Android studio项目运行出现调用虚拟方法的问题

Question

Android studio项目运行出现Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference这样的问题，可是按钮我也绑定了，为啥还有这种情况，以下是我的代码

package com.example.musicplayer;

import androidx.appcompat.app.AppCompatActivity;

import android.annotation.SuppressLint;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Button;

public class MainActivity2 extends AppCompatActivity {
    MediaPlayer mediaPlayer;
    Button button_play,button_pause,button_stop;
    @SuppressLint("MissingInflatedId")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mediaPlayer=MediaPlayer.create(this,R.raw.young_for_you_gala);
        button_play=findViewById(R.id.button_play);
        button_pause=findViewById(R.id.button_pause);
        button_stop=findViewById(R.id.button_stop);
        button_play.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!mediaPlayer.isPlaying()){
                    mediaPlayer.start();
                }
            }
        });
        button_pause.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mediaPlayer.pause();
            }
        });
        button_stop.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mediaPlayer.stop();
            }
        });
    }
}

这个是我的界面设计代码

```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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity2">

    <Button
        android:id="@+id/button_play"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="85dp"
        android:text="开始"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button_pause"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="44dp"
        android:text="暂停"
        app:layout_constraintStart_toStartOf="@+id/button_play"
        app:layout_constraintTop_toBottomOf="@+id/button_play" />

    <Button
        android:id="@+id/button_stop"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="44dp"
        android:text="停止"
        app:layout_constraintStart_toStartOf="@+id/button_pause"
        app:layout_constraintTop_toBottomOf="@+id/button_pause" />
</androidx.constraintlayout.widget.ConstraintLayout>

```

Answer 1

你贴的布局文件是activity_main吗？不会是activity_main2吧？