Polymorphism vs Strategy pattern

Question

What is the difference between the Strategy pattern and Polymorphism in Java?

I'm confused that whatever is achieved via Strategy Pattern is basically possible by polymorphism. Correct me if I'm wrong in this regard.

Please, also provide me example to eradicate my confusion.

Perspective

It's apples and oranges since you can't have a Strategy Pattern without polymorphism as the interface must be implemented, and so your question is confusing to me. – Steapsin

I think he just wants an example in which the strategy pattern is more useful than polymorphism, this happens when you want to reuse different strategies for multiple object. Otherwise it really doesn't make sense and you should use polymorphism. – Garrotte

Related What pattern does Collections.sort with a Comparator use – Kenny

Answer 1

For me, the link from CKing post and the example in Wikipedia are clear enough, but I'll try to give you a new example. As they said, Strategy Pattern is mostly a way to change the behaviour of an algorithm at runtime. Of course you can achieve this in many different ways (such as holding a value and using switch-case, but it wouldn't be as nice as Strategy Pattern).

Let's say you're developing a turn-based strategy game with two kind of Units: Infantry and Tank (subclasses of Unit). Your terrain could be Plains, Railroad or Forests.

class Unit{
    MovementStrategy ms;      
    final int baseMovement;
    int x,y;

    public Unit(int baseMovement){
        this.baseMovement = baseMovement;
    }

    abstract void fire();

    void moveForward(){
        x = x + ms.getHexagonsToMove(baseMovement);
    }

    void setMovementStrategy(MovementStrategy ms){
        this.ms = ms;
    }
}

Any Unit subclass must implement fire() method because it's going to be completely different for them (Tank shots heavy long-distance round and Infantry shot several short distance light bullets). In this example we use normal polymorphism/inheritance since fire() method will be really different for any unit, and it won't change during the game.

class Infantry extends Unit{
    public Infantry(){
        super(2);
    }

    void fire(){
        //whatever
    }
}

class Tank extends Unit{
    public Tank(){
        super(5);
    }

    void fire(){
        //whatever
    }
}

Units also are able to move, and have a field baseMovement that holds the number of hexagons it can walk. We're developing a strategy game, not a real world simulation, so we don't care how they move, we just want to add a value to their coordinates (in my example I only use X coordinate in order to get a simpler code). If all the terrain was the same, we wouldn't need any Strategy object... but we need to change the behaviour of move() method at runtime!

So, we implement a different MovementStrategy class for each of our kinds of Terrain, and we program our game to trigger a setMovementStrategy() to any unit that move on each hexagon. And we don't even need to write anything else in our Unit subclasses.

interface MovementStrategy{
    public int getHexagonsToMove(int base);
}

class PlainMovementStrategy implements MovementStrategy{
    public int getHexagonsToMove(int base){
        return base;
    }
}

class RailroadMovementStrategy implements MovementStrategy{
    public int getHexagonsToMove(int base){
        return base*3;
    }
}

class ForestMovementStrategy implements MovementStrategy{
    public int getHexagonsToMove(int base){
        return (int)(base/2);
    }
}   

Now, when any Unit move inside a Forest, we call

unit.setMovementStrategy(new ForestMovementStrategy());

And as soon it goes to a Plain, we do:

unit.setMovementStrategy(new PlainMovementStrategy());

Now we're able to change how far away our units move depending on the Terrain, and we don't need to rewrite in any of the subclasses.

I hope this helps you a better understanding of the difference.

Vocative

Just Wondering: is there a benefit to using an enum for this particular example over creating a new object every single time the location changes movement strategies? – Deegan

Enum could work here, you can even add abstract methods to an enumeration. If you follow that route though, I suggest having the enum implement an interface. That way you can always swap out the enum implementation with another implementaiton without much refactoring of the calling class. – Killick

@Vocative "As they said, Strategy Pattern is mostly a way to change the behaviour of an algorithm at runtime.". Should you opt for simple inheritance instead if the algorithms don't change at runtime? – Thais

Answer 2

I'm confused that whatever is achieved via Strategy Pattern is basically possible by polymorphism.

You can't drive a car without a steering wheel. That does not mean that a steering wheel is a car. Similarly, the Strategy pattern relies on polymorphism but that does not mean that they are the same thing.

The purpose of the Strategy pattern is to promote the use of composition (has-a) over inheritance (is-a). Instead of your class inheriting behavior from a super class, you define the behavior in a separate class and your class has-a reference to it.

As far as an example goes, take a look at this answer that does a good job.

