I am having trouble understanding the OOP Polymorphic principl of Dynamic Binding ( Late Binding ) in Java. I looked for question pertaining to java, and wasn’t sure if a overall answer to how dynamic binding works would pertain to Java Dynamic Binding, I wrote this question.

Given:

class Person
{
     private String name;

     Person(intitialName)
     {
          name = initialName;
     }

     // irrelevant methods is here.

     // Overides Objects method
     public void writeOutput()
     {
          println(name);
     }
}

class Student extends Person
{

     private int studentNumber;

     Student(String intitialName, int initialStudentNumber)
     {
     super(intitialName);
     studentNumber = initialStudentNumber;
     }

     // irrellevant methods here...

     // overides Person, Student and Objects method
     public void writeOutput()
     {
      super.writeOutput();
      println(studentNumber);
     }   
  }

class Undergaraduate extends Student
{

     private int level;

     Undergraduate(String intitialName, int initialStudentNumber,int initialLevel)
     {
     super(intitialName,initialStudentNumber);
     level = initialLevel;
     }

     // irrelevant methods is here.

     // overides Person, Student and Objects method
     public void writeOutput()
     {
      super.writeOutput();
      println(level);
     }

  }

I am wondering. if I had an array called person declared to contain objects of type Person:

  Person[] people = new Person[2];
  person[0] = new Undergraduate("Cotty, Manny",4910,1);
  person[1] = new Student("DeBanque, Robin", 8812);

Given that person[] is declared to be of type Person, you would expect, for example, in the third line where person[0] is initialized to a new Undergraduate object,to only gain the instance variable from Person and Persons Methods since doesn’t the assignment to a new Undergraduate to it’s ancestor denote the Undergraduate object to access Person – it’s Ancestors, methods and isntance variables…

Thus …with the following code I would expect

  person[0].writeOutput();  // calls Undergraduate::writeOutput()
  person[1].writeOutput();  // calls Student::writeOutput() 

person[0] to not have Undergraduate’s writeOutput() overidden method, nor have person[1] to have Student’s overidden method – writeOutput().

If I had

Person mikeJones = new Student("Who?,MikeJones",44,4);
mikeJones.writeOutput();

The Person::writeOutput() method would be called.

Why is this not so? Does it have to do with something I don’t understand about relating to arrays? Does the declaration Person[] people = new Person[2] not bind the method like the previous code would?