// This example demonstrates why you may want to explicitly
// define the default constructor when you write a constructor
// for a class. Some important code is commented out here!

class Person {

   // The person class has just a name, and age, and a friend
   // for each person it it.
      String name;
      int age;
      Person friend;

   // A constructor for the person class that takes in the
   // desired initial settings for the person's name, age,
   // and friend and assigns those values to the appropriate
   // fields.
      Person(String n, int a, Person f) {
         name = n;
         age = a;
         friend = f;
      }

   // Because a constructor has been defined, the default
   // constructor no longer exists. Recall that the default
   // constructor has no behavior; the newly created object
   // will have the default values in its instance variables
   // and no computation will have occured. To reintroduce
   // the default constructor explicitly, uncomment the
   // following line of code. Java will be able to distinguish
   // between which of the two constructors to use because
   // they have different parameter lists; the version of the
   // constructor that has the appropriate parameter list given
   // the arguments given to the constructor will be used.
      // Person(){}

   // A method to print out the contents of a person object, so
   // that we can look at the contents.
      void describe() {
         System.out.print("name " + name + " age " + age);
         if (friend == null)
            System.out.println(" no best friend");
         else
            System.out.println(" friend " + friend.name);
         return;
      }

}

public class Constructor {
   public static void main(String[] args) {

   // Create two person objects that you know the name, age,
   // and friend of; three arguments are passed into the Person
   // constructor so the first version will be used.
      Person ann = new Person("Ann", 12, null);
      Person bob = new Person("Bob", 11, ann);
      ann.friend = bob;

   // Create a person object that you do not know any information
   // about. As the code is now, it will not run and you will get
   // an error saying:
   //    "No constructor matching Person() found in class Person."
   // You can correct this by reintroducing the default constructor
   // (un-comment it out above). Note that this line of code would
   // have run correctly until the constructor taking parameters
   // was defined and the default constructor was eliminated.
   // Once the default constructor has been redefined, it will be
   // used because no arguments are being passed into the Person
   // constructor here.
      Person cary = new Person();

      ann.describe();      // prints "name Ann age 12 friend Bob"
      bob.describe();      // prints "name Bob age 11 friend Ann"
      cary.describe();     // prints "name null age 0 no best friend"

      System.out.println("Done!");   // prints "Done!"

   }
}