Lecture-17Instructor Name:

Object Oriented Programming

Today’s Lecture

Polymorphism

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Polymorphism

What is Polymorphism? Polymorphism is the ability of an object to take on many

forms. The most common use of polymorphism in OOP occurs when a

parent class reference is used to refer to a child class object. Any Java object that can pass more than one IS-A test is

considered to be polymorphic. In Java, all Java objects are polymorphic since any object will

pass the IS-A test for their own type and for the class Object. It is important to know that the only possible way to access

an object is through a reference variable. A reference variable can be of only one type. Once declared, the type of a reference variable cannot be changed.

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Polymorphism

What is Polymorphism? The reference variable can be reassigned to other objects

provided that it is not declared final. The type of the reference variable would determine the

methods that it can invoke on the object. A reference variable can refer to any object of its declared

type or any subtype of its declared type. A reference variable can be declared as a class or interface

type.

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Polymorphism

Static & Dynamic Type Trying to solve problem of developing a complete polymorphic

display method leads into discussion of static and dynamic types and method lookup.

At first attempt to solve this problem might be to move display method in Photopost and MessagePost

Do you think your application will compile anymore? We get errors in the MessagePost and PhotoPost classes,

because we cannot access the superclass fields. We get an error in the NewsFeed class, because it cannot find

the display method.5

Polymorphism

Static & Dynamic Type The reason for the first sort of error is that the fields in Post

have private access and so are inaccessible to any other class—including subclasses.

In order to keep maintaining encapsulation the easiest way to solve this is to define public accessor methods for them.

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Polymorphism

Static & Dynamic TypeRecall the display method called in NewsFeed Class

for(Post post : posts) {

post.display();

System.out.println();

} The compiler informs display does not exist in post. Every Post object in the collection is in fact a MessagePost or

a PhotoPost object, and both have display methods This should mean that post.display() ought to work, because,

whatever it is—MessagePost or PhotoPost—we know that it does have a display method.

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Polymorphism

Static & Dynamic Type To understand in detail

Car c1 = new Car(); type of c1 is Car

Consider the following statement: Vehicle v1 = new Car();

What is the type of v1? The type of the variable v1 is Vehicle; The type of the object stored in v1 is Car. Through sub-typing and substitution rules --Allowed, we now have situations where the type of the variable and

the type of the object stored in it are not exactly the same8

Polymorphism

Static & Dynamic Type We come up with two concepts Static & Dynamic types The static type of a variable v is the type as declared in the

source code in the variable declaration statement. The dynamic type of a variable v is the type of the object that

is currently stored in v More precise: the static type of v1 is Vehicle, the dynamic

type of v1 is Car. In Our example the static type of post is Post, while the

dynamic type is either MessagePost or PhotoPost. We do not know which one of these it is, assuming that we

have entered both MessagePost and PhotoPost objects into the feed. 9

Polymorphism

Static & Dynamic Type The compiler reports an error because, for type checking, the

static type is used. The dynamic type is often only known at runtime, so the

compiler has no other choice but to use the static type if it wants to do any checks at compile time. The static type of

post is Post, and Post does not have a display method. The behavior of the compiler is reasonable in this respect,

because it has no guarantee that all subclasses of Post will, indeed, define a display method, and this is impossible to check in practice.

The technique to handle such kind of problem is method overriding.

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Polymorphism

Dynamic Method Lookup Type checking uses the static type, but at runtime, the

methods from the dynamic type are executed. This is a fairly important statement. To understand it better, we look in more detail at how

methods are invoked. This procedure is known as method lookup, method

binding, or method dispatch..

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Polymorphism

Method Lookup with Simple Object

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Polymorphism

Method Lookup with InheritanceThis scenario illustrates how objects inherit methods.

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Polymorphism

Method Lookup with Polymorphism & Overriding

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Polymorphism

Redefining Display Method

public void display(){

super.display();System.out.println(" ["+ filename + "]");System.out.println(" "+ caption);

}

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Access Modifiers

Protected Access In OOP a level of access lies between the complete restriction

of private access and the full availability of public access Declaring a field or a method protected allows direct access to

it from (direct or indirect) subclasses.protected long getTimeStamp(){

return timestamp;}

Protected access allows access to the fields or methods within a class itself and from all its subclasses, but not from other classes 16

Access Modifiers

Access Levels

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Access Modifiers

Access Levels While protected access can be applied to any member of a

class, it is usually reserved for methods and constructors.

It is not usually applied to fields, because that would be a

weakening of encapsulation.

Wherever possible, mutable fields in super classes should

remain private.

There are, however, occasional valid cases where direct access

by subclasses is desirable. 18

Access Modifiers

Access Control & Inheritance The following rules for inherited methods are enforced:

Methods declared public in a superclass also must be

public in all subclasses.

Methods declared protected in a superclass must either be

protected or public in subclasses; they cannot be private.

Methods declared without access control (no modifier was

used) can be declared more private in subclasses.

Methods declared private are not inherited at all, so there

is no rule for them. 19

Final Keyword

Final Keyword in Java The final keyword in java is used to restrict the user. The java

final keyword can be used in many context. Final can be:1. variable2. method3. class

The final keyword can be applied with the variables, a final variable that have no value it is called blank final variable or uninitialized final variable.

It can be initialized in the constructor only. The blank final variable can be static also which will be

initialized in the static block only. I f you make any variable as final, you cannot change the

value of final variable(It will be constant). 20

Final Keyword

Final Keyword – Example Codeclass Bike9{   final int speedlimit=90;//final variable   void run(){    speedlimit=400;   }   public static void main(String args[]){   Bike9 obj=new  Bike9();   obj.run();   }  }//end of class  

Output – Compile time error

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Final Keyword

Final Methods & Final Classes If a method is declared final , it not be overridden

If a class is final , it can not be extended

Final method is inherited but it can not override

If declare any parameter as final we can not change the value

of it

A constructor can not be declare as final

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Static Keyword

Static Keyword The static keyword in java is used for memory management

mainly. We can apply java static keyword with variables, methods,

blocks and nested class. The static keyword belongs to the class than instance of the

class. The static can be:

1. variable (also known as class variable)

2. method (also known as class method)

3. block

4. nested class 23

Static Keyword

Static Variable If declare any variable as static, it is known static variable. The static variable can be used to refer the common property

of all objects (that is not unique for each object) e.g. company name of employees,college name of students etc.

The static variable gets memory only once in class area at the time of class loading.

Advantage of Static Variable is that It makes your program memory efficient (i.e it saves memory).

Suppose there are 500 students in my college, now all instance data members will get memory each time when object is created.All student have its unique rollno and name so instance data member is good.Here, college refers to the common property of all objects.If we make it static,this field will get memory only once.

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Static Keyword

Static Variable

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Static Keyword

Static Methods If you apply static keyword with any method, it is

known as static method.– A static method belongs to the class rather than

object of a class.– A static method can be invoked without the need for

creating an instance of a class.– static method can access static data member and

can change the value of it.Restrictions There are two main restrictions for the static method. They

are:1. The static method can not use non static data member or

call non-static method directly.2. this and super cannot be used in static context.

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Static Keyword

Static Block Is used to initialize the static data member.

It is executed before main method at the time of class

loading.

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