android studio中运行以下代码为什么会出现找不到符号

Question

public class MainActivity extends AppCompatActivity {
private Button btn_1;
private Button btn_2;

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


    btn_1=(Button)findViewById(R.id.anniu);
    btn_1.setOnClickListener(new View.OnClickListener()
    {
        @Override
                public void onClick(View view)
        {
            ViewGroup activity_main=(ViewGroup)findViewById(R.id.activity_main);
            btn_2=new Button(MainActivity.this);
            btn_2.setText("按钮2");
            activity_main.addView(btn_2);
        }
    });
}

}

错误：找不到符号
符号：变量activity_main
位置：类id