title: Incompatible types: Mainactivity cannot be converted to OnClickListener
body: <p><a href="https://i.stack.imgur.com/XG6C3.png" rel="nofollow noreferrer">Click here. This is link to image error I'm getting</a></p>
  
  <p><a href="https://i.stack.imgur.com/XG6C3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/XG6C3.png" alt="enter image description here"></a></p>
  
  <p>I have started with most basic calculator app that adds and subtracts two numbers.<br>
  I'm getting this incompatible error for <code>SetOnClickListener()</code>.
  Before this I got an error saying "ActivityBar is deprecated", later I changed it to
  AppCompactActivity and the error cleared.</p>
  
  <p>I'm really new to this and have almost no clue to what the code means I just got it online.</p>