Design Patterns: Coffee Vending Machine

The question was asked in an interview to design a coffee vending machine.

Most importantly it is required for us to create the object with the help of creational design patterns. We would try to go through all the creational patterns which are widely known to us.

Let's start with Builder design pattern.

Builder design pattern is applied when the object constructed is huge and application would want to split the responsibility of object creation to various different objects. It can even give you the flexibility to sneak in the different implementations of a certain object based on need.

Does this DP fit to design the coffee vending machine?

Let's look over at the requirement. Coffee vending machine dispatches only few basic things i.e. various tea, various coffees, and hot water. None of the objects are really complex in nature. So it would be waste to invest so much efforts to apply builder pattern for the given problem.

Okay, let's take up another pattern i.e. Singleton design pattern.

Singleton design patterns knowing or unknowing have been in use for us. The idea behind this pattern to allow only one instance across the application. One instance must serve the need of all object requirement.

Instead analyzing this overly, we can conclude that single instance can not serve our purpose in this case. So we can not apply singleton design pattern for our application.

Moving on to Factory Abstract design pattern. (during the interview, I was using this design pattern to implement coffee vending machine)
Factory abstract design application build various different objects based on the requirement. Application would not really not know the product yet, the object created would serve the purpose of the operation. In this case application would require one class/product for each type of product type.

Does this DP fit to design the coffee vending machine?

If vending machine is used to get hot water, an object of hot water class must be returned. In this where the options could be unlimited Factory Abstract limits these options. To add another option, application would always need to introduce new class and add the application's main object which returns the output.

Could Factory Method design pattern suffice the need for this?
Based on my understanding abstract factory and factory method are almost the same. The difference lies between the responsibility of object creation. In case of factory method, object creation responsibility is designated to a factory method.

So applying this method would also be difficult in our scenario.

Let's look at the last known creational DP i.e. Prototype DP. In this design pattern we maintain a registry of object at some place and clone the object as per the need. It is very similar to on demand object creation. So in this DP also we would need to create as many classes as the options in the vending machine. Again, we would be writing way too much code for our work.

As we could not achieve the best design pattern mechanism with the help of creational design pattern. Let's move to other side of design patterns.

Let's look at the requirement in a different way.

Coffee Vending machine is using Coffee, Tea, Water, Milk, and Sugar to create all the available options. It might be needed that option might be needed to add lemon in the water and sugar to create lemon tea. In this case, we can use mix of two patterns. One abstract factory pattern and another is decorator patterns to decorate the object based on need.

Utilization of Abstract Factory pattern. Create 3 separate classes for Water, Coffee, and Tea. These 3 are the main component of the vending machine.

Now decorate the Coffee object with milk to make it milk coffee. Coffee object in itself can be utilized for espresso. Whereas in case tea, we can add various style based on the need.

milkCoffee = new Coffee(new Milk());
milkTea = new Tea(new Milk(new Masala()))
gingerTea = new Tea(new Milk(new Ginger()))
gingerMasalaTea = new Tea(new Milk(new Ginger(new Masala())))

Styling the tea and coffee can be solved with decorator pattern.

Why did we use different objects for tea and coffee?
Tea, Coffee, and Water are different products. So based on usual product understanding, we kept them separate and added styles later which are ginger to tea, masala to tea, and so on. Though it is possible to add the styles to the water to create coffee and tea also.
milkCoffee = new Water(Coffee(new Milk()));

If you think that your understanding deviates from mine. Let me know. We can discuss that in details.