What is polymorphism, what is it for, and how is it used?

https://stackoverflow.com/questions/1031273

06-07-2019
|

Question

Solution

If you think about the Greek roots of the term, it should become obvious.

Poly = many: polygon = many-sided, polystyrene = many styrenes ^(a), polyglot = many languages, and so on.
Morph = change or form: morphology = study of biological form, Morpheus = the Greek god of dreams able to take any form.

So polymorphism is the ability (in programming) to present the same interface for differing underlying forms (data types).

For example, in many languages, integers and floats are implicitly polymorphic since you can add, subtract, multiply and so on, irrespective of the fact that the types are different. They're rarely considered as objects in the usual term.

But, in that same way, a class like BigDecimal or Rational or Imaginary can also provide those operations, even though they operate on different data types.

The classic example is the Shape class and all the classes that can inherit from it (square, circle, dodecahedron, irregular polygon, splat and so on).

With polymorphism, each of these classes will have different underlying data. A point shape needs only two co-ordinates (assuming it's in a two-dimensional space of course). A circle needs a center and radius. A square or rectangle needs two co-ordinates for the top left and bottom right corners and (possibly) a rotation. An irregular polygon needs a series of lines.

By making the class responsible for its code as well as its data, you can achieve polymorphism. In this example, every class would have its own Draw() function and the client code could simply do:

shape.Draw()

to get the correct behavior for any shape.

This is in contrast to the old way of doing things in which the code was separate from the data, and you would have had functions such as drawSquare() and drawCircle().

Object orientation, polymorphism and inheritance are all closely-related concepts and they're vital to know. There have been many "silver bullets" during my long career which basically just fizzled out but the OO paradigm has turned out to be a good one. Learn it, understand it, love it - you'll be glad you did :-)

^(a) I originally wrote that as a joke but it turned out to be correct and, therefore, not that funny. The momomer styrene happens to be made from carbon and hydrogen, C₈H₈, and polystyrene is made from groups of that, (C₈H₈)_n.

Perhaps I should have stated that a polyp was many occurrences of the letter p although, now that I've had to explain the joke, even that doesn't seem funny either.

Sometimes, you should just quit while you're behind :-)

OTHER TIPS

Polymorphism is when you can treat an object as a generic version of something, but when you access it, the code determines which exact type it is and calls the associated code.

Here is an example in C#. Create four classes within a console application:

public abstract class Vehicle
{
    public abstract int Wheels;
}

public class Bicycle : Vehicle
{
    public override int Wheels()
    {
        return 2;
    }
}

public class Car : Vehicle
{
    public override int Wheels()
    {
        return 4;
    }
}

public class Truck : Vehicle
{
    public override int Wheels()
    {
        return 18;
    }
}

Now create the following in the Main() of the module for the console application:

public void Main()
{
    List<Vehicle> vehicles = new List<Vehicle>();

    vehicles.Add(new Bicycle());
    vehicles.Add(new Car());
    vehicles.Add(new Truck());

    foreach (Vehicle v in vehicles)
    {
        Console.WriteLine(
            string.Format("A {0} has {1} wheels.",
                v.GetType().Name, v.Wheels));
    }
}

In this example, we create a list of the base class Vehicle, which does not know about how many wheels each of its sub-classes has, but does know that each sub-class is responsible for knowing how many wheels it has.

We then add a Bicycle, Car and Truck to the list.

Next, we can loop through each Vehicle in the list, and treat them all identically, however when we access each Vehicles 'Wheels' property, the Vehicle class delegates the execution of that code to the relevant sub-class.

This code is said to be polymorphic, as the exact code which is executed is determined by the sub-class being referenced at runtime.

I hope that this helps you.

From Understanding and Applying Polymorphism in PHP, Thanks Steve Guidetti.

Polymorphism is a long word for a very simple concept.

Polymorphism describes a pattern in object oriented programming in which classes have different functionality while sharing a common interface.

The beauty of polymorphism is that the code working with the different classes does not need to know which class it is using since they’re all used the same way. A real world analogy for polymorphism is a button. Everyone knows how to use a button: you simply apply pressure to it. What a button “does,” however, depends on what it is connected to and the context in which it is used — but the result does not affect how it is used. If your boss tells you to press a button, you already have all the information needed to perform the task.

