title: @Nullable annotation usage
body: <p>I saw some method in java declared as:</p>
  
  <pre><code>void foo(@Nullable Object obj)
  { ... }
  </code></pre>
  
  <p>What's the meaning of <code>@Nullable</code> here? Does it mean the input could be <code>null</code>?</p>
  
  <p>Without the annotation, the input can still be null, so I guess that's not just it?</p>