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

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.