Modelling an elevator using Object-Oriented Analysis and Design

Question

There are a set of questions that seem to be commonly-used in interviews and classes when it comes to object-oriented design and analysis  This is one of them  unfortunately  my OOP professor in college never actually gave an answer to it  and so I ve been wondering   The problem is as follows  design a basic set of objects methods to be used to simulate an elevator bank  What are the objects and their attributes methods   For the sake of argument  let us assume that our building has twenty floors  the bottom floor is the lobby  and the second floor connects to the parking garage  therefore  people will enter exit the building on either the bottom floor or the second floor   There is one elevator bank that services all the floors  there are three elevator shafts in the elevator bank  and one elevator per shaft   What would be the correct way to model this in an object-oriented model

User · Answer

Main thing to worry about is how would you notify the elevator that it needs to move up or down. and also if you are going to have a centralized class to control this behavior and how could you distribute the control.

It seems like it can be very simple or very complicated. If we don't take concurrency or the time for an elevator to get to one place, then it seems like it will be simple since we just need to check the states of elevator, like is it moving up or down, or standing still. But if we make Elevator implement Runnable, and constantly check and synchronize a queue (linkedList). A Controller class will assign which floor to go in the queue. When the queue is empty, the run() method will wait (queue.wait() ), when a floor is assigned to this elevator, it will call queue.notify() to wake up the run() method, and run() method will call goToFloor(queue.pop()). This will make the problem too complicated. I tried to write it on paper, but dont think it works. It seems like we don't really need to take concurrency or timing issue into account here, but we do need to somehow use a queue to distribute the control.

Any suggestion?

User · Answer

First there is an elevator class  It has a direction  up  down  stand  maintenance   a current floor and a list of floor requests sorted in the direction  It receives request from this elevator   Then there is a bank  It contains the elevators and receives the requests from the floors  These are scheduled to all active elevators  not in maintenance    The scheduling will be like    if available pick a standing elevator for this floor  else pick an elevator moving to this floor  else pick a standing elevator on an other floor  else pick the elevator with the lowest load    Each elevator has a set of states     Maintenance  the elevator does not react to external signals  only to its own signals   Stand  the elevator is fixed on a floor  If it receives a call  And the elevator is on that floor  the doors open  If it is on another floor  it moves in that direction  Up  the elevator moves up  Each time it reaches a floor  it checks if it needs to stop  If so it stops and opens the doors  It waits for a certain amount of time and closes the door  unless someting is moving through them  Then it removes the floor from the request list and checks if there is another request  If so the elevator starts moving again  If not it enters the state stand  Down  like up but in reverse direction    There are additional signals    alarm  The elevator stops  And if it is on a floor  the doors open  the request list is cleared  the requests moved back to the bank   door open  Opens the doors if an elevator is on a floor and not moving  door closes  Closed the door if they are open     EDIT  Some elevators don t start at bottom first floor esp  in case of skyscrapers   min floor  amp  max floor are two additional attributes for Elevator

User · Answer

See   Lu Luo  A UML Documentation for a Elevator System Distributed Embedded Systems  Fall 2000 Ph D  Project Report Carneghie Mellon University   link

User · Answer

I ve seen many variants of this problem  One of the main differences  that determines the difficulty  is whether there is some centralized attempt to have a  smart and efficient system  that would have load balancing  e g   send more idle elevators to lobby in morning   If that is the case  the design will include a whole subsystem with really fun design    A full design is obviously too much to present here and there are many altenatives   The breadth is also not clear  In an interview  they ll try to figure out how you would think  However  these are some of the things you would need    Representation of the central controller  assuming there is one   Representations of elevators Representations of the interface units of the elevator  these may be different from  elevator to elevator   Obviously also call buttons on every floor  etc   Representations of the arrows or indicators on each floor  almost a  view  of the elevator model   Representation of a human and cargo  may be important for factoring in maximal loads  Representation of the building  in some cases  as certain floors may be blocked at times  etc

User · Answer

I ve seen many variants of this problem  One of the main differences  that determines the difficulty  is whether there is some centralized attempt to have a  smart and efficient system  that would have load balancing  e g   send more idle elevators to lobby in morning   If that is the case  the design will include a whole subsystem with really fun design    A full design is obviously too much to present here and there are many altenatives   The breadth is also not clear  In an interview  they ll try to figure out how you would think  However  these are some of the things you would need    Representation of the central controller  assuming there is one   Representations of elevators Representations of the interface units of the elevator  these may be different from  elevator to elevator   Obviously also call buttons on every floor  etc   Representations of the arrows or indicators on each floor  almost a  view  of the elevator model   Representation of a human and cargo  may be important for factoring in maximal loads  Representation of the building  in some cases  as certain floors may be blocked at times  etc

