title: Java, Type Substitution
body: <p>In Java, type substitution is<br></p>
  
  <ol>
  <li>A subclass object can substitute super-class object</li>
  <li>A super-class object can substitute subclass object</li>
  <li>A super-class variable can refer subclass object</li>
  <li>A subclass variable can refer super-class object<br>
  I am confuse with options<br>
  which option is right?</li>
  </ol>