title: What is the purpose of question mark(?) after a variable in Groovy
body: <p>I am new to grails I found in many examples that a variable may end with question mark (?) 
  like this</p>
  
  <pre><code>boolean equals(other) {
      if(other?.is(this))
      return true
  }
  </code></pre>
  
  <p>above code contains If condition in that other is ending with a ? so I want to know the meaning of that representation</p>