title: what does "??" mean in Kotlin
body: <p>I am a student studying about mvvm. </p>
  
  <p>And I saw some code.</p>
  
  <p>so what does "??" mean in kotlin.</p>
  
  <p>I searched for this but I couldn't find it.</p>
  
  <p>How can I fix it if I'm wrong? -> (s.toString ?? "")</p>
  
  <p><strong>1.</strong></p>
  
  <pre><code>fun wrap(a: Int?): Int?? {
      return a
  }
  
  fun desc(a: Int??) {
      if (a == null) {
          println("None")
      } else {
          if (a == null) {
              println("Some(None)")
          } else {
              println("Some(Some($a))")
          }
      }
  }
  
  fun main(args: Array&lt;String&gt;) {
      val a: Int?? = wrap(null)
      desc(a) 
  }
  </code></pre>
  
  <hr>
  
  <p><strong>2.</strong> </p>
  
  <pre><code>&lt;layout/&gt;
      &lt;data&gt;
          &lt;variable&gt;
              name="signid"
              type="SignidViewModel" /&gt;
      &lt;/data&gt;
  
      &lt;EditText
          android:onTextChanged='@{(s,start,end,before) -&gt; signid.Check_ID(s.toString ?? "")}' /&gt;
  &lt;/layout&gt;
  </code></pre>