lect 4 inheritance polymorphism final access modifiers

7/28/2019 Lect 4 Inheritance Polymorphism Final Access Modifiers

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(i)Encapsulation

(ii)Inheritance

(iii)Polymorphism


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Mechanism to bind the code and the data itmanipulates to protect it from outside interference andmisuse.

Class must make its fields/instance varaibles private . Class should provide public methods(getters and

setters) for accessing those private members. For eg:- When u driver a car u actually dont know

how the engine works

User of ur class must not worry about implementationdetails .He can use the interfaces provided by uthrough public methods.

Tight encapsulation is highly desirable

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Class Customer{ private int height; private int weight; public int getHeight(){ Return this.height;

} public int getWeight(){ Return this.weight; } public void setHeight(int height){ // user of Customer class is not

//worried about how the programmer sets the value

this.height=height; }public void setWeight(int weight){

this.weight=weight; } }

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Resuabilty of code

No Multiple inheritance

Multi level Inheritance


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Deadly Diamond of deathA

Int Add(int a,int b)

B

Overriding


C

Overriding


D

super.add();



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Class Person { String name; String address; String contact; }

Class Employee extends Person{ String designation; Int empid; Int salary; } Class Customer extends Person{ String email; String noofvisits; }


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Private Member :-can be accessed onlywithin the class .They are not inherited .

Default Member :- can be accessed byclasses in same package only

Protected Member :-can be accessed byclasses in same package + all subclasses in

other packages but that too throughinheritance Public Member :-can be assessed by all

classes in any package.


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Access modifiers are applicable to only members ofclass.

A non inner class can only be public or default Access modifiers are not applicable to local variable

of method

For eg:- class Customer{ // default or public only possibleprivate String name; // valid

public String getName(){ // validprivate int i ; // I is local variable so invalid code } }


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Class Animal { }

Class Dog extends Animal {

Int height,weight; } Class BullDog extends Dog{

}

// Dog IS-A animal

// BullDog IS-A Dog and animal both

// Dog HAS-A height and weight


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Application require high cohesion

More specialization

More specialized the class , the more it isuseful in other projects and application

Also easier to maintain


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class BudgetReport {

void connectToRDBMS(){ }

void generateBudgetReport() { } void saveToFile() { }

void print() { }

}


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class BudgetReport { Options getReportingOptions() { } void generateBudgetReport(Options o) { } } class ConnectToRDBMS { DBconnection getRDBMS() { } } class PrintStuff { PrintOptions getPrintOptions() { } } class FileSaver { SaveOptions getFileSaveOptions() { } }


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Can be applied to classes , methods and

variables

Final Class- cannot be inherited

Eg String

Final Method- cannot be overridden

Final Variable -Constant


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class Animal{ final void eat(){ // subclass of animal cannot override

it }

void sleep(){ } } final class Horse extends Animal{

// Horse class cannot be etended

Public final WEIGHT=10; // WEIGHT is constant Overriding nonfinal sleep method Public void sleep{ } }


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Use of same thing for different Purpose Real World Example


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(i)Compile TimeMethod Overloading(in same class as well as through inheritance)For eg:In same classclass A{int add(int a,int b){ }int add(int a,int b,int c){ }}Through InheritanceClass A{Int add(int a,int b){ }}Class B extends A{Int add(int a,int b,int c){ } // different signature}


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(ii)Run Time Method Overriding (Only through inheritance) =

Dynamic method dispatch For eg:Class A{int add(int a,int b) {return (a+b); }}Class B extends A {int add(int a,int b) { // same signaturereturn 2*(a+b);}}


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For Over-riding (i)signature must be same (ii)return type must be same or subtype of the

type returned (iii)Access modifier must be same or less

stricter than that in original

(iv)can throw an exception if originalmethod does not throw any. But if originalmethod throws an exception it can throwonly its subclass exc.


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c1 c =new c2(); // Reference is of Parent class but object is of

subclass Why it is allowed? Suppose u want a method which takes as an

argument one object of anysubclass of Animalclass for eg Dog ,Cat and Horse . At compile time

it is impossible to decide which object will bepassed as an argument.Also if u add a newsubclass of Animal , the method will still work


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If u dont have a source code for a class and

there is one method named Abc() inside that

class which u want to behave in ur own way.All the other methods and variable u dont

want to change .What will u do??..

lect 4 inheritance polymorphism final access modifiers

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