title: Duck example strategy pattern - Head first design pattern
body: <p>I want to ask something about duck example on this book that made me confused and I feel contradictions.</p>
  
  <ol>
  <li><p>Problem
  <img src="https://i.stack.imgur.com/BKZRI.png" alt="enter image description here"></p></li>
  <li><p>the conclusions
  <img src="https://i.stack.imgur.com/fPXch.png" alt="enter image description here"></p></li>
  </ol>
  
  <p>He said "<strong>when joe added new behavior to the duck superclass, he was also adding behavior that was not appropiate for sume Duck subclasses</strong>"</p>
  
  <p>BUT in the conclusion he added <code>performFly()</code> and <code>performQuack();</code> what is the different because i think it same with <code>he was also adding behavior that was not appropiate for sume Duck subclasses</code>?</p>
  
  <p>**image taken from the book Head first design pattern
  ** this question doesn't state this book is not good, this book is really good in my opinion. this is just me who is asking something that I didn't get from the book.</p>