Java Dynamic Binding
- by Chris Okyen
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?
