I am facing a design problem. This must only be solved by applying oops concepts. I am describing the problem below.
Problem: Suppose You have a class called X . It has two Paid (Chargeable) methods like m, n. Their may be many consumers classes of these methods. Someone pays for m, someone pays for n and someone pays for both m, n.
Now I have to design my X class in such a way that consumers can only see that method for which they make payment. How can we do this via OOPS concepts? I can make appropriate changes in my X class to achieve this design. Sample class is written below.
class X { // service class
public m(){ // do some stuff
}
public n(){ // do some stuff
}
}
Create 3 interfaces: one containing the m method, one containing n and a third containing both (the third interface can extend the two others). Then make your class X implement those interfaces.
You will then be able to expose the appropriate interface to your consumers, depending on their needs, while still using the same X class.
interface M { // exposed to customers paying for m
void m();
}
interface N { // exposed to customers paying for n
void n();
}
interface Mn extends M, N {} // exposed to customers paying for both
class X implements Mn {
#Override
public m(){ // do some stuff
}
#Override
public n(){ // do some stuff
}
}
I think you are not taking advantage of the class state. Class can store information in its instance fields about the user, and change its behavior accordingly.
One possible option would be:
class Payment {
int paymentType = 0; // fill with constructor for i.e.
public pay(int sum){
// some common behavior
switch(this.paymentType){
case 1:
// pay 1 logic
break;
case 2:
// pay 2 logic
break;
}
// some other common behavior
}
}
In another design you might use the Strategy pattern to have family of decoupled algorithms.
In the above code I assumed we are talking about some logically related code. If the code has nothing in common, you might even split it into other classes.
Update: I wouldn't advice on using it, but you can implement the Template Method pattern. The problem is you are going to overuse inheritance.
abstract class Payment {
public Pay(int sum){
// some common code
this.doPay(sum);
}
abstract protected doPay(int sum);
}
class PaymentOne : Payment {
protected doPay(int sum){
// pay 1 logic
}
}
class PaymentTwo : Payment {
protected doPay(int sum){
// pay 2 logic
}
}
You'd better use polymorphism concept
As example, based on assumption that m and n has different types:
class X{ // service class
public Pay(NType n){ // do some stuff
}
public Pay(MType m){ // do some stuff
}
public Pay(NType n, MType m){ // do some stuff
Pay(n);
Pay(m);
}
}
Related
I have an interface, say IVehicle, which is implemented in 100s of classes, some of them are variety of 4 wheeler and some are two wheeler dervied types.
I need to introduce a new method for all the 4 wheeler classes, lets say there are 50 of them. My challenge is to reduce the effort as much as I can.
I suggested, to introduce a new interface / abstract class with a method definition. But this require to change every 4 wheeler class declaration and extend with an extra parent.
Is there any possible way?
If you really want to avoid changing all those classes and want a solution that can be considered to be OO, one thing you can do is decorate those classes where they are used and need this extra behaviour.
I'll use C# for example code as you mentioned you're looking for C#/Java solution.
interface IVehicle
{
void DoThisNormalThing();
// ...
}
interface IBetterVehicle : IVehicle
{
void DoThisNeatThing();
}
class FourWheelVehicle : IVehicle
{
public void DoThisNormalThing()
{
// ...
}
// ...
}
class BetterFourWheelVehicle : IBetterVehicle
{
private readonly _vehicle;
public BetterFourWheelVehicle(IVehicle vehicle)
{
_vehicle = vehicle;
}
public void DoThisNormalThing()
{
_vehicle.DoThisNormalThing();
}
public void DoThisNeatThing()
{
// ...
}
// ...
}
Then usage:
var vehicle = new FourWheelVehicle();
var betterVehicle = new BetterFourWheelVehicle(vehicle);
betterVehicle.DoThisNeatThing();
This can be done using extension methods as well (and would result in a little less code and fewer allocated objects), but as this question is tagged with [oop] I wouldn't say extension methods are an OO construct. They're much more aligned with procedural style as they turn your objects into bags of procedures.
