System Design / State• Patterns: State, Strategy (Dispatching)

Elevator System

Hard

Problem Summary

Design a controller for a bank of elevators optimizing request handling, speed, and energy across multiple floors.

Functional Scope

Elevators must go Up, Down, or stay Idle.
Dispatch algorithm should pick the optimal elevator for internal and external requests.
Handle state transitions cleanly (doors opening, closing, moving, emergency stops).
Support request queues with direction optimization (e.g. SCAN algorithm).

Entity-Relationship (ER) Schema

ElevatorController [1] <---> [*] ElevatorCar
ElevatorCar [1] <---> [1] ElevatorState
ElevatorCar [1] <---> [*] Request

Design Approach

Use the State pattern to decouple direction & floor transition logics from the main Elevator class. Maintain separate sorted sets (upQueue and downQueue) for SCAN-based request handling.

Core Classes & Models

ElevatorController (Manages multiple cars)ElevatorCar (Current state, floor, direction)Request (Source floor, target floor, direction)ElevatorState (Interface for IdleState, MovingState, StoppedState)

Code Blueprint

public enum Direction { UP, DOWN, IDLE }
public interface ElevatorState {
    void handleRequest(ElevatorCar car, int floor);
}