In the programming world, polymorphism is used to make applications more modular and extensible. Instead of messy conditional statements describing different courses of action, you create interchangeable objects that you select based on your needs. That is the basic goal of polymorphism.

If anybody says CUT to these people

The Surgeon
The Hair Stylist
The Actor

What will happen?

The Surgeon would begin to make an incision.
The Hair Stylist would begin to cut someone's hair.
The Actor would abruptly stop acting out of the current scene, awaiting directorial guidance.

So above representation shows What is polymorphism (same name, different behavior) in OOP.

If you are going for an interview and interviewer asks you tell/show a live example for polymorphism in the same room we are sitting at, say-

Answer - Door / Windows

Wondering How?

Through Door / Window - a person can come, air can come, light can come, rain can come, etc.

To understand it better and in a simple manner I used above example.. If you need reference for code follow above answers.

Simple Explanation by analogy

The President of the United States employs polymorphism. How? Well, he has many advisers:

Military Advisers
Legal Advisers
Nuclear physicists (as advisers)
Medical advisers
etc etc.

Everyone Should only be responsible for one thing: Example:

The president is not an expert in zinc coating, or quantum physics. He doesn't know many things - but he does know only one thing: how to run the country.

It's kinda the same with code: concerns and responsibilities should be separated to the relevant classes/people. Otherwise you'd have the president knowing literally everything in the world - the entire Wikipedia. Imagine having the entire wikipedia in a class of your code: it would be a nightmare to maintain.

Why is that a bad idea for a president to know all these specific things?

If the president were to specifically tell people what to do, that would mean that the president needs to know exactly what to do. If the president needs to know specific things himself, that means that when you need to make a change, then you'll need to make it in two places, not just one.

For example, if the EPA changes pollution laws then when that happens: you'd have to make a change to the EPA Class and also the President class. Changing code in two places rather than one can be dangerous - because it's much harder to maintain.

Is there a better approach?

There is a better approach: the president does not need to know the specifics of anything - he can demand the best advice, from people specifically tasked with doing those things.

He can use a polymorphic approach to running the country.

Example - of using a polymorphic approach:

All the president does is ask people to advise him - and that's what he actually does in real life - and that's what a good president should do. his advisors all respond differently, but they all know what the president means by: Advise(). He's got hundreds of people streaming into his office. It doesn't actually matter who they are. All the president knows is that when he asks them to "Advise" they know how to respond accordingly:

public class MisterPresident
{
    public void RunTheCountry()
    {
        // assume the Petraeus and Condi classes etc are instantiated.
        petraeus.Advise(); // # Petraeus says send 100,000 troops to Fallujah
        condolezza.Advise(); // # she says negotiate trade deal with Iran
        healthOfficials.Advise(); // # they say we need to spend $50 billion on ObamaCare
    }
}

This approach allows the president to run the country literally without knowing anything about military stuff, or health care or international diplomacy: the details are left to the experts. The only thing the president needs to know is this: "Advise()".

What you DON"T want:

public class MisterPresident
{
    public void RunTheCountry()
    {
        // people walk into the Presidents office and he tells them what to do
        // depending on who they are.

        // Fallujah Advice - Mr Prez tells his military exactly what to do.
        petraeus.IncreaseTroopNumbers();
        petraeus.ImproveSecurity();
        petraeus.PayContractors();

        // Condi diplomacy advice - Prez tells Condi how to negotiate

        condi.StallNegotiations();
        condi.LowBallFigure();
        condi.FireDemocraticallyElectedIraqiLeaderBecauseIDontLikeHim();

        // Health care

        healthOfficial.IncreasePremiums();
        healthOfficial.AddPreexistingConditions();
    }
}

NO! NO! NO! In the above scenario, the president is doing all the work: he knows about increasing troop numbers and pre-existing conditions. This means that if middle eastern policies change, then the president would have to change his commands, as well as the Petraeus class as well. We should only have to change the Petraeus class, because the President shouldn't have to get bogged down in that sort of detail. He doesn't need to know about the details. All he needs to know is that if he makes one order, everything will be taken care of. All the details should be left to the experts.