Let's say you need to build an application that manages cheques. Each cheque contains data about the amount of money, the date, the payee and an additional payment date which may or may not be present. Additionally, each cheque must be related to a current account which belongs to a certain bank.
Now, our application should allow cheques printing under these conditions:
Each bank managed by the app has a different cheque layout (i.e. each field has a different x,y position).
The cheque layout changes slightly if the payment date is present, even with the same related bank object. But, from bank to bank these changes may not be the same (e.g. bank A may vary position for the date field, while bank B changes position for the payee field)
With these restrictions in place, it's difficult to come up with a simple inheritance schema as there is no consistent behavior to factor out accross the different types of cheques there are. One possible solution would be to avoid inheritance and create a class for every cheque - bank combination:
class ChequeNoPaymentDateForBankA
class ChequeWithPaymentDateForBankA
class ChequeNoPaymentDateForBankB
class ChequeWithPaymentDateForBankB, etc.
Each of these classes implement the print() method which takes the fields positions from a Bank object and builds up the cheque layout. So far so good, but this approach leaves me with a strange feeling as there is no room for code reuse. I wonder if I'm misinterpreting the problem and perhaps there is a better way. As this is not a new problem domain at all, I'm sure this is a reinvent-the-wheel effort. Any insights will be kindly appreciated.
Usually in these situations I move from inheritance to delegation. That is, instead of putting the common code in a superclass (which, as you say, is problematic becuase there are two dimensions), I put the common in a field (one field per dimension) and delegate to that field.
Assuming you're speaking about Java:
public interface Bank {
public void print();
}
public class BankA implements Bank {
public void print() { ... }
}
public class BankB implements Bank {
public void print() { ... }
}
public interface PaymentSchedule {
public void print();
}
public class WithPaymentDate implements PaymentSchedule {
public void print() { ... }
}
public class NoPaymentDate implements PaymentSchedule {
public void print() { ... }
}
public class Cheque {
private final Bank bank;
private final PaymentSchedule schedule;
public Cheque(Bank b, PaymentSchedule s) {
bank = b;
schedule = s;
}
public void print() {
bank.print();
schedule.print();
}
}
That's the general structure of the solution.
Depending on the exact details of your print() algorithm you may need to pass some more data into the print methods and/or to pass this data into the constructors of the classes (of the Bank or PaymentSchedule subclasses) and store it in fields.
I would start by separating the domain model (cheques, banks, etc) from the view (the way the cheques are printed). This is the basic idea behind the MVC pattern and one of its aims is to allow the same domain model to be displayed in different ways (which seems to be your case). So, I would first create the domain classes, something like:
class Cheque
{
protected $bank;
protected $money;
...
}
class Bank {...}
Note that these classes are the "M" of the MVC triad and implement the logic of your domain model, not the behavior related to the rendering process. The next step would be to implement the View classes used to render a cheque. Which approach to take heavily depends on how complex your rendering is, but I would start by having a ChequeView class that renders the common parts and that delegates to other sub-view the specific parts that can change (in this case the date):
abstract class ChequeView
{
protected $cheque;
protected $dateView;
public function __construct($cheque)
{
$this->cheque = $cheque;
$this->dateView = //Whatever process you use to decide if the payment date is shown or not
}
public function render()
{
$this->coreRender();
$this->dateView->render();
}
abstract protected function coreRender();
}
class BankACheckView extends ChequeView
{
protected function coreRender() {...}
}
class BankBCheckView extends ChequeView
{
protected function coreRender() {...}
}
abstract class DateView
{
abstract function render()
}
class ShowDateView extends DateView
{
function render() {...}
}
class NullDateView extends DateView
{
function render() {...}
}
And, if there is code to reuse across subclasses, you can of course factor them in ChequeView and make coreRender() call them.
