title: Implementing a composition relationship in an abstract class definition
body: <p>I'm going through past exam papers for an OOP exam and I've come across a question:
  Could the below class definition be used to implement a composition relationship between the classes Person and Brain?</p>
  
  <pre><code>abstract class Person{
     private Brain brain;
     Person(Brain humanBrain) {
        brain = humanBrain;
     }
  }
  </code></pre>
  
  <p>I was under the impression that from the code given there is already a composition relationship between the two however as the question also asks for code samples I presume I'm wrong. An explanation of how to implement the composition or moreover why this already isn't considered a composition relationship between the two would be appreciated.</p>