Question

Consider that we have a Car object. The acceleration and breaking features are implemented using strategy pattern. But what if we want to introduce nitro gas feature to an existing car object ? What is the design pattern that I can use ?

I want to add the nitro feature(Attribute) after creating the car object.

Answer 1

In statically typed languages you can not add methods to an object at runtime. Compiler, when it encounters a statement like: car.nitroAccelerate(), checks whether a car object implements any interface that has nitroAccelerate method. If you could add (or remove) methods during runtime such checks would be impossible.

Dynamic languages allow to add methods during runtime. But this has a drawback, when you put car.nitroAccelerate() in the code, you need to carefully analyze if the car object in this point has such method.

You can use decorator to modify existing methods at runtime, but doing so, you are not modifying an existing object, just creating a new one that wraps the old one.

So if you do something like:

Car fasterCar = new CarWithNitro(car);

and some piece of your code still holds a reference to the original car, this original car would not be faster, because the act of wrapping does not modify the original.