title: Swift : Difference in '!' and '?' in swift
body: <p>I declared below:</p>
  
  <pre><code>@IBOutlet var hw_label : UILabel!
  </code></pre>
  
  <p>If I write as above than it's running successfully,</p>
  
  <p>But when i declare as below :</p>
  
  <pre><code>    @IBOutlet var hw_label : UILabel!
  
                and 
      hw_label.text = "Hello World"
  </code></pre>
  
  <p>It display error as Image.<img src="https://i.stack.imgur.com/rQ6aX.png" alt="Error Image"></p>
  
  <p>I Just want to know that what is the difference between '!' and '?'.</p>
  
  <p>or why we have to use this..?</p>