Design a vending machine
Design a vending machine, that allows customers to buy different products with coins.
What patterns are going to use? How would you implement it?
Short Analysis
We need a vending machine, that:
- accepts coins (one by one) and shows the value of the accepted coins
- accepts a product order, if there are accepted coins
- releases the unaccepted coin
- releases the ordered product
- returns the change (if there is)
- releases the accepted coins by user request (when “Terminate” button pressed)
- provides service interface for loading coins/ products
- provides service interface for withdrawing coins/ products
Problems
List all problems, that come to your mind, and we need to solve:
- We should provide multiple products, that can be bought
- Each product should have a price
- We should have starting balance, so that we can return change
- We should accept different coins
- We should have a temp balance with the accepted coins, if the product is not released yet
- We should calculate the total value of the accepted coins
- We should release all unaccepted coins
- We should return change, if there is any
- We should “calculate” how to return change (the combination of coins)
- We should add the acquire the order balance, when the product is released
- It should be easy to modify the product list (add or remove products)
Design Patterns we may want to consider
Singleton
- You may want to limit the number of instances to 1 and ensure the serial access to the machine.
Builder
- You may use Builder pattern to “construct”/ “initialize” the machine
Factory
- To get an instance of the machine
Abstract Factory
- If you want to support multiple families of products
Strategy/ Policy
- To calculate the combination of coins, that form the change
- To implement security policy (in order to access the machine service interface)
Memento
You may use memento, if you want to keep the state in case the power stops ot etc.
Command
- You may use command to “cover” to issue commands when a button is pressed
- You can combine command + memento if the power stops and you want to continue when the power is back on