This allows the president to do what he does best: set general policy, look good and play golf :P.

How is it actually implemented - through a base class or a common interface

That in effect is polymorphism, in a nutshell. How exactly is it done? Through "implementing a common interface" or by using a base class (inheritance) - see the above answers which detail this more clearly. (In order to more clearly understand this concept you need to know what an interface is, and you will need to understand what inheritance is. Without that, you might struggle.)

In other words, Petraeus, Condi and HealthOfficials would all be classes which "implement an interface" - let's call it the IAdvisor interface which just contains one method: Advise(). But now we are getting into the specifics.

This would be ideal

    public class MisterPresident
    {
            // You can pass in any advisor: Condi, HealthOfficials,
            //  Petraeus etc. The president has no idea who it will 
            // be. But he does know that he can ask them to "advise" 
            // and that's all Mr Prez cares for.

        public void RunTheCountry(IAdvisor governmentOfficer)
        {             
            governmentOfficer.Advise();              
        }
    }


    public class USA
    {
        MisterPresident president;

        public USA(MisterPresident president)
        {
            this.president = president;
        }

        public void ImplementPolicy()
        {
            IAdvisor governmentOfficer = getAdvisor(); // Returns an advisor: could be condi, or petraus etc.
            president.RunTheCountry(governmentOfficer);
        }
    }

Summary

All that you really need to know is this:

The president doesn't need to know the specifics - those are left to others.
All the president needs to know is to ask who ever walks in the door to advice him - and we know that they will absolutely know what to do when asked to advise (because they are all in actuality, advisors (or IAdvisors :) )

I really hope it helps you. If you don't understand anything post a comment and i'll try again.

Polymorphism is the ability to treat a class of object as if it is the parent class.

For instance, suppose there is a class called Animal, and a class called Dog that inherits from Animal. Polymorphism is the ability to treat any Dog object as an Animal object like so:

Dog* dog = new Dog;
Animal* animal = dog;

Polymorphism:

It is the concept of object oriented programming.The ability of different objects to respond, each in its own way, to identical messages is called polymorphism.

Polymorphism results from the fact that every class lives in its own namespace. The names assigned within a class definition don’t conflict with names assigned anywhere outside it. This is true both of the instance variables in an object’s data structure and of the object’s methods:

Just as the fields of a C structure are in a protected namespace, so are an object’s instance variables.
Method names are also protected. Unlike the names of C functions, method names aren’t global symbols. The name of a method in one class can’t conflict with method names in other classes; two very different classes can implement identically named methods.

Method names are part of an object’s interface. When a message is sent requesting that an object do something, the message names the method the object should perform. Because different objects can have methods with the same name, the meaning of a message must be understood relative to the particular object that receives the message. The same message sent to two different objects can invoke two distinct methods.

The main benefit of polymorphism is that it simplifies the programming interface. It permits conventions to be established that can be reused in class after class. Instead of inventing a new name for each new function you add to a program, the same names can be reused. The programming interface can be described as a set of abstract behaviors, quite apart from the classes that implement them.

Examples:

Example-1: Here is a simple example written in Python 2.x.

class Animal:
    def __init__(self, name):    # Constructor of the class
        self.name = name
    def talk(self):              # Abstract method, defined by convention only
        raise NotImplementedError("Subclass must implement abstract method")

class Cat(Animal):
    def talk(self):
        return 'Meow!'

class Dog(Animal):
    def talk(self):
        return 'Woof! Woof!'

animals = [Cat('Missy'),
           Dog('Lassie')]

for animal in animals:
    print animal.name + ': ' + animal.talk()

Example-2: Polymorphism is implemented in Java using method overloading and method overriding concepts.

Let us Consider Car example for discussing the polymorphism. Take any brand like Ford, Honda, Toyota, BMW, Benz etc., Everything is of type Car.

But each have their own advanced features and more advanced technology involved in their move behavior.

Now let us create a basic type Car

Car.java

public class Car {

    int price;
    String name;
    String color;

    public void move(){
    System.out.println("Basic Car move");
    }

}

Let us implement the Ford Car example.

Ford extends the type Car to inherit all its members(properties and methods).

