how to achieve dynamic polymorphism without extending a class

Question

I was asked in an interview how you can achieve dynamic polymorphism without extending a class. How can this be done?

Answer 1

Decorator design pattern that exploits encapsulation is what you're looking for.

Polymorphism through inheritance:

class Cat {   void meow() {     // meow...   } } class Lion extends Cat { } 

Polymorphism through encapsulation (Decorator pattern):

interface Cat {   void meow();       } class Lion implements Cat {   private Cat cat;   void meow() {     this.cat.meow();   } } 

ps. More about decorators: http://www.yegor256.com/2015/02/26/composable-decorators.html

Answer 2

The simple solution is to write a class that implements an interface rather than extending a base class.

Another solution is to create a dynamic proxy ... which is essentially a clever way of implementing an interface without explicitly writing the class. See the Proxy javadoc for details.

And yes, these are (or can be) examples of the decorator pattern, though the key thing here is the implementation techniques rather than the design pattern.