title: How come I can't use toLowerCase() after using a StringBuffer?
body: <p>I read that in Java, String is immutable, so we can't really use toLowerCase intuitively, i.e the original string is unmodified:</p>
  
  <pre><code>String s = "ABC";
  s.toLowerCase();
  
  &gt; "ABC"
  </code></pre>
  
  <p>But even using StringBuffer(which supports mutable Strings) does not work</p>
  
  <pre><code>StringBuffer so = new StringBuffer("PoP");
  
  so.toLowerCase()
  
  &gt; Static Error: No method in StringBuffer has name 'toLowerCase'
  </code></pre>
  
  <p>I appreciate any tips or advice.</p>