In case your rendering turns to be too complex, this design may not scale. In that case I would go for splitting your view in meaningful subparts (e.g. HeaderView, AmountView, etc) so that rendering a cheque becomes basically rendering its different sub-parts. In this case the ChequeView may end basically working as a Composite. Finally, if you reach this case and setting up the ChequeView turns out to be a complex task you may want to use a Builder.
Edit based on the OP comments
The Builder is mostly used when the instantiation of the final object is a complex process (e.g. there are many things to sync between the sub-parts in order to get a consistent whole). There is generally one builder class and different clients, that send messages (potentially in different orders and with different arguments) to create a variety of final objects. So, while not prohibited, it is not usual to have one builder per type of object that you want to build.
If you are looking for a class that represents the creation of a particular instance you may want to check the Factory family of patterns (maybe the Abstract Factory resembles closely to what you had in mind).
HTH
Referring to the below link:
http://www.javaworld.com/javaworld/jw-11-1998/jw-11-techniques.html?page=2
The composition approach to code reuse provides stronger encapsulation
than inheritance, because a change to a back-end class needn't break
any code that relies only on the front-end class. For example,
changing the return type of Fruit's peel() method from the previous
example doesn't force a change in Apple's interface and therefore
needn't break Example2's code.
Surely if you change the return type of peel() (see code below) this means getPeelCount() wouldn't be able to return an int any more? Wouldn't you have to change the interface, or get a compiler error otherwise?
class Fruit {
// Return int number of pieces of peel that
// resulted from the peeling activity.
public int peel() {
System.out.println("Peeling is appealing.");
return 1;
}
}
class Apple {
private Fruit fruit = new Fruit();
public int peel() {
return fruit.peel();
}
}
class Example2 {
public static void main(String[] args) {
Apple apple = new Apple();
int pieces = apple.peel();
}
}
With a composition, changing the class Fruit doesn't necessary require you to change Apple, for example, let's change peel to return a double instead :
class Fruit {
// Return String number of pieces of peel that
// resulted from the peeling activity.
public double peel() {
System.out.println("Peeling is appealing.");
return 1.0;
}
}
Now, the class Apple will warn about a lost of precision, but your Example2 class will be just fine, because a composition is more "loose" and a change in a composed element does not break the composing class API. In our case example, just change Apple like so :
class Apple {
private Fruit fruit = new Fruit();
public int peel() {
return (int) fruit.peel();
}
}
Whereas if Apple inherited from Fruit (class Apple extends Fruit), you would not only get an error about an incompatible return type method, but you'd also get a compilation error in Example2.
** Edit **
Lets start this over and give a "real world" example of composition vs inheritance. Note that a composition is not limited to this example and there are more use case where you can use the pattern.
Example 1 : inheritance
An application draw shapes into a canvas. The application does not need to know which shapes it has to draw and the implementation lies in the concrete class inheriting the abstract class or interface. However, the application knows what and how many different concrete shapes it can create, thus adding or removing concrete shapes requires some refactoring in the application.
interface Shape {
public void draw(Graphics g);
}
class Box implement Shape {
...
public void draw(Graphics g) { ... }
}
class Ellipse implements Shape {
...
public void draw(Graphics g) { ... }
}
class ShapeCanvas extends JPanel {
private List<Shape> shapes;
...
protected void paintComponent(Graphics g) {
for (Shape s : shapes) { s.draw(g); }
}
}
Example 2 : Composition
An application is using a native library to process some data. The actual library implementation may or may not be known, and may or may not change in the future. A public interface is thus created and the actual implementation is determined at run-time. For example :
interface DataProcessorAdapter {
...
public Result process(Data data);
}
class DataProcessor {
private DataProcessorAdapter adapter;
public DataProcessor() {
try {
adapter = DataProcessorManager.createAdapter();
} catch (Exception e) {
throw new RuntimeException("Could not load processor adapter");
}
}
public Object process(Object data) {
return adapter.process(data);
}
}
static class DataProcessorManager {
static public DataProcessorAdapter createAdapter() throws ClassNotFoundException, InstantiationException, IllegalAccessException {
String adapterClassName = /* load class name from resource bundle */;
Class<?> adapterClass = Class.forName(adapterClassName);
DataProcessorAdapter adapter = (DataProcessorAdapter) adapterClass.newInstance();
//...
