question about designing parking lot system when  an interviewer asked such a question you need to  

first understand what he actually wants and what  he is looking in the candidate and so you need  

to discuss first all the requirements with  the interviewer before directly jumping into  

designing the parking lot system since the parking  lot system design is a very vague question do not  

assume anything on your own but it specifically  clear out everything with the interviewer doing  

the first 5 to 10 minutes of the interview in  general you need to understand following when an  

interviewer is asking you such question the very  first thing he's looking at is how good are you at  

requirements collection whether you even clearly  list out all the possible requirements and whether  

you can successfully scope down the problem into  something which you can solve in 30 to 45 minutes  

another thing that the interviewer is looking  for is your system design it and especially  

object oriented design skills he is looking for  how well you can design the whole system in terms  

of components and classes how will your model  parking-lot system into different classes and  

objects identify constraints and design interfaces  how well versed are you in the object-oriented  

design concepts like abstraction encapsulation  inheritance and polymorphism can you define the  

links and association between the classes and  objects in your system another thing that you  

need to consider is that when you are designing  a parking-lot system you should be discussing the  

design in terms of different design patterns the  design patterns are well established solution to  

the most commonly encountered problems which  occurred during software development these  

represents best practices used by experienced  object-oriented software developers this shows the  

interviewer about experience in object oriented  analysis and design and provides a positive  

feedback to your overall interview performance  so the interviewer is looking for whether you  

can identify different object-oriented design  patterns all programming patterns while designing  

the parking-lot system you will be discussing your  requirement for the Binod system the interview we  

might show some different requirements to you for  example he can add two or more entrances or exits  

to the parking guard system in case the parking  lot system has two or more intense Azure exits the  

interviewer is also looking at how you can tackle  the concurrency in your design sometimes the  

interviewer gave you requirements about selecting  a parking spot near some entrance to see how you  

can figure out finding the solution for nearest  parking spot and the important thing that you  

need to understand is that this type of question  is not related to designing a distributed system  

and so you should not solve it as a distributed  system design unless you have clearly discussed  

this with the interviewer in the requirement  collection phase if you try to solve it as a  

distributed system design then the interviewer can  interpret it as you have not sufficient experience  

to scope down upon them correctly and instead of  simplifying you are all complicating the system  

design so let's first collect the requirements  so the very first requirement is that you should  

like this video and you should subscribe to the  stand if you haven't done that anyway jokes apart  

let's consider that we have a very huge parking  lot if you google you will find that there are  

many parking lots with parking sizes more than  10,000 for example the word largest parking  

lot is in West Edmonton in Alberta Canada which  has more than 20,000 parking spots for vehicles  

the seattle sea-tac airport has more than 13,000  parking spots for the vehicles so we are talking  

about parking spaces in the range of 10,000 to  30,000 so the first requirement is big parking  

lots 10k to 30k spots these huge parking lots will  have more than one internships and exits in our  

case let's assume that our parking lot has four  entrances and four exits for influences four exits  

the third requirement is the customers collect  ticket at the entrances and the parking spot is  

assigned on the ticket the fourth requirement is  the parking spot assigned to a vehicle should be  

near to the entrance so the parking spot should be  nearest to the entrance so what it means that if  

a customer is entering into the parking lot from  this entrance then the parking got assigned should  

be the nearest from this entrance and if another  customer is actually entering the parking lot from  

some other interns like this one then the parking  spot assigned to that customer should be nearest  

form this interns the fifth comment is the system  should not allow more vehicles then the capacity  

of the parking lot so there is a limit or capacity  and we can assume simply than the capacity that  

we have in our parking lot system is let's say  30,000 the sixth requirement is that there will  

be different types of parking spot for example  there will be parking spots for handicapped they  

will be parking spots for compact cars there  will be parking spots for large cars etc so in  

our design we will consider there are four types  of parking spots parking spots and these will be  

handicapped parking compact car large car large  vehicle compact vehicle and motorcycle the seventh  

requirement is the system should support hourly  parking rate and the final parking fee should be  

based on the total time that a vehicle is spent  in the parking lot the customers can pay parking  

fee both with cash and credit card so cash and  credit cards they are accepted there should be a  

monitoring system as well so the monitoring system  would be responsible for monitoring how many cars  

are entering and exiting etc and of course there  could be video surveillance as well as part of  

the monitoring system now the last but by the  important requirement that most interviewers  

actually forget is that you need to design the  system in such a way so that it can be applied  

for this toleration of parking system for more  and more parking lots with minimal changes in  

the code and design this means you can take your  system and apply it to a different parking lot  

maybe for a different shopping mall parking lot  or an airport parking lot with minimal changes  

mostly due to the configurations as of that new  parking lot describing the overall structure of  

the parking lot the number of flows the capacity  and the different types of parking spots that the  

new parking guard system actually support so the  basic idea is that you are working for a company  

which actually stalls in different parking lot  systems and so what you would like to do is you  

would like to have your one system designed and  then you what you would like to do is that you  

just want to apply your system that you have  designed and you want to reuse that system for  

this tradition of different parking lot systems  and the only thing that you would like to now  

change is the configuration information this is  code the usability now if you check out the online  

sources you will find that they discuss so many  different superficial requirements but please note  

if you discuss those superficial requirements then  an experienced interviewer can actually catch you  

there for example if there are multiple parking  flows let's say in Seattle Airport where we have  

like six seven different parking flows then the  requirement that there should be a panel at the  