Ford.java

public class Ford extends Car{
  public void move(){
    System.out.println("Moving with V engine");
  }
}

The above Ford class extends the Car class and also implements the move() method. Even though the move method is already available to Ford through the Inheritance, Ford still has implemented the method in its own way. This is called method overriding.

Honda.java

public class Honda extends Car{
  public void move(){
    System.out.println("Move with i-VTEC engine");
  }
}

Just like Ford, Honda also extends the Car type and implemented the move method in its own way.

Method overriding is an important feature to enable the Polymorphism. Using Method overriding, the Sub types can change the way the methods work that are available through the inheritance.

PolymorphismExample.java

public class PolymorphismExample {
  public static void main(String[] args) {
    Car car = new Car();
    Car f = new Ford();
    Car h = new Honda();

    car.move();
    f.move();
    h.move();

  }
}

Polymorphism Example Output:

In the PolymorphismExample class main method, i have created three objects- Car, Ford and Honda. All the three objects are referred by the Car type.

Please note an important point here that A super class type can refer to a Sub class type of object but the vice-verse is not possible. The reason is that all the members of the super class are available to the subclass using inheritance and during the compile time, the compiler tries to evaluate if the reference type we are using has the method he is trying to access.

So, for the references car,f and h in the PolymorphismExample, the move method exists from Car type. So, the compiler passes the compilation process without any issues.

But when it comes to the run time execution, the virtual machine invokes the methods on the objects which are sub types. So, the method move() is invoked from their respective implementations.

So, all the objects are of type Car, but during the run time, the execution depends on the Object on which the invocation happens. This is called polymorphism.

Usually this refers the the ability for an object of type A to behave like an object of type B. In object oriented programming this is usually achieve by inheritance. Some wikipedia links to read more:

EDIT: fixed broken links.

Polymorphism is this:

class Cup {
   int capacity
}

class TeaCup : Cup {
   string flavour
}

class CoffeeCup : Cup {
   string brand
}

Cup c = new CoffeeCup();

public int measure(Cup c) {
    return c.capacity
}

you can pass just a Cup instead of a specific instance. This aids in generality because you don't have to provide a specific measure() instance per each cup type

I know this is an older question with a lot of good answers but I'd like to include a one sentence answer:

Treating a derived type as if it were it's base type.

There are plenty of examples above that show this in action, but I feel this is a good concise answer.

(I was browsing another article on something entirely different.. and polymorphism popped up... Now I thought that I knew what Polymorphism was.... but apparently not in this beautiful way explained.. Wanted to write it down somewhere.. better still will share it... )

http://www.eioba.com/a/1htn/how-i-explained-rest-to-my-wife

read on from this part:

..... polymorphism. That's a geeky way of saying that different nouns can have the same verb applied to them.

The term polymorphism comes from:

poly = many

morphism = the ability to change

In programming, polymorphism is a "technique" that lets you "look" at an object as being more than one type of thing. For instance:

A student object is also a person object. If you "look" (ie cast) at the student, you can probably ask for the student ID. You can't always do that with a person, right? (a person is not necessarily a student, thus might not have a student ID). However, a person probably has a name. A student does too.

Bottom line, "looking" at the same object from different "angles" can give you different "perspectives" (ie different properties or methods)

So this technique lets you build stuff that can be "looked" at from different angles.

Why do we use polymorphism? For starters ... abstraction. At this point it should be enough info :)

Generally speaking, it's the ability to interface a number of different types of object using the same or a superficially similar API. There are various forms:

Function overloading: defining multiple functions with the same name and different parameter types, such as sqrt(float), sqrt(double) and sqrt(complex). In most languages that allow this, the compiler will automatically select the correct one for the type of argument being passed into it, thus this is compile-time polymorphism.
Virtual methods in OOP: a method of a class can have various implementations tailored to the specifics of its subclasses; each of these is said to override the implementation given in the base class. Given an object that may be of the base class or any of its subclasses, the correct implementation is selected on the fly, thus this is run-time polymorphism.
Templates: a feature of some OO languages whereby a function, class, etc. can be parameterised by a type. For example, you can define a generic "list" template class, and then instantiate it as "list of integers", "list of strings", maybe even "list of lists of strings" or the like. Generally, you write the code once for a data structure of arbitrary element type, and the compiler generates versions of it for the various element types.