Kenny

Answer 3

• Basic difference : Polymorphism is programming language concept, and Strategy pattern is one of behavioral design pattern of GoF.

• Polymorphism is the provision of a single interface for several different underlying data types.

  • Example: The steering wheel(i.e., the interface) is same no matter what type of actual steering mechanism is used. That is, the steering wheel works the same whether your car has manual steering, power steering, or rack-and-pinion steering. Therefore once you know how to operate the steering wheel, you can drive any type of car.

  • In programming, Polymorphism implemented in two ways:

    • Early-Binding/Static/Compile-Time Polymorphism (ex: function overloading)
    • Late-Binding/Dynamic/Run-Time Polymorphism (ex: function overriding)

• A Strategy pattern defines a set of algorithms that can be used interchangeably.

  • The Strategy pattern is a dynamic pattern (How do you want run a behavior in software?).

  • Example of core java: java.util.Comparator#compare(), executed by among others Collections#sort().

  • Modes of transportation is analogous to strategy design pattern. We use car, bike, bus, local train and so on.. different strategies to go office day by day.

Cooking

java.util.Comparator#compare() is a great example!! Now I feel ashamed for not think of that first. I use it widely in my projects. – Vocative

If java.util Comparator#compare is an example of the strategy pattern, then is the strategy pattern and polymorphism one and the same thing? If yes, then what is the difference between Polymorphism and Strategy pattern? – Kenny

Answer 4

If you are establishing an analogy where:

• in one case you have several overridable methods;
• in the other case you have a Strategy interface with several implementations,

then the difference is the degree of coupling, which is very strong in the first case, whereas in the second case any foreign code can participate in your class's logic by contributing its Strategy implementation.

Masera

Answer 5

Q: What is the difference between the Strategy pattern and Polymorphism in Java?

The questions is certainly confusing since initially there seems not to be any relation between these two ideas.

Polymorphism is a much broader concept in programming and yes the strategy pattern in Java uses a form of polymorphism catalogued as inclusion polymorphism to accomplish its intent, but this is by no means the only type of polymorphism that exists, neither it is the only way to implement the strategy pattern as I will demonstrate soon.

Also polymorphism is not something that only exists in Java or in object-oriented programming languages. Different forms of polymorphism exist in all the programming paradigms and not in all languages you are forced to use polymorphism to implement a strategy pattern (e.g. functional languages).

For further discussion on this topic please read this other answer where we have discussed whether polymorphism is possible without inheritance and I provide interesting references and examples to other types of polymorphism like parametric and ad-hoc polymorphism.

Ideally this will reveal to you that polymorphism is a bigger concept that goes beyond the boundaries of object-oriented programming and even beyond inheritance and subtyping.

Q: I'm confused that whatever is achieved via Strategy Pattern is basically possible by polymorphism. Correct me if I'm wrong in this regard.

From my point of view the relations between these two concepts are: that the strategy pattern leverages the power of polymorphism available in languages like Java to implement its intent, and that polymorphism can be considered in itself a pattern.

For instance, consider this quote from the GoF book:

If we assumed procedural languages, we might have included design patterns called 'inheritance', 'encapsulation' and 'polymorphism'.

It is just that we rarely think of polymorphism as a pattern, first because it implies many things and because it is implemented differently in different languages and also because it typically presents as some form of language feature.

In his book Elemental Design Patterns, Jason Mc C. Smith comments on the GoF quote above saying:

Pattern are language-independent concepts; they take form and become concrete solutions when you implement them within a particular language with a given set of language features and constructs [...] This means that it is a bit strange to talk about "Java design pattern", "C++ design patterns", "Websphere design pattern" and so on, even though we all do it. It's a mildly lazy form of shorthand for what we really mean, or should mean: design patterns as implemented in Java, C++, WebSphere and so on, regardless of language or API.

So, as you can see, you are thinking in the Strategy pattern from the perspective of the Java implementation, but in other language paradigms such pattern may have been implemented in different ways, probably without using inheritance at all, for example, in pure functional programming languages this is most certainly implemented using high order functions and function composition.

As such, this would be an strategy pattern implementation without resorting to inclusion polymorphism at all. In a function composition strategy we may still be using other forms of polymorphism (e.g. parametric), but that is not a requirement for the strategy pattern

Q: Please, also provide me example to eradicate my confusion.

As discussed above, in Java we are probably forced to use inclusion polymorphism to implement a Strategy pattern, but as explained above as well, patterns are not something that belong to a specific language, so if we think of the strategy pattern as a concept living outside any language boundaries then you will easily see other languages implement this in different ways.

