Consuming from more than one BlockingQueue

Question

I am trying to implement Elevator system which contains Elevator class that takes requests from ElevatorController and moves accordingly. ElevatorController maintains 2 blocking queues, one for upward requests and other for downward requests.

I want the elevator to look at both the queues and serve if any request appears in one of the queues.

If i use upwardQueue.take(), the elevator will wait for some request to appear it the upwardQueue. But at the same time, if a request appears in downwardQueue how can the elevator consume it?

Basically, how can the elevator consume requests from both the queues?

Answer 1

The ideal solution is to create a class specifically for requests.

public class FloorRequest {

  private final int floor;
  private final Direction direction;

  public FloorRequest(int floor, Direction direction) {
    this.floor = floor;
    this.direction = direction;
  }  

  public int getFloor() { return floor; }
  public Direction getDirection() { return direction; }

  public enum Direction {
    UP, DOWN
  }

}

Then you would use a single BlockingQueue of FloorRequest elements.

BlockingQueue<FloorRequest> queue = ...;
while (/* some condition */) {
  FloorRequest request = queue.take();
  switch (request.getDirection()) {
    case UP:
      // do something
      break;
    case DOWN:
      // do something else
      break;
  }
}