return adapter;
}
}
So, as you can see, the composition may offer some advantage over inheritance in the sense that it allows more flexibility in the code. It allows the application to have a solid API while the underlaying implementation may still change during it's life cycle. Composition can significantly reduce the cost of maintenance if properly used.
For example, when implementing test cases with JUnit for Exemple 2, you may want to use a dummy processor and would setup the DataProcessorManager to return such adapter, while using a "real" adapter (perhaps OS dependent) in production without changing the application source code. Using inheritance, you would most likely hack something up, or perhaps write a lot more initialization test code.
As you can see, compisition and inheritance differ in many aspects and are not preferred over another; each depend on the problem at hand. You could even mix inheritance and composition, for example :
static interface IShape {
public void draw(Graphics g);
}
static class Shape implements IShape {
private IShape shape;
public Shape(Class<? extends IShape> shape) throws InstantiationException, IllegalAccessException {
this.shape = (IShape) shape.newInstance();
}
public void draw(Graphics g) {
System.out.print("Drawing shape : ");
shape.draw(g);
}
}
static class Box implements IShape {
#Override
public void draw(Graphics g) {
System.out.println("Box");
}
}
static class Ellipse implements IShape {
#Override
public void draw(Graphics g) {
System.out.println("Ellipse");
}
}
static public void main(String...args) throws InstantiationException, IllegalAccessException {
IShape box = new Shape(Box.class);
IShape ellipse = new Shape(Ellipse.class);
box.draw(null);
ellipse.draw(null);
}
Granted, this last example is not clean (meaning, avoid it), but it shows how composition can be used.
Bottom line is that both examples, DataProcessor and Shape are "solid" classes, and their API should not change. However, the adapter classes may change and if they do, these changes should only affect their composing container, thus limit the maintenance to only these classes and not the entire application, as opposed to Example 1 where any change require more changes throughout the application. It all depends how flexible your application needs to be.
If you would change Fruit.peel()'s return type, you would have to modify Apple.peel() as well. But you don't have to change Apple's interface.
Remember: The interface are only the method names and their signatures, NOT the implementation.
Say you'd change Fruit.peel() to return a boolean instead of a int. Then, you could still let Apple.peel() return an int. So: The interface of Apple stays the same but Fruit's changed.
If you would have use inheritance, that would not be possible: Since Fruit.peel() now returns a boolean, Apple.peel() has to return an boolean, too. So: All code that uses Apple.peel() has to be changed, too. In the composition example, ONLY Apple.peel()'s code has to be changed.
The key word in the sentence is "interface".
You'll almost always need to change the Apple class in some way to accomodate the new return type of Fruit.peel, but you don't need to change its public interface if you use composition rather than inheritance.
If Apple is a Fruit (ie, inheritance) then any change to the public interface of Fruit necessitates a change to the public interface of Apple too. If Apple has a Fruit (ie, composition) then you get to decide how to accomodate any changes to the Fruit class; you're not forced to change your public interface if you don't want to.
Return type of Fruit.peel() is being changed from int to Peel. This doesn't meant that the return type of Apple.peel() is being forced to change to Peel as well. In case of inheritance, it is forced and any client using Apple has to be changed. In case of composition, Apple.peel() still returns an integer, by calling the Peel.getPeelCount() getter and hence the client need not be changed and hence Apple's interface is not changed ( or being forced to be changed)
Well, in the composition case, Apple.peel()'s implementation needs to be updated, but its method signature can stay the same. And that means the client code (which uses Apple) does not have to be modified, retested, and redeployed.
This is in contrast to inheritance, where a change in Fruit.peel()'s method signature would require changes all way into the client code.
