Object-oriented design

These exercises focus on class design rather than distributed systems. The examples use Python.

The six questions

Hash map

class Item:
    def __init__(self, key, value): ...

class HashTable:
    def __init__(self, size):
        self.table = [[] for _ in range(size)]   # chained buckets
    def _hash_function(self, key):
        return hash(key) % self.size
    def set(self, key, value): ...   # append or replace in bucket
    def get(self, key): ...          # walk the bucket

LRU cache

• Node: holds the result.
• LinkedList: move_to_front, append_to_front, remove_from_tail.
• Cache: a hash map (query → node) plus the linked list; evicts the tail at capacity.

Call center

• Rank enum: operator, supervisor, director.
• Employee (abstract) holds rank and current call.
• A dispatch deque routes calls to the lowest-rank free employee; escalate if busy.

Deck of cards

• Suit enum (heart, diamond, clubs, spade); Card (abstract) with value, suit, availability.
• Deck, hand, and deal logic. Blackjack-style scoring is a common extension.

Parking lot

• VehicleSize enum: motorcycle, compact, large.
• Vehicle tracks license_plate, spot_size, and the spots it occupies.
• ParkingLot assigns spots by size and frees them on exit.

Chat server

• UserService: add_user, remove_user, and friend-request methods.
• User, Message, and conversation management.

How to structure the design

1. State the entities and their relationships.
2. Draw a class diagram (Mermaid classDiagram).
3. Write the key methods.
4. Agree on the amount of code before implementing.