Design Elevator (Lift) in Java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.TreeSet;

public class MyLift {
    public static void main(String[] args) {
        System.out.println("Welcome to MyLift");
        
        // RequestListenerThread to read requested floor and add to Set
        Thread requestListenerThread = new Thread(new RequestListener(), "RequestListenerThread");
        
        // RequestProcessorThread to read Set and process requested floor
        Thread requestProcessorThread = new Thread(new RequestProcessor(), "RequestProcessorThread");
        
        Elevator.getInstance().setRequestProcessorThread(requestProcessorThread);        
        requestListenerThread.start();
        requestProcessorThread.start();       
    }
}

class Elevator {
    
    private static Elevator elevator = null;
    
    private TreeSet requestSet = new TreeSet();
    
    private int currentFloor = 0;

    private Direction direction = Direction.UP;

    private Elevator() {};
    
    private Thread requestProcessorThread;

    /**
     * @return singleton instance
     */
    static Elevator getInstance() {
        if (elevator == null) {
            elevator = new Elevator();
        }
        return elevator;
    }

    /**
     * Add request to Set
     * 
     * @param floor
     */
    public synchronized void addFloor(int f) {
        requestSet.add(f);
        
        if(requestProcessorThread.getState() == Thread.State.WAITING){
            // Notify processor thread that a new request has come if it is waiting
            notify();
        }else{
            // Interrupt Processor thread to check if new request should be processed before current request or not. 
            requestProcessorThread.interrupt();
        }
        
    }

    /**
     * @return next request to process based on elevator current floor and direction
     */
    public synchronized int nextFloor() {

        Integer floor = null;

        if (direction == Direction.UP) {
            if (requestSet.ceiling(currentFloor) != null) {
                floor = requestSet.ceiling(currentFloor);
            } else {
                floor = requestSet.floor(currentFloor);
            }
        } else {
            if (requestSet.floor(currentFloor) != null) {
                floor = requestSet.floor(currentFloor);
            } else {
                floor = requestSet.ceiling(currentFloor);
            }
        }
        if (floor == null) {
            try {
                System.out.println("Waiting at Floor :" + getCurrentFloor());
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        } else {
            // Remove the request from Set as it is the request in Progress.
            requestSet.remove(floor);
        }
        return (floor == null) ? -1 : floor;
    }

    public int getCurrentFloor() {
        return currentFloor;
    }
    
    /**
     * Set current floor and direction based on requested floor
     * 
     * @param currentFloor
     * @throws InterruptedException 
     */
    public void setCurrentFloor(int currentFloor) throws InterruptedException {
        if (this.currentFloor > currentFloor) {
            setDirection(Direction.DOWN);
        } else {
            setDirection(Direction.UP);
        }
        this.currentFloor = currentFloor;
        
        System.out.println("Floor : " + currentFloor);
        
        Thread.sleep(3000);
    }

    public Direction getDirection() {
        return direction;
    }

    public void setDirection(Direction direction) {
        this.direction = direction;
    }

    public Thread getRequestProcessorThread() {
        return requestProcessorThread;
    }

    public void setRequestProcessorThread(Thread requestProcessorThread) {
        this.requestProcessorThread = requestProcessorThread;
    }

    public TreeSet getRequestSet() {
        return requestSet;
    }

    public void setRequestSet(TreeSet requestSet) {
        this.requestSet = requestSet;
    }
    
}

class RequestProcessor implements Runnable {

    @Override
    public void run() {
        while (true) {
            Elevator elevator = Elevator.getInstance();
            int floor = elevator.nextFloor();
            int currentFloor = elevator.getCurrentFloor();
            try{
                if (floor >= 0) {
                    if (currentFloor > floor) {
                        while (currentFloor > floor) {
                            elevator.setCurrentFloor(--currentFloor);
                        }
                    } else {
                        while (currentFloor < floor) {
                            elevator.setCurrentFloor(++currentFloor);
                        }
                    }
                    System.out.println("Welcome to Floor : " + elevator.getCurrentFloor());
                }
                
            }catch(InterruptedException e){
                // If a new request has interrupted a current request processing then check -
                // -if the current request is already processed
                // -otherwise add it back in request Set
                if(elevator.getCurrentFloor() != floor){
                    elevator.getRequestSet().add(floor);
                }
            }
        }
    }
}

class RequestListener implements Runnable {

    @Override
    public void run() {

        while (true) {
            String floorNumberStr = null;
            try {
                // Read input from console
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
                floorNumberStr = bufferedReader.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }

            if (isValidFloorNumber(floorNumberStr)) {
                System.out.println("User Pressed : " + floorNumberStr);
                Elevator elevator = Elevator.getInstance();
                elevator.addFloor(Integer.parseInt(floorNumberStr));
            } else {
                System.out.println("Floor Request Invalid : " + floorNumberStr);
            }
        }
    }

    /**
     * This method is used to define maximum floors this elevator can process.
     * @param s - requested floor
     * @return true if requested floor is integer and upto two digits. (max floor = 99)
     */
    private boolean isValidFloorNumber(String s) {
        return (s != null) && s.matches("\\d{1,2}");
    }

}

enum Direction {
    UP, DOWN
}