A super minimal implementation of a thread-pool-executor service / scheduler in Java. The goal is to provide a simple, easy to understand implementation. The implementation is not intended to be used in production, but rather to be used as a learning tool.

Problem

• There are 7 elevators in a building with 55 floors.

• Each floor has exactly one button to request an elevator.

• Elevators have the capacity of carrying an arbitrary, given number of people.

• Elevators don't change direction and don't stop mid-trip.

• People can only choose their destination freely, when they are on the ground-floor:

  • up: ground-floor [0] − can only travel to ⟶ non-ground-floor [1;54]
  • down: non-ground-floor [1;54] − can only travel to ⟶ ground-floor [0]

• The requests are handled globally.

• The elevator-scheduler assigns a chosen request to an elevator based on some arbitrary algorithm.

• If there are no elevators available, the request is queued up.

Design decisions

• Decided to use vanilla Java without any non-default libraries for the sake of simplicity.
• Got rid of the suggested "up"/"down" directions since they can be inferred from the current floor.
• Chose a poison-pill approach to terminate the scheduler (also see: this similar implementation)
• Chose JDK-19 to make use of type inferrence with var and other nifty features.