internets of each floor showing the number of  free parking spots available in that floor is a  

conflicting the government with other requirement  where we have said that the parking spot will be  

assigned to a vehicle at the time of entrance  so we don't really need this requirement in our  

system why because we already know the parking  spot which is assigned to him at the entrance  

so if you had a requirement that each parking flow  should have a panels showing the available parking  

spots then this is a conflicting requirement  similarly the other online resources which  

discuss some requirements regarding for example  the color of the vehicle however an interviewer  

can ask you how the parking system will benefit  from such requirements how knowing the color of  

a car actually hurts indeed overall design of  the parking system so you need to understand  

this thing that during the Carmen collection you  should not discuss some superficial requirements  

with the interviewer which are hard for you to  justify that why you need them in designing the  

parking lot system it also shows that you are not  experiencing enough to collect the right set of  

requirements and you are over complicating  the requirements instead of simplifying the  

requirements to come up with a simple design we  will design that parking lot system using some  

design patterns there are three broad categories  of design patterns the first one is traditional  

design patterns the creation of design patterns  deals with how the objects are instantiated the  

second type of design patterns structural design  patterns the structural design patterns discuss  

how different objects and classes are composed  in order to form larger structure and the third  

design patterns are behavioral design patterns  the behavioral design patterns deals with these  

possibilities of the objects and how they interact  with each other before going into a design of  

the parking lot we are also going to discuss the  design approach that we will be taking to design  

the parking lot system so there are two different  design approaches that people usually take the  

first one is top-down design and the second one  is the bottom-up design in the top-down design  

we first construct the high-level object and  then we identify these smaller sub components  

in it and then we design those sub components  and then in each sub component we identify its  

sub components and then we had then design those  sub components whereas in a bottom-up approach we  

first design the smallest component and then we  use those smallest components to design a bigger  

component and so on and so on please note that the  bottom-up approach is aligned with object oriented  

design and so we should be using bottom-up design  approach while designing a bug in gaad's system  

at this point we should identify different  objects and actors in our system so we have  

the parking-lot system itself then we have entry  and exit terminals entry exit terminals both the  

entry and exit terminals will have printers the  exit terminal will also have a payment processor  

then in our parking lot system we have a parking  spot another actor is the ticket itself then we  

will have the database we will also have the  monitoring system let me know in the comments  

below if you think there are other actors as well  which we need in our parking lot system I will go  

further now into areas of these objects however  before going to that I would like to discuss  

one other thing if you have seen other online  sources you will find that they also describe  

the hick 11 actor in the parking lot system and  so in their design they usually have a vehicle  

class and then they drive different classes  from the vehicle class like compact vehicle  

large vehicle burst Prakash cetera please note  a good interviewer will ask you a question that  

why even you need a vehicle glass in your system  and why are you defining of a heater class he can  

ask you questions like how using a vehicle class  helps you in designing the world system what are  

the different options or methods that they were  heated class need that are helpful for you in  

designing your system so please understand the  difference between a vehicle parking spot and a  

vehicle class the parking lot system will be using  the vehicle parking spot but it will not be using  

a vehicle class s there is no reason to use a  vehicle class along with its subclasses so do  

not diss véhicule class along with its subclasses  if you don't have a clear story about the usage  

in the overall system now let's design different  interfaces classes and components in our parking  

lot system using bottom-up approach so a vehicle  parking spot is an actor or an object in our  

system there could be different types of hiding  spot example we could have a parking spot for  

handicapped we could have a compact parking spot  we could have a large parking spot we could have  

motorcycle parking spot as we have discussed in  the requirements now there are two different ways  

to represent those different types of parking  spots one way is to use in M types to represent  

each type of parking spot however this shows that  you are not that proficient in the object oriented  

design as this approach has some flaws for example  if you need to add another parking spot type then  

it could require changes at various places in the  code which would violate the open/close design  

principle this principle dictates that existing  and well tested classes should not be modified  

whether new feature needs to be built the second  approach which is more preferable approach is that  

we actually drive each different types of parking  spot from apparent parking spot interface or class  

so this diagram shows all the different types of  parking spot available in our system in terms of  

different classes so we have a parent parking  spot class and which has an ID and deserve as  

a property and there will be methods to get the  property like gate ID and there would be a gate  

inside function on the reserved property as well  and now if you see we have handicapped parking  

spot compact parking spot large parking spot and  motorcycle five spots are different classes which  

are derived from this parking spot and now in that  case if we have to add another type of parking  

spot what we would need to do is just actually  exchange this parking spot class by some other  

parking spot class that's it please note that the  parking spot class will be an extra class what it  

means that we cannot instantiate an object of  a parking spot we could instantiate an object  

of handicapped parking spot or large or compact  parking spot a motorcycle parking spot but not  

the parent parking spot class object we will have  parking ticket where we will have parking ticket  

ID in it parking spot ID the ticket will contains  information about this part of the buying slot  

number where the car needs to be parked then the  pion spot type and the issue time which is the  

time when the vehicle enter the parking lot if  we discuss in our requirement that there are two  

types of terminals that we have in our parking lot  system which are entry terminal and exit terminal  

and so we will have a terminal class which would  have a get ID as a the class function and then  

from this terminal class we would actually drive  two different class f1 for the into terminal one  

for the exit terminal and you can see right  now the entry terminal have caretaker at and  

with a parking spot type and then we have an exit  terminal X and which accept the ticket and it is  

a ticket it takes a ticket as input and these  normal I'm saying classes right now these are