User · Answer

Thing s to be Consider While Designing the Elevator System   Elevator Floor Location Identifier Number of steps Rotation speed Daterange InstallationDate MaintainenceDate Department Identifier AllowedWeight Detail   Description Poison Ratio  Statistics  Start Stop SetDirection SetRotationSpeed EmergencyStop   Stop   Alert EmergencyAccidentSenser Handler   Each button press results in an elevator request which has to be served  Each of these requests is tracked at a global place  The number of elevators in the building will be determined by the user  The building will contain a fixed number of floors  The number of passengers that can fit into the elevator will be fixed  The passengers will be counted as they leave the elevator at their destination floor  The destination floor will be determined using a  random  Poisson interval  When all of the passengers in the elevator have reached their destination floors  the elevator will return to the lobby to pickup more passengers

User · Answer

Donald Knuth s The Art of Computer Programming Vol 1 has a demonstration of elevator and the data-structures  Knuth presents a very thorough discussion and program   Knuth 1997   Information Structures   The Art of Computer Programming Vol  1 pp 302-308

User · Answer

Detailed Answer   http   www angelfire com trek software elevator html

User · Answer

Main thing to worry about is how would you notify the elevator that it needs to move up or down. and also if you are going to have a centralized class to control this behavior and how could you distribute the control.

It seems like it can be very simple or very complicated. If we don't take concurrency or the time for an elevator to get to one place, then it seems like it will be simple since we just need to check the states of elevator, like is it moving up or down, or standing still. But if we make Elevator implement Runnable, and constantly check and synchronize a queue (linkedList). A Controller class will assign which floor to go in the queue. When the queue is empty, the run() method will wait (queue.wait() ), when a floor is assigned to this elevator, it will call queue.notify() to wake up the run() method, and run() method will call goToFloor(queue.pop()). This will make the problem too complicated. I tried to write it on paper, but dont think it works. It seems like we don't really need to take concurrency or timing issue into account here, but we do need to somehow use a queue to distribute the control.

Any suggestion?

User · Answer

I ve seen many variants of this problem  One of the main differences  that determines the difficulty  is whether there is some centralized attempt to have a  smart and efficient system  that would have load balancing  e g   send more idle elevators to lobby in morning   If that is the case  the design will include a whole subsystem with really fun design    A full design is obviously too much to present here and there are many altenatives   The breadth is also not clear  In an interview  they ll try to figure out how you would think  However  these are some of the things you would need    Representation of the central controller  assuming there is one   Representations of elevators Representations of the interface units of the elevator  these may be different from  elevator to elevator   Obviously also call buttons on every floor  etc   Representations of the arrows or indicators on each floor  almost a  view  of the elevator model   Representation of a human and cargo  may be important for factoring in maximal loads  Representation of the building  in some cases  as certain floors may be blocked at times  etc

User · Answer

First there is an elevator class  It has a direction  up  down  stand  maintenance   a current floor and a list of floor requests sorted in the direction  It receives request from this elevator   Then there is a bank  It contains the elevators and receives the requests from the floors  These are scheduled to all active elevators  not in maintenance    The scheduling will be like    if available pick a standing elevator for this floor  else pick an elevator moving to this floor  else pick a standing elevator on an other floor  else pick the elevator with the lowest load    Each elevator has a set of states     Maintenance  the elevator does not react to external signals  only to its own signals   Stand  the elevator is fixed on a floor  If it receives a call  And the elevator is on that floor  the doors open  If it is on another floor  it moves in that direction  Up  the elevator moves up  Each time it reaches a floor  it checks if it needs to stop  If so it stops and opens the doors  It waits for a certain amount of time and closes the door  unless someting is moving through them  Then it removes the floor from the request list and checks if there is another request  If so the elevator starts moving again  If not it enters the state stand  Down  like up but in reverse direction    There are additional signals    alarm  The elevator stops  And if it is on a floor  the doors open  the request list is cleared  the requests moved back to the bank   door open  Opens the doors if an elevator is on a floor and not moving  door closes  Closed the door if they are open     EDIT  Some elevators don t start at bottom first floor esp  in case of skyscrapers   min floor  amp  max floor are two additional attributes for Elevator

