawable.black, R.drawable.black, R.drawable.black};


        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.my_row);

            recyclerView = findViewById(R.id.recyclerView);

            s1 = getResources() .getStringArray(R.array.Navigation_List);
            s2 = getResources() .getStringArray(R.array.description);

            MyAdapter myAdapter = new MyAdapter(this, s1, s2, images);
            recyclerView.setLayoutManager(new LinearLayoutManager(this));
            recyclerView.setAdapter(myAdapter);

            //call recycler
        }


        public void  Recycler(){

        }






    }

适配器：

package com.edu.pfrfitness;

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

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

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {

    String data1[], data2[];
    int images[];
    Context context;

    public MyAdapter(Context ct, String s1[], String s2[], int img[]) {
            context = ct;
            data1 = s1;
            data2 = s2;
            images = img;


    }

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

        LayoutInflater inflater = LayoutInflater.from(context);
        View view = inflater.inflate(R.layout.my_row, parent, false);

        return new MyViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {

        holder.edwardText1.setText(data1[position]);
        holder.edwardText2.setText(data2[position]);
        holder.edwardImageView.setImageResource(images[position]);
    }

    @Override
    public int getItemCount() {
        return images.length;
    }

    public class MyViewHolder extends RecyclerView.ViewHolder {

        TextView edwardText1, edwardText2;
        ImageView edwardImageView;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
            edwardText1 = itemView.findViewById(R.id.edwardText1);
            edwardText2 = itemView.findViewById(R.id.edwardText2);
            edwardImageView = itemView.findViewById(R.id.edwardImageView);
        }
    }


}

我的行就像每一行的样子：

<?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="wrap_content"
    android:layout_margin="10dp">


    <androidx.cardview.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        tools:ignore="MissingConstraints">

        <androidx.constraintlayout.widget.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">

            <ImageView
                android:id="@+id/edwardImageView"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent"
                tools:srcCompat="@tools:sample/avatars" />

            <TextView
                android:id="@+id/edwardText1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="20dp"
                android:layout_marginTop="10dp"
                android:text="Navigation_List"
                android:textSize="24sp"
                android:textStyle="bold"
                app:layout_constraintStart_toEndOf="@+id/edwardImageView"
                app:layout_constraintTop_toTopOf="parent" />

            <TextView
                android:id="@+id/edwardText2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="description"
                android:textSize="16sp"
                app:layout_constraintStart_toStartOf="@+id/edwardText1"
                app:layout_constraintTop_toBottomOf="@+id/edwardText1" />
        </androidx.constraintlayout.widget.ConstraintLayout>
    </androidx.cardview.widget.CardView>
</androidx.constraintlayout.widget.ConstraintLayout>

【问题讨论】：

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标签： java android android-studio android-recyclerview nullpointerexception

【解决方案1】：

错误说明： 根据您尝试在空对象上调用 setLayoutManager 函数的错误。所以，你的变量“recyclerView”显然是空的。

原因： 原因是在你的活动中你这样做：

setContentView(R.layout.my_row);

然后您使用文件“my_row.xml”来创建您的活动布局。但是，在这个布局中，没有 ID 为 recyclerView 的视图。所以，当你这样做时：

recyclerView = findViewById(R.id.recyclerView);

然后变量recyclerView设置为null。

解决方案： 我猜你错误地使用文件“my_row.xml”作为活动的布局，因为这个布局显然是为适配器的项目创建的。您应该通过在 Activity 的 onCreate 中将其传递到 setContent 中来为 Activity 的布局使用另一个布局。

【讨论】：