I've got the error: "Cannot resolve symbol 'activity_main' and could not figure out how to fix it.Here is my code :

Question

I'm getting error at the second and the third "activity_main". The first is alright.I'm a beginner and I don't know very much. I tried to add the library "android.widget.TextView" but it shows "unused import statement". If you could help me..

import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RelativeLayout;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.activity_main);
        bgElement.setBackgroundColor(Color.RED);
        myButtonListenerMethod();
    }

    public void myButtonListenerMethod() {
        Button button = (Button) findViewById(R.id.button);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
              RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.activity_main);
               int color = ((ColorDrawable) bgElement.getBackground()).getColor();
               if (color == Color.RED) {
                   bgElement.setBackgroundColor(Color.BLUE);
               }
                else {
                   bgElement.setBackgroundColor(Color.RED);
               }
            }
        });
    }
}