title: cannot resolve symbol R, setContentView() android studio
body: <p>I have found numerous posts about this issue and read through them. I have tried rebuilding the project, do a project clean and a gradle-clean as well still didn't help.</p>
  
  <p>Perhaps there's something wrong with my codes?</p>
  
  <p>The error I get lies in "setContentView(R.layout.read_comments)" cannot resolve symbol R </p>
  
  <p>Here is my code:</p>
  
  <pre><code>package com.example.mysqltest;
  
  import android.app.Activity;
  import android.os.Bundle;
  
  public class ReadComments extends Activity{
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          // TODO Auto-generated method stub
          super.onCreate(savedInstanceState);
          setContentView(R.layout.read_comments);
      }
  }
  </code></pre>