Let's use an analogy. For a given musical script every musician which plays it gives her own touch in the interpretation.

Musician can be abstracted with interfaces, genre to which musician belongs can be an abstrac class which defines some global rules of interpretation and every musician who plays can be modeled with a concrete class.

If you are a listener of the musical work, you have a reference to the script e.g. Bach's 'Fuga and Tocata' and every musician who performs it does it polymorphicaly in her own way.

This is just an example of a possible design (in Java):

public interface Musician {
  public void play(Work work);
}

public interface Work {
  public String getScript();
}

public class FugaAndToccata implements Work {
  public String getScript() {
    return Bach.getFugaAndToccataScript();
  }
}

public class AnnHalloway implements Musician {
  public void play(Work work) {
    // plays in her own style, strict, disciplined
    String script = work.getScript()
  }
}

public class VictorBorga implements Musician {
  public void play(Work work) {
    // goofing while playing with superb style
    String script = work.getScript()
  }
}

public class Listener {
  public void main(String[] args) {
    Musician musician;
    if (args!=null && args.length > 0 && args[0].equals("C")) {
      musician = new AnnHalloway();
    } else {
      musician = new TerryGilliam();
    }
    musician.play(new FugaAndToccata());
}

I've provided a high-level overview of polymorphism for another question:

Polymorphism in c++

Hope it helps. An extract...

...it helps to start from a simple test for it and definition of [polymorphism]. Consider the code:

Type1 x;
Type2 y;

f(x);
f(y);

Here, f() is to perform some operation and is being given the values x and y as inputs. To be polymorphic, f() must be able to operate with values of at least two distinct types (e.g. int and double), finding and executing type-appropriate code.

( continued at Polymorphism in c++ )

Polymorphism is an ability of object which can be taken in many forms. For example in human class a man can act in many forms when we talk about relationships. EX: A man is a father to his son and he is husband to his wife and he is teacher to his students.

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. In this example that is written in Java, we have three type of vehicle. We create three different object and try to run their wheels method:

public class PolymorphismExample {

    public static abstract class Vehicle
    {
        public int wheels(){
            return 0;
        }
    }

    public static class Bike extends Vehicle
    {
        @Override
        public int wheels()
        {
            return 2;
        }
    }

    public static class Car extends Vehicle
    {
        @Override
        public int wheels()
        {
            return 4;
        }
    }

    public static class Truck extends Vehicle
    {
        @Override
        public int wheels()
        {
            return 18;
        }
    }

    public static void main(String[] args)
    {
        Vehicle bike = new Bike();
        Vehicle car = new Car();
        Vehicle truck = new Truck();

        System.out.println("Bike has "+bike.wheels()+" wheels");
        System.out.println("Car has "+car.wheels()+" wheels");
        System.out.println("Truck has "+truck.wheels()+" wheels");
    }

}

The result is:

For more information please visit https://github.com/m-vahidalizadeh/java_advanced/blob/master/src/files/PolymorphismExample.java. I hope it helps.

Polymorphism is the ability of the programmer to write methods of the same name that do different things for different types of objects, depending on the needs of those objects. For example, if you were developing a class called Fraction and a class called ComplexNumber, both of these might include a method called display(), but each of them would implement that method differently. In PHP, for example, you might implement it like this:

//  Class definitions

class Fraction
{
    public $numerator;
    public $denominator;

    public function __construct($n, $d)
    {
        //  In real life, you'd do some type checking, making sure $d != 0, etc.
        $this->numerator = $n;
        $this->denominator = $d;
    }

    public function display()
    {
        echo $this->numerator . '/' . $this->denominator;
    }
}

class ComplexNumber
{
    public $real;
    public $imaginary;

    public function __construct($a, $b)
    {
        $this->real = $a;
        $this->imaginary = $b;
    }

    public function display()
    {
        echo $this->real . '+' . $this->imaginary . 'i';
    }
}


//  Main program

$fraction = new Fraction(1, 2);
$complex = new ComplexNumber(1, 2);

echo 'This is a fraction: '
$fraction->display();
echo "\n";

echo 'This is a complex number: '
$complex->display();
echo "\n";

Outputs:

This is a fraction: 1/2
This is a complex number: 1 + 2i

Some of the other answers seem to imply that polymorphism is used only in conjunction with inheritance; for example, maybe Fraction and ComplexNumber both implement an abstract class called Number that has a method display(), which Fraction and ComplexNumber are then both obligated to implement. But you don't need inheritance to take advantage of polymorphism.

