What is the real significance(use) of polymorphism_问答_开发者

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 polymorphis开发者_JAVA技巧m 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.

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.

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.

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.)