Object composition (revisited)

. Problem: Design a Circle class to demonstrate how a small object
  is used to create a larger object

. Solution: Use Point class as a part of Circle class.

  - Using an object of one class as part of another class is a
    powerful way of designing complex objects. 

    . We are said to be using 'object composition' when we do that.

    . 'object inheritance' is another way of designing complex
      objects using simpler ones and we will see it later.

. For object composition,

  - see Point.java
        Circle.java
    

. See how toString is invoked indirectly in the println call in
  the main of Circle.

. Also note that toString of Circle relies on the toString of Point
  doing its job.  Separation of concerns or following the
  abstraction principle.

. Now that you know how composition works, you can apply this concept
  as you build complex objects in any depth.  For example, a student
  object containing an array of courses the student is taking.  A
  course consisting of many objects, one of which being the professor
  who is teaching, where the professor itself is represented as an
  object having an array of students who are advisees, etc.  The
  course also has an array of students who are enrolled in the course.
  The course also has a classroom object.  You get the idea.    It can
  get quite complex! And object composition at work at every level in
  the design of a system of that complexity.