OOP interfaces.
In my own experience I find interfaces very useful when it comes to design and implement multiple inter-operating modules with multiple developers. For example, if there are two developers, one working on backend and other on frontend (UI) then they can start working in parallel once they have interfaces finalized. Thus, if everyone follows the defined contract then the integration later becomes painless. And thats what interfaces precisely do - define the contract!
Basically it avoids this situation :
Interfaces are very useful when you need a class to operate on generic methods implemented by subclasses.
public class Person
{
public void Eat(IFruit fruit)
{
Console.WriteLine("The {0} is delicious!",fruit.Name);
}
}
public interface IFruit
{
string Name { get; }
}
public class Apple : IFruit
{
public string Name
{
get { return "Apple"; }
}
}
public class Strawberry : IFruit
{
public string Name
{
get { return "Strawberry"; }
}
}
Interfaces are very useful, in case of multiple inheritance.
An Interface totally abstracts away the implementation knowledge from the client.
It allows us to change their behavior dynamically. This means how it will act depends on dynamic specialization (or substitution).
It prevents the client from being broken if the developer made some changes
to implementation or added new specialization/implementation.
It gives an open way to extend an implementation.
Programming language (C#, java )
These languages do not support multiple inheritance from classes, however, they do support multiple inheritance from interfaces; this is yet another advantage of an interface.
Basically Interfaces allow a Program to change the Implementation without having to tell all clients that they now need a "Bar" Object instead of a "Foo" Object. It tells the users of this class what it does, not what it is.
Example:
A Method you wrote wants to loop through the values given to it. Now there are several things you can iterate over, like Lists, Arrays and Collections.
Without Interfaces you would have to write:
public class Foo<T>
{
public void DoSomething(T items[])
{
}
public void DoSomething(List<T> items)
{
}
public void DoSomething(SomeCollectionType<T> items)
{
}
}
And for every new iteratable type you'd have to add another method or the user of your class would have to cast his data. For example with this solution if he has a Collection of FooCollectionType he has to cast it to an Array, List or SomeOtherCollectionType.
With interfaces you only need:
public class Foo<T>
{
public void DoSomething(IEnumerable<T> items)
{
}
}
This means your class only has to know that, whatever the user passes to it can be iterated over. If the user changes his SomeCollectionType to AnotherCollectionType he neither has to cast nor change your class.
Take note that abstract base classes allow for the same sort of abstraction but have some slight differences in usage.
I am new to OOP. Though I understand what polymorphism is, but I can't get the real use of it. I can have functions with different name. Why should I try to implement polymorphism in my application.
Classic answer: Imagine a base class Shape. It exposes a GetArea method. Imagine a Square class and a Rectangle class, and a Circle class. Instead of creating separate GetSquareArea, GetRectangleArea and GetCircleArea methods, you get to implement just one method in each of the derived classes. You don't have to know which exact subclass of Shape you use, you just call GetArea and you get your result, independent of which concrete type is it.
Have a look at this code:
#include <iostream>
using namespace std;
class Shape
{
public:
virtual float GetArea() = 0;
};
class Rectangle : public Shape
{
public:
Rectangle(float a) { this->a = a; }
float GetArea() { return a * a; }
private:
float a;
};
class Circle : public Shape
{
public:
Circle(float r) { this->r = r; }
float GetArea() { return 3.14f * r * r; }
private:
float r;
};
int main()
{
Shape *a = new Circle(1.0f);
Shape *b = new Rectangle(1.0f);
cout << a->GetArea() << endl;
cout << b->GetArea() << endl;
}
An important thing to notice here is - you don't have to know the exact type of the class you're using, just the base type, and you will get the right result. This is very useful in more complex systems as well.
Have fun learning!
Have you ever added two integers with +, and then later added an integer to a floating-point number with +?
Have you ever logged x.toString() to help you debug something?
I think you probably already appreciate polymorphism, just without knowing the name.
