我是新的java和android，我得到这个错误：原因：java. lang.空指针异常：尝试对空对象引用调用虚拟方法"android. view. View android. widget. Button. findViewById（int）"
下面是我的www.example.com代码：MainActivity.java code:

package com.example.intentpassingoneactivitytoanother;
    import androidx.appcompat.app.AppCompatActivity;
    import android.content.Intent;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.widget.Toast;
    public class MainActivity extends AppCompatActivity {
       Button button;
        @Override
    protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
          button.findViewById(R.id.btn);
      button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent=new Intent(MainActivity.this, SecondActivity.class);
            startActivity(intent);
       // Toast.makeText(MainActivity.this, "This is toast", Toast.LENGTH_SHORT).show();
    }
    });
    }
    }

下面是我的activity_main. xml代码：

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout 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">
    <Button
      android:id="@+id/btn"
      android:layout_gravity="center"
      android:layout_marginHorizontal="120dp"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="NextButton"/>
    </LinearLayout>