At least in dynamically-typed languages like PHP (I don't know about C++ or Java), polymorphism allows the developer to call a method without necessarily knowing the type of object ahead of time, and trusting that the correct implementation of the method will be called. For example, say the user chooses the type of Number created:

$userNumberChoice = $_GET['userNumberChoice'];

switch ($userNumberChoice) {
    case 'fraction':
        $userNumber = new Fraction(1, 2);
        break;
    case 'complex':
        $userNumber = new ComplexNumber(1, 2);
        break;
}

echo "The user's number is: ";
$userNumber->display();
echo "\n";

In this case, the appropriate display() method will be called, even though the developer can't know ahead of time whether the user will choose a fraction or a complex number.

In object-oriented programming, polymorphism refers to a programming language's ability to process objects differently depending on their data type or class. More specifically, it is the ability to redefine methods for derived classes.

Polymorphism literally means, multiple shapes. (or many form) : Object from different classes and same name method , but workflows are different. A simple example would be:

Consider a person X.

He is only one person but he acts as many. You may ask how:

He is a son to his mother. A friend to his friends. A brother to his sister.

Polymorphism in OOP means a class could have different types, inheritance is one way of implementing polymorphism.

for example, Shape is an interface, it has Square, Circle, Diamond subtypes. now you have a Square object, you can upcasting Square to Shape automatically, because Square is a Shape. But when you try to downcasting Shape to Square, you must do explicit type casting, because you can't say Shape is Square, it could be Circle as well. so you need manually cast it with code like Square s = (Square)shape, what if the shape is Circle, you will get java.lang.ClassCastException, because Circle is not Square.

Polymorphism:

Different execution according to the instance of the class, not the type of reference variable.

An interface type reference variable can refer to any of the class instances that implement that interface.

Polymorphism is the ability to use an object in a given class, where all components that make up the object are inherited by subclasses of the given class. This means that once this object is declared by a class, all subclasses below it (and thier subclasses, and so on until you reach the farthest/lowest subclass) inherit the object and it's components (makeup).

Do remember that each class must be saved in separate files.

The following code exemplifies Polymorphism:

The SuperClass:

public class Parent {
    //Define things that all classes share
    String maidenName;
    String familyTree;

    //Give the top class a default method
    public void speak(){
         System.out.println("We are all Parents");
    }
}

The father, a subclass:

public class Father extends Parent{
    //Can use maidenName and familyTree here
    String name="Joe";
    String called="dad";

    //Give the top class a default method
    public void speak(){
        System.out.println("I am "+name+", the father.");
    }
}

The child, another subclass:

public class Child extends Father {
    //Can use maidenName, familyTree, called and name here

    //Give the top class a default method
    public void speak(){
        System.out.println("Hi "+called+". What are we going to do today?");
    }
}

The execution method, references Parent class to start:

public class Parenting{
    public static void main(String[] args) {
        Parent parents = new Parent();
        Parent parent = new Father();
        Parent child = new Child();

        parents.speak();
        parent.speak();
        child.speak();
    }
}

Note that each class needs to be declared in separate *.java files. The code should compile. Also notice that you can continually use maidenName and familyTree farther down. That is the concept of polymorphism. The concept of inheritance is also explored here, where one class is can be used or is further defined by a subclass.

Hope this helps and makes it clear. I will post the results when I find a computer that I can use to verify the code. Thanks for the patience!

Polymorphism allows the same routine (function, method) to act on different types.

Since many existing answers are conflating subtyping with polymorphism, here are three ways (including subtyping) to implement polymorphism.

Parameteric (generic) polymorphism allows a routine to accept one or more type parameters, in addition to normal parameters, and runs itself on those types.
Subtype polymorphism allows a routine to act on any subtype of its parameters.
Ad hoc polymorphism generally uses routine overloading to grant polymorphic behavior, but can refer to other polymorphism implementations too.