client: someone who knows how to use something
        (like a smartphone or ArrayList)
        
implementer: someone who knows how to build something
             (like smartphone or ArrayList)


/* what does it mean to "implement" something?
 *
 * someone at Java wrote a file that started like this!
 * they "implemented" ArrayList so all of us can just use it
 */
public class ArrayList {
   ... 
}


class:  blueprint for the objects
        (think of a plan for how to build a smartphone)
   state: traits of each object
      name (for wifi and bluetooth, etc.)
      what apps you have
      
   behavior: what the object can do
      make calls
      play games or use apps
      send texts
     
object: the actual thing you make from the blueprint
        (think of the actual smartphone itself)
   brought into existence by "new ArrayList()",
   which calls a constructor that we wrote