In a strictly typed language, polymorphism is important in order to have a list/collection/array of objects of different types. This is because lists/arrays are themselves typed to contain only objects of the correct type.
Imagine for example we have the following:
// the following is pseudocode M'kay:
class apple;
class banana;
class kitchenKnife;
apple foo;
banana bar;
kitchenKnife bat;
apple *shoppingList = [foo, bar, bat]; // this is illegal because bar and bat is
// not of type apple.
To solve this:
class groceries;
class apple inherits groceries;
class banana inherits groceries;
class kitchenKnife inherits groceries;
apple foo;
banana bar;
kitchenKnife bat;
groceries *shoppingList = [foo, bar, bat]; // this is OK
Also it makes processing the list of items more straightforward. Say for example all groceries implements the method price(), processing this is easy:
int total = 0;
foreach (item in shoppingList) {
total += item.price();
}
These two features are the core of what polymorphism does.
Advantage of polymorphism is client code doesn't need to care about the actual implementation of a method.
Take look at the following example.
Here CarBuilder doesn't know anything about ProduceCar().Once it is given a list of cars (CarsToProduceList) it will produce all the necessary cars accordingly.
class CarBase
{
public virtual void ProduceCar()
{
Console.WriteLine("don't know how to produce");
}
}
class CarToyota : CarBase
{
public override void ProduceCar()
{
Console.WriteLine("Producing Toyota Car ");
}
}
class CarBmw : CarBase
{
public override void ProduceCar()
{
Console.WriteLine("Producing Bmw Car");
}
}
class CarUnknown : CarBase { }
class CarBuilder
{
public List<CarBase> CarsToProduceList { get; set; }
public void ProduceCars()
{
if (null != CarsToProduceList)
{
foreach (CarBase car in CarsToProduceList)
{
car.ProduceCar();// doesn't know how to produce
}
}
}
}
class Program
{
static void Main(string[] args)
{
CarBuilder carbuilder = new CarBuilder();
carbuilder.CarsToProduceList = new List<CarBase>() { new CarBmw(), new CarToyota(), new CarUnknown() };
carbuilder.ProduceCars();
}
}
Polymorphism is the foundation of Object Oriented Programming. It means that one object can be have as another project. So how does on object can become other, its possible through following
Inheritance
Overriding/Implementing parent Class behavior
Runtime Object binding
One of the main advantage of it is switch implementations. Lets say you are coding an application which needs to talk to a database. And you happen to define a class which does this database operation for you and its expected to do certain operations such as Add, Delete, Modify. You know that database can be implemented in many ways, it could be talking to file system or a RDBM server such as MySQL etc. So you as programmer, would define an interface that you could use, such as...
public interface DBOperation {
public void addEmployee(Employee newEmployee);
public void modifyEmployee(int id, Employee newInfo);
public void deleteEmployee(int id);
}
Now you may have multiple implementations, lets say we have one for RDBMS and other for direct file-system
public class DBOperation_RDBMS implements DBOperation
// implements DBOperation above stating that you intend to implement all
// methods in DBOperation
public void addEmployee(Employee newEmployee) {
// here I would get JDBC (Java's Interface to RDBMS) handle
// add an entry into database table.
}
public void modifyEmployee(int id, Employee newInfo) {
// here I use JDBC handle to modify employee, and id to index to employee
}
public void deleteEmployee(int id) {
// here I would use JDBC handle to delete an entry
}
}
Lets have File System database implementation
public class DBOperation_FileSystem implements DBOperation
public void addEmployee(Employee newEmployee) {
// here I would Create a file and add a Employee record in to it
}
public void modifyEmployee(int id, Employee newInfo) {
// here I would open file, search for record and change values
}
public void deleteEmployee(int id) {
// here I search entry by id, and delete the record
}
}
Lets see how main can switch between the two
public class Main {
public static void main(String[] args) throws Exception {
Employee emp = new Employee();
... set employee information
DBOperation dboper = null;
// declare your db operation object, not there is no instance
// associated with it
if(args[0].equals("use_rdbms")) {
dboper = new DBOperation_RDBMS();
// here conditionally, i.e when first argument to program is
// use_rdbms, we instantiate RDBM implementation and associate
// with variable dboper, which delcared as DBOperation.
