title: What is meant by immutable?
body: <p>This could be the dumbest question ever asked but I think it is quite confusing for a Java newbie. </p>
  
  <ol>
  <li>Can somebody clarify what is meant by <em>immutable</em>? </li>
  <li>Why is a <code>String</code> immutable?</li>
  <li>What are the advantages/disadvantages of the immutable objects?</li>
  <li>Why should a mutable object such as <code>StringBuilder</code> be preferred over String and vice-verse?</li>
  </ol>
  
  <p>A nice example (in Java) will be really appreciated.</p>