1. Lecture 0311 – Polymorphism
Nancy Harris

2. New Teams Eskridge Irons Fisher-Duke Onat Bardas Deck Naylor Foster
Coradazzi
Heatwole
Olvera
McMillen
Ernst
Hubbard
Hittie
Cottingham
Field, Kevin
Jiggetts
Sheppard
Moran
Greene
Staleva
Underwood
Shami
Moomau
Gagon
Tucker
Galaska
Sibley
Mawyer
Goldenberg
Frye
Judson
Allen
Bailey
Hart
Kelchner
Russell
Copeland
Mersiovsky
Tessitore
Miles
Zhang
Massetti

3. Inheritance - Preventing a Method from Being Overridden
The final modifier will prevent the overriding of a superclass method in a subclass.
public final void message()
If a subclass attempts to override a final method, the compiler generates an error.
This ensures that a particular superclass method is used by subclasses rather than a modified version of it.

4. Protected Members
Using protected instead of private makes some tasks easier.
However, any class that is derived from the class, or is in the same package, has unrestricted access to the protected member.
It is always better to make all fields private and then provide public methods for accessing those fields.
If no access specifier for a class member is provided, the class member is given package access by default.
Any method in the same package may access the member.

5. Polymorphism
from the Greek
poly = many
morph = forms

6. We have seen polymorphism
Think clocks.
You can create a Clock or an AlarmClock.
You can create a Clock and instantiate an AlarmClock.
The method used (like the updateTime method) will be based on the object type.
But the object declaration determines which methods are available to us.

7. Polymorphism
Polymorphic references are resolved at run time; this is called dynamic binding
Careful use of polymorphic references can lead to elegant, robust software designs
Polymorphism can be accomplished using inheritance or using interfaces(to be discussed later)

8. References and Inheritance
An object reference can refer to an object of its class, or to an object of any class related to it by inheritance
For example, if the Holiday class is used to derive a child class called Christmas, then a Holiday reference could be used to point to a Christmas object
Holiday day;
day = new Christmas();
Holiday
Christmas

9. References and Inheritance
An Object reference can be used to refer to any object
An ArrayList is designed to hold Object references
See API for ArrayList
But we have also seen that we can force the ArrayList to only hold objects of a particular type.

10. Polymorphism via Inheritance
It is the type of the object being referenced, not the declared type, that determines which method is invoked
Suppose the Holiday class has a method called celebrate, and the Christmas class overrides it
Now consider the following invocation:
day.celebrate();
If day refers to a Holiday object, it invokes the Holiday version of celebrate; if it refers to a Christmas object, it invokes the Christmas version

11. Polymorphism via Inheritance
Consider the following class hierarchy:
PassFailActivity
PassFailExam
GradedActivity
FinalExam

12. Copyright © 2005, Pearson Addison-Wesley. All rights reserved.
Polymorphism
A GradedActivities variable can be used to reference a FinalExam object.
GradedActivity exam = new FinalExam(50, 7);
This statement creates a FinalExam object and stores the object’s address in the exam variable.
This is an example of polymorphism.
The term polymorphism means the ability to take many forms.
In Java, a reference variable is polymorphic because it can reference objects of types different from its own, as long as those types are subclasses of its type.
Starting Out With Java Control Structures to Objects
By Tony Gaddis
Copyright © 2005, Pearson Addison-Wesley. All rights reserved.

13. Copyright © 2005, Pearson Addison-Wesley. All rights reserved.
Polymorphism
Other legal polymorphic references:
GradedActivity exam1 = new FinalExam(50, 7);
GradedActivity exam2 = new PassFailActivity(70);
GradedActivity exam3 = new PassFailExam(100, 10, 70);
The GradedActivity class has three methods: setScore, getScore, and getGrade.
A GradedActivity variable can be used to call only those three methods.
GradedActivity exam = new PassFailExam(100, 10, 70);
System.out.println(exam.getScore()); // This works.
System.out.println(exam.getGrade()); // This works.
System.out.println(exam.getPointsEach()); // ERROR!
Starting Out With Java Control Structures to Objects
By Tony Gaddis
Copyright © 2005, Pearson Addison-Wesley. All rights reserved.

14. Polymorphism via Inheritance
Consider all kinds of grading
See Polymorphic.java
See GradedActivity.java
See PassFailActivity.java
See FinalExam.java
See PassFailExam.java

15. Polymorphism in Java Class Membership
The compiler uses the declared class
At run-time an object knows its actual class         
Search Order:
Think of a method call as being a “message” to the object.
The message may contain parameters or it may not.
The message is always asking the object to do something.
If a "message" is sent to an object of the derived class then the derived class is searched (for the existence of such a method) first and the base class is searched second. (Note: The search will move up the class hierarchy until found.)
If the "message" is sent to an object of the base class then only the base class is searched.
At compile time we know which messages are valid; at run time we use the particular version of the message that corresponds to the object reacting to the message.

16. Taking it a step further
Abstract classes – templates for class families

17. Rules for Abstract Classes
An abstract class cannot be instantiated
A class that can have instances is said to be concrete
An abstract class provides a prototype for other classes to follow

18. Subclasses of an Abstract Class
will inherit the variables and methods of the abstract class
will have the same basic characteristics
are free to redefine variables and methods and add new ones
must override any abstract methods of its parent (unless it itself is abstract).

19. Abstract Classes generally contain at least one abstract method
are any classes containing at least one abstract method
can contain non-abstract methods
If there is an abstract method, then the class must be declared as abstract, but…
If a class is declared as abstract it may or may not have non-abstract or abstract methods.

20. Abstract Methods have the word abstract in their declaration
do not have a body
end their declarations with a semi-colon
must be overridden in concrete children
are generally methods whose bodies will change from one subclass to another

21. Standard UML Notation + public - private # protected
{abstract} (name is italicized)

22. Abstract Classes
The child of an abstract class must override the abstract methods of the parent, or it too will be considered abstract
An abstract method cannot be defined as final (because it must be overridden) or static (because it has no definition yet)
The use of abstract classes is a design decision – it helps us establish common elements in a class that is too general to instantiate

23. Let’s look at the Student class
What will happen if:
We remove the word abstract in the class header?
We change the abstract method to a non-abstract method?
We try to instantiate a StaffMember object?
We decide not to “pay” Volunteers?
We create a new class from the StaffMember object which is an abstract class also?

24. Student class family Student.java CompSciStudent.java
CompSciStudentDemo.java