Transcript What is Polymorphism?

What is Polymorphism?
Polymorphism is a process which
allows a general class to specify
methods that will be common to all of
its derivatives, while allowing
subclasses to define the specific
implementation of some or all of
those methods.
What is the use of
Polymorphism?
The use of polymorphism is that
programs take an simplified
appearance. They contain less
branching logic in favor of simpler
sequential code.This simplification
facilitates testing,debugging and
program maintenance.
What are the ways in which you
can implement polymorphism?
1. Using switch Statement.
2. Using Dynamic Method Binding
Using switch is time consuming
and error prone- How?
• It is problem in case the programmer for get to test
all possible cases in switch.
• If a switch based system is modified by adding
new types, the programmer might forget to insert
the new cases in existing switch statement.
• Every addition or deletioin of a class demands that
every switch statement in the system be modified.
Dynamic Method Binding
Dynamic Method Binding is the
mechanism by which a call to an
overridden function is resolved at run
time rather than compile time.
Dynamic method binding is important
because this is how Java implements
run-time polymorphism.
What Principle does java uses to
resolve calls to overridden
methods at run time?
The principle is: A superclass
reference variable can refer to a
subclass object. Java uses this fact to
resolve calls to overridden methodss
at run time.
How java support Dynamic
Method Binding?
When an overridden method is called
through a superclass reference, Java
determines which version of that
method to execute based upon the
type of object being referred to at the
call occurs. This determination is
made at run time rather than compile
time. Precisely we can say as, it is
the type of the object being referred
to and not the type of the reference
variable.
Final methods and classes
1.Classes declared as final cannot be
inherited.
2.Methods declared as final cannot be
overridden.
Final keyword has 3 uses
1.It can be used to create the
equivalent of a named constant.
2.To prevent overriding
3.To prevent inheritence.
To prevent overriding
To disallow a method from being
overridden, specify final as a modifier
at the start of its declaration.
Methods declared as final cannot be
overridden.
Methods declared as final can
sometimes provide a performance
enhancement. How?
When a final function is called,
often the java compiler can copy the
bytecode for the subroutine directly
inline with the compiled code of the
calling method. Thus eliminating the
costly overhead associated with a
method call.
Late Binding & Early Binding
Late Binding: Call to a method
dynamically at runtime is late
binding.
Early Binding:Call to a method which
is resolvd at compile time is early
binding.
Using final to prevent inheritence
If we want to prevent a class from
being inherited, then precede the class
declaration with final.
Note: While using final keyword
It is illegal to declare a class as both
abstract and final, since an abstract
class is incomplete itself and relies
upon the subclasses provide complete
implementations.
What is Abstract class?
An abstract class is a superclass that
only defines a generalized form of
method that will be shared by all of
its subclasses, leaving it to each
subclass to fill in the details.
Where the abstract class is used?
Abstract class is used when a
superclass is unable to create a
meaningful implementation for a
method.
Notes: Abstract Class
• Abstract keyword is used to declare either abstract
class or abstract method.
• Any class that contains one or more abstract
methods must also be declared abstract. To
declare a class abstract, you simply use the
abstract keyword in front of the class keyword at
the class declaration.
• There can be no object of an abstract class,
because an abstract class is not fully defin
• We cannot declare abstract constructor or
abstract static methods.
• Any subclass of an abstract class must
either implement all of the abstract methods
in the superclass or be itself declared
abstract
• Abstract class can include concrete methods
• Although abstract classes cannot be used to
instantiate objects they can be used to create
object references, because java’s approach
to run-time polymorphism is implemented
through the use of superclass references.