I have a method need to implement which ask me to return the ship in a queue according to some conditions

/** Returns the next ship waiting to enter the port. The queue should not change.
     *
     * The rules for determining which ship in the queue should be returned next are as
     * follows:
     * <ul>
     *     <li>If a ship is carrying dangerous cargo, it should be returned. If more than one
     *     ship is carrying dangerous cargo return the one added to the queue first.</li>
     *
     *     <li>If a ship requires medical assistance, it should be returned. If more than one
     *     ship requires medical assistance, return the one added to the queue first.</li>
     *
     *     <li>If a ship is ready to be docked, it should be returned. If more than one ship is
     *     ready to be docked, return the one added to the queue first.</li>
     *
     *     <li>If there is a container ship in the queue, return the one added to the queue first.</li>
     *
     *     <li>If this point is reached and no ship has been returned, return the ship that was
     *     added to the queue first.</li>
     *
     *     <li>If there are no ships in the queue, return null.</li>
     * </ul>
     * @return next ship in queue
     * */
    public Ship peek() {
        Queue<Ship> newShips = new LinkedList<>(ships);     // Make a copy of the queue and change here

        for (Ship s : newShips) {
            if (s.getFlag() == NauticalFlag.BRAVO) {    //ship carrying dangerous cargo
                return s;
            } else if (s.getFlag() == NauticalFlag.WHISKEY) {   // ship requires medical assistance
                return s;
            } else if (s.getFlag() == NauticalFlag.HOTEL) { // ship is ready to be docked
                return s;
            } else if (s.getName() == "ContainerShip") {    // Container Ship 
                return s;
            } else {
                return poll(); // ship that was added to the queue first
            }
        }
        if (newShips.isEmpty()) {
            return null;
        }
    }

I am stuck on returning the ship in the if else statement in the for loop. And also how can I know if the condition for example carrying dangerous cargo = NauticalFlag.BRAVO have more than one ship in the queue and return the one added to queue first.

CodePudding user response：

You must iterate through the queue everytime and look for the flags by priority, this is the way to do it in a clean way:

public Ship peek() {
  Queue<Ship> newShips = new LinkedList<>(ships);     // Make a copy of the queue and change here

  return findFirstShip(newShips, NauticalFlag.BRAVO)
      .orElse(findFirstShip(newShips, NauticalFlag.WHISKEY)
          .orElse(findFirstShip(newShips, NauticalFlag.HOTEL)
              .orElse(findFirstShip(newShips, NauticalFlag.CONTAINER)
                  .orElseGet(newShips::poll))));

}

private Optional<Ship> findFirstShip(Queue<Ship> newShips, NauticalFlag flag) {
  return newShips.stream().filter(ship -> ship.getFlag() == flag).findFirst();
}

CodePudding user response：

I would do this as a linear search of minimization algorithm, like this:

public class Ship
{
    /** The known ships. This needs to be populated for the example to work. */
    static List<Ship> ships = new ArrayList<> ();

    /** Type/status of this ship. */
    private NauticalFlag flag;

    /** Ship name or type name */
    private String name;

    public NauticalFlag getFlag ()
    {
        return flag;
    }

    public String getName ()
    {
        return name;
    }

    /** This is the original version from the question */
    public Ship peek1 ()
    {
        // Make a copy of the queue and change here
        final Queue<Ship> newShips = new LinkedList<> (ships);

        for (final Ship s : newShips)
        {
            if (s.getFlag () == NauticalFlag.BRAVO)
            { // ship carrying dangerous cargo
                return s;
            }
            else if (s.getFlag () == NauticalFlag.WHISKEY)
            { // ship requires medical assistance
                return s;
            }
            else if (s.getFlag () == NauticalFlag.HOTEL)
            { // ship is ready to be docked
                return s;
            }
            else if (s.getName () == "ContainerShip")
            { // Container Ship
                return s;
            }
            else
            {
                return newShips.poll (); // ship that was added to the queue first
            }
        }
        if (newShips.isEmpty ())
        {
            return null;
        }
        return newShips.poll ();
    }

    /** This is how I would do it as a linear search (or minimization). */
    public Ship peek2 ()
    {
        Ship result = null;

        for (final Ship s : ships)
        {
            if (priority (s) > priority (result))
            {
                result = s;
            }
        }
        return result;
    }

    /**
     * Define the preference function here. Note how null is handled first to prevent
     * nullPointerException problems.
     */
    public int priority (Ship s)
    {
        if (s == null)
        {
            return 0;
        }
        if (s.getFlag () == NauticalFlag.BRAVO)
        { // ship carrying dangerous cargo
            return 5;
        }
        else if (s.getFlag () == NauticalFlag.WHISKEY)
        { // ship requires medical assistance
            return 4;
        }
        else if (s.getFlag () == NauticalFlag.HOTEL)
        { // ship is ready to be docked
            return 3;
        }
        else if (s.getName () == "ContainerShip")
        { // Container Ship
            return 2;
        }
        return 1;
    }
}