// this is where runtime binding of polymorphism kicks in
// JVM is allowing this assignment because DBOperation_RDBMS
// has a "is a" relationship with DBOperation.
} else if(args[0].equals("use_fs")) {
dboper = new DBOperation_FileSystem();
// similarly here conditionally we assign a different instance.
} else {
throw new RuntimeException("Dont know which implemnation to use");
}
dboper.addEmployee(emp);
// now dboper is refering to one of the implementation
// based on the if conditions above
// by this point JVM knows dboper variable is associated with
// 'a' implemenation, and it will call appropriate method
}
}
You can use polymorphism concept in many places, one praticle example would be: lets you are writing image decorer, and you need to support the whole bunch of images such as jpg, tif, png etc. So your application will define an interface and work on it directly. And you would have some runtime binding of various implementations for each of jpg, tif, pgn etc.
One other important use is, if you are using java, most of the time you would work on List interface, so that you can use ArrayList today or some other interface as your application grows or its needs change.
Polymorphism allows you to write code that uses objects. You can then later create new classes that your existing code can use with no modification.
For example, suppose you have a function Lib2Groc(vehicle) that directs a vehicle from the library to the grocery store. It needs to tell vehicles to turn left, so it can call TurnLeft() on the vehicle object among other things. Then if someone later invents a new vehicle, like a hovercraft, it can be used by Lib2Groc with no modification.
I guess sometimes objects are dynamically called. You are not sure whether the object would be a triangle, square etc in a classic shape poly. example.
So, to leave all such things behind, we just call the function of derived class and assume the one of the dynamic class will be called.
You wouldn't care if its a sqaure, triangle or rectangle. You just care about the area. Hence the getArea method will be called depending upon the dynamic object passed.
One of the most significant benefit that you get from polymorphic operations is ability to expand.
You can use same operations and not changing existing interfaces and implementations only because you faced necessity for some new stuff.
All that we want from polymorphism - is simplify our design decision and make our design more extensible and elegant.
You should also draw attention to Open-Closed Principle (http://en.wikipedia.org/wiki/Open/closed_principle) and for SOLID (http://en.wikipedia.org/wiki/Solid_%28Object_Oriented_Design%29) that can help you to understand key OO principles.
P.S. I think you are talking about "Dynamic polymorphism" (http://en.wikipedia.org/wiki/Dynamic_polymorphism), because there are such thing like "Static polymorphism" (http://en.wikipedia.org/wiki/Template_metaprogramming#Static_polymorphism).
You don't need polymorphism.
Until you do.
Then its friggen awesome.
Simple answer that you'll deal with lots of times:
Somebody needs to go through a collection of stuff. Let's say they ask for a collection of type MySpecializedCollectionOfAwesome. But you've been dealing with your instances of Awesome as List. So, now, you're going to have to create an instance of MSCOA and fill it with every instance of Awesome you have in your List<T>. Big pain in the butt, right?
Well, if they asked for an IEnumerable<Awesome>, you could hand them one of MANY collections of Awesome. You could hand them an array (Awesome[]) or a List (List<Awesome>) or an observable collection of Awesome or ANYTHING ELSE you keep your Awesome in that implements IEnumerable<T>.
The power of polymorphism lets you be type safe, yet be flexible enough that you can use an instance many many different ways without creating tons of code that specifically handles this type or that type.
Tabbed Applications
A good application to me is generic buttons (for all tabs) within a tabbed-application - even the browser we are using it is implementing Polymorphism as it doesn't know the tab we are using at the compile-time (within the code in other words). Its always determined at the Run-time (right now! when we are using the browser.)