In some hypothetical functional language I may have a function that reads some data from a file, maybe the file is encrypted and you need to provide a decryption strategy:

function readFile(path: String, decrypt: string -> string) {
    return decrypt(loadFromDisk(path));
}

And that decrypt argument is a function that serves the purpose of a strategy pattern, it encapsulates an interchangeable algorithm.

Now you can do

readFile("customers.txt", aes)
readFile("finance.txt", blowfish)

Where aes and blowfish are decryption function strategies.

There are dozens of languages that work like this, SML, Haskell, JavaScript, etc.

Aridatha

Answer 6

First of all. Polymorphism can mean two different things. Most commonly polymorphism refers to a polymorphic type. However, you are asking for the pattern.

Polymorphic code can change itself each time it runs while the function of the code stays the same. An easy example is to do 1+3=4 instead of 5-1=4. Both achieves the same result using different code. This is useful for code that does not want to be recognized i.e computer viruses or cryptographic code.

Strategy pattern on the other hand uses a family of algorithms that can be interchanged. This might be used when translating text. First some code determines the language. if the language is swedish or spanish the text will be processed by, different functions of the same family, translateSwedish() or translateSpanish().

To round things up. Polymorphic code uses different code that achieves the same result. While Strategy uses different code to achieve better results.

Disgrace

Answer 7

Consider this

we have animals and a strategy pattern object to describe how they move... for instance

fly/swim/walk

Given the large number of animals that use any of these methods (ie thousands of different animals fly), we need to use the same code for many different animals. That code should only exist in one place, so that it is easily changed and doesn't take up any unneeded space.

In this example, a straightforward polymorphism approach will result in massive code duplication. A more complex approach which places a intermediate class between animal and say robin fail to take in to consideration that how a animal moves is not really what defines it. Furthermore, it is possible that a animal has other strategy objects and they cannot all be made polymorphic through intermediate classes.

Garrotte

Answer 8

One definition for Polymorphism is the provision of a single interface to entities of different types.

With that in mind say you have a “bird” interface and all of your bird classes must implement “laysEggs()” method, well no biggie that works. And as you keep coding your “bird paradise program” you now add the “fly()” and realize that overloading and overriding for penguin and kiwi is unnecessary as in real life they can't fly, yet you must still implement that method. This can become tedious and pointless as you are faced with Ostrich and others that can't fly. And even worst off when adding the method “swim()” because even fewer birds can swim. As you may already know, the strategy pattern solves this problem.

In lame-man’s terms you can think of Polymorphism as a conglomerate of practices, while the Strategy Pattern is the best practice for a specific case. Example: the Strategy Pattern is to be used when an algorithm's behavior needs to be selected at Runtime (via interchangeable algorithms). And although it is sort of true that whatever is achieved via Strategy Pattern is basically possible by polymorphism, without knowledge of the Strategy Pattern this would leave you with the “reinvent the wheel” problem to solve this specific problem. In conclusion, they are very different even if one is based on the other. I'll leave you to see “Ender Muab'Dib” code as it is well explained if you still want a code example from me just ask, cheers and hope I helped.

Discipline

Answer 9

Polymorphism is a principle and Strategy is a design pattern

From oracle documentation page

The dictionary definition of polymorphism refers to a principle in biology in which an organism or species can have many different forms or stages. This principle can also be applied to object-oriented programming and languages like the Java language. Subclasses of a class can define their own unique behaviors and yet share some of the same functionality of the parent class.

Polymorphism can be achieved in compile time ( method overloading) and run time (method overriding).

Strategy_pattern

1. Defines a family of algorithms,
2. Encapsulates each algorithm, and
3. Makes the algorithms interchangeable within that family.

Strategy can use run-time polymorphism principle to achieve the desired functionality.

Strategy pattern had one more component called Context in it's URL diagram. Refer to below SE posts:

Real World Example of the Strategy Pattern

Does this Java Strategy pattern have a redundant Context class?

Few more useful articles:

strategy by sourcemaking

Ordination

In short, a strategy pattern can use polymorphism. It's not a question of one vs. the other. – Beiderbecke

Answer 10

Here is the concise response:

1. Polymorphism is one of the fundamental characteristics of object-oriented programming.
2. Strategy pattern is a language-independent pattern that can be implemented by all programming languages paradigms, such as imperative, functional, and object-oriented.

Therefore, if you are going to implement Strategy pattern by using Java, that is OOP, then you just need to use polymorphism.

In other words, Strategy pattern is one of many forms of polymorphism.

Snobbish