In android studio I am stuck with the error "R cannot be resolved"

Question

Ever since i opened the java file i have been unable to fix this error. I always set up my code in the same way and this is the first time i have gotten this error, I've tried opening a new file and pasting my code in their, but it does not fix it.

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    int turn=1;
    public void tile1(ImageView i) {
        ImageView tile1 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile1.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile1.setImageResource(R.drawable.grape);
        }
    }
    public void tile2(ImageView i) {
        ImageView tile2 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile2.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile2.setImageResource(R.drawable.grape);
        }
    }
    public void tile3(ImageView i) {
        ImageView tile3 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile3.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile3.setImageResource(R.drawable.grape);
        }
    }
    public void tile4(ImageView i) {
        ImageView tile4 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile4.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile4.setImageResource(R.drawable.grape);
        }
    }
    public void tile5(ImageView i) {
        ImageView tile5 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile5.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile5.setImageResource(R.drawable.grape);
        }
    }
    public void tile6(ImageView i) {
        ImageView tile6 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile6.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile6.setImageResource(R.drawable.grape);
        }
    }
    public void tile7(ImageView i) {
        ImageView tile7 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile7.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile7.setImageResource(R.drawable.grape);
        }
    }
    public void tile8(ImageView i) {
        ImageView tile8 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile8.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile8.setImageResource(R.drawable.grape);
        }
    }
    public void tile9(ImageView i) {
        ImageView tile9 = (ImageView) findViewById(R.id.turn);
        if (turn == 1) {
            turn = 2;
            tile9.setImageResource(R.drawable.apple);
        } else {
            turn = 1;
            tile9.setImageResource(R.drawable.grape);
        }
    }
}