title: Calling java functions from Clojure
body: <p>I can use (.toUpperCase "GOOD") in clojure, as "GOOD" is java string, and java string has toUpperCase method. </p>
  
  <p>I also can use (java.io.File/separator) from clojure as a way of calling java functions.</p>
  
  <p>But, why can't I call (java.lang/Object wait 3) or (java.lang.System/println "hi")?</p>
  
  <ul>
  <li>Can't we use all the java functions from Clojure? </li>
  <li>If not, is there any rule for calling them? If so, where are the reference for those functions?</li>
  </ul>