User · Answer

First there is an elevator class  It has a direction  up  down  stand  maintenance   a current floor and a list of floor requests sorted in the direction  It receives request from this elevator   Then there is a bank  It contains the elevators and receives the requests from the floors  These are scheduled to all active elevators  not in maintenance    The scheduling will be like    if available pick a standing elevator for this floor  else pick an elevator moving to this floor  else pick a standing elevator on an other floor  else pick the elevator with the lowest load    Each elevator has a set of states     Maintenance  the elevator does not react to external signals  only to its own signals   Stand  the elevator is fixed on a floor  If it receives a call  And the elevator is on that floor  the doors open  If it is on another floor  it moves in that direction  Up  the elevator moves up  Each time it reaches a floor  it checks if it needs to stop  If so it stops and opens the doors  It waits for a certain amount of time and closes the door  unless someting is moving through them  Then it removes the floor from the request list and checks if there is another request  If so the elevator starts moving again  If not it enters the state stand  Down  like up but in reverse direction    There are additional signals    alarm  The elevator stops  And if it is on a floor  the doors open  the request list is cleared  the requests moved back to the bank   door open  Opens the doors if an elevator is on a floor and not moving  door closes  Closed the door if they are open     EDIT  Some elevators don t start at bottom first floor esp  in case of skyscrapers   min floor  amp  max floor are two additional attributes for Elevator

User · Answer

I ve seen many variants of this problem  One of the main differences  that determines the difficulty  is whether there is some centralized attempt to have a  smart and efficient system  that would have load balancing  e g   send more idle elevators to lobby in morning   If that is the case  the design will include a whole subsystem with really fun design    A full design is obviously too much to present here and there are many altenatives   The breadth is also not clear  In an interview  they ll try to figure out how you would think  However  these are some of the things you would need    Representation of the central controller  assuming there is one   Representations of elevators Representations of the interface units of the elevator  these may be different from  elevator to elevator   Obviously also call buttons on every floor  etc   Representations of the arrows or indicators on each floor  almost a  view  of the elevator model   Representation of a human and cargo  may be important for factoring in maximal loads  Representation of the building  in some cases  as certain floors may be blocked at times  etc

User · Answer

See   Lu Luo  A UML Documentation for a Elevator System Distributed Embedded Systems  Fall 2000 Ph D  Project Report Carneghie Mellon University   link

User · Answer

Thing s to be Consider While Designing the Elevator System   Elevator Floor Location Identifier Number of steps Rotation speed Daterange InstallationDate MaintainenceDate Department Identifier AllowedWeight Detail   Description Poison Ratio  Statistics  Start Stop SetDirection SetRotationSpeed EmergencyStop   Stop   Alert EmergencyAccidentSenser Handler   Each button press results in an elevator request which has to be served  Each of these requests is tracked at a global place  The number of elevators in the building will be determined by the user  The building will contain a fixed number of floors  The number of passengers that can fit into the elevator will be fixed  The passengers will be counted as they leave the elevator at their destination floor  The destination floor will be determined using a  random  Poisson interval  When all of the passengers in the elevator have reached their destination floors  the elevator will return to the lobby to pickup more passengers

User · Answer

Donald Knuth s The Art of Computer Programming Vol 1 has a demonstration of elevator and the data-structures  Knuth presents a very thorough discussion and program   Knuth 1997   Information Structures   The Art of Computer Programming Vol  1 pp 302-308

User · Answer

First there is an elevator class  It has a direction  up  down  stand  maintenance   a current floor and a list of floor requests sorted in the direction  It receives request from this elevator   Then there is a bank  It contains the elevators and receives the requests from the floors  These are scheduled to all active elevators  not in maintenance    The scheduling will be like    if available pick a standing elevator for this floor  else pick an elevator moving to this floor  else pick a standing elevator on an other floor  else pick the elevator with the lowest load    Each elevator has a set of states     Maintenance  the elevator does not react to external signals  only to its own signals   Stand  the elevator is fixed on a floor  If it receives a call  And the elevator is on that floor  the doors open  If it is on another floor  it moves in that direction  Up  the elevator moves up  Each time it reaches a floor  it checks if it needs to stop  If so it stops and opens the doors  It waits for a certain amount of time and closes the door  unless someting is moving through them  Then it removes the floor from the request list and checks if there is another request  If so the elevator starts moving again  If not it enters the state stand  Down  like up but in reverse direction    There are additional signals    alarm  The elevator stops  And if it is on a floor  the doors open  the request list is cleared  the requests moved back to the bank   door open  Opens the doors if an elevator is on a floor and not moving  door closes  Closed the door if they are open     EDIT  Some elevators don t start at bottom first floor esp  in case of skyscrapers   min floor  amp  max floor are two additional attributes for Elevator

User · Answer

Detailed Answer   http   www angelfire com trek software elevator html

[oop] Modelling an elevator using Object-Oriented Analysis and Design

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