C#•3mo ago

Advice on making sure an object cannot be added to another list

I've got a program with boats, companies and fleets, if a ship is added to one fleet it cannot be added to another fleet, could someone give me some guideness as to how to achive this?

52 Replies

Pobiega•3mo ago

Easiest way is to make the boats keep track of what fleet they belong to. ie, if a boatA.Fleet is null then yeah, we can add it to a fleet

TheUnquiet•3mo ago

should i add an id field to each fleet ?

Pobiega•3mo ago

this is also how a relational database would keep track of it, by the way. well either that, or keep a direct refernce to the fleet object If we assume that companies, fleets and boats are all classes

TheUnquiet•3mo ago

yes

Pobiega•3mo ago

then just have a property of type Fleet in your boat class

TheUnquiet•3mo ago

all fleets have a dictionary of boats where the key is the boat name and the value is the boat object itself

Pobiega•3mo ago

thats fine I suggest you have both thou fleets keep track of boats, but boats also keep track of fleets

TheUnquiet•3mo ago

this is an assignment, i have to follow a specific document tho

Pobiega•3mo ago

this lets you navigate the tree two ways can you show the requirements?

TheUnquiet•3mo ago

sure but they are in dutch ill translate and send them your way

Pobiega•3mo ago

translate them. most browsers have built in autotranslate

TheUnquiet•3mo ago

Ships are managed by shipping companies, with each shipping company having different fleets, each fleet consisting of a number of ships. There are different types of ships, we distinguish: • Container ship: length, width, tonnage, name, number of containers, cargo value • Ro-Ro ship: length, width, tonnage, name, number of cars, number of trucks, cargo value • Cruise ship: length, width, tonnage , name, number of passengers, route • Ferry: length, width, tonnage, name, number of passengers, route • Oil tanker: length, width, tonnage, name, cargo value, volume (liters), cargo (oil, benzene, diesel or naphtha) •Gas tanker: length, width, tonnage, name, cargo value, volume (liters), cargo (LPG, LNG or ammonia) •Tugboat: length, width, tonnage, nameCargo value: is the value of the cargo expressed in euros. Tonnage: is the volume of the ship Route: for a ferry this is a fixed route between 2 ports, for a cruise ship this is a variable list of ports that are visited. The order of the ports is important for the route of a cruise ship. Ports can easily be represented by their name. A fleet has a name (e.g. North Sea Fleet) and has a number of ships. It should be possible to add ships to a fleet as well as to remove ships. A ship may of course only appear once in a fleet. It must also be possible to look up a ship very quickly based on the name of the ship. And it must also be possible to provide an overview of the ships included in a fleet. The shipping company has various fleets (e.g. North Atlantic fleet, North Sea fleet, Caribbean fleet, etc.). It should be possible to add or remove a fleet. Of course, the same fleet only appears once. A fleet must also be quickly searchable based on its name. A shipping company operates in several ports, so it should be possible to add ports to a shipping company (and also to remove ports). It should also be possible to provide an overview. The shipping company has several fleets (e.g. North Atlantic fleet, North Sea fleet, Caribbean fleet ...). It should be possible to add or remove a fleet. Of course, the same fleet only appears once. A fleet must also be quickly searchable based on its name. A shipping company operates in several ports, so it should be possible to add ports to a shipping company (and also to remove ports). It should also be possible to provide an overview the ports belonging to a shipping company in alphabetical order. Ports are simply represented by just a name (e.g. Antwerp, Rotterdam, ...). It should also be possible to place a ship in another fleet (e.g. place ship “De Plank” in fleet “Pacific Ocean”). Verder moet het mogelijk zijn om een overzicht te geven van : •De totale cargowaarde van deschepen die tot een rederij behoren.
•Het totaal aantal passagiers. •De tonnage per vloot op te lijsten (van groot naar klein). •Het totaal aantal volume die de tankers kunnen vervoeren. •De beschikbare sleepboten.Een rederij moet ook in staat zijn om info over een bepaald schip weer te geven (op basis van de naam van het schip).Opmerking : zorg ervoor dat alles efficiënt verloopt (denk eraan dat de lijsten in realiteit groot kunnen zijn en de opvragingen veelvuldig).

Pobiega•3mo ago

So far I don't see anything that prevents double bookkeeping

TheUnquiet•3mo ago

I think adding that extra field is okay I should add a field 'Fleet' to the Boat class, how will the rest work?

Pobiega•3mo ago

public class Fleet
{
    private List<Boat> _boats = new();

    public void AddBoat(Boat boat)
    {
        if (boat.Fleet != null)
        {
            throw new InvalidOperationException("Boat is already assigned to a fleet");
        }

        boat.Fleet = this;
        _boats.Add(boat);
    }
}

public class Boat
{
    public Fleet Fleet { get; internal set; }
}

public class Fleet
{
    private List<Boat> _boats = new();

    public void AddBoat(Boat boat)
    {
        if (boat.Fleet != null)
        {
            throw new InvalidOperationException("Boat is already assigned to a fleet");
        }

        boat.Fleet = this;
        _boats.Add(boat);
    }
}

public class Boat
{
    public Fleet Fleet { get; internal set; }
}

something like this you said you used a dictionary, so just change that around as needed

TheUnquiet•3mo ago

interesting the internal set is there because we don't want anyone outside the class to change the fleet that is assigned? am I correct?

Pobiega•3mo ago

well yesn't what you described would be private set but we cant use that, as we want Fleet to be able to set it

TheUnquiet•3mo ago

oh, what's the internal set do?

Pobiega•3mo ago

so public is everyone private is "just me" protected is "me and my children" internal is "everyone in my assembly" assembly being the current project

TheUnquiet•3mo ago

oh, what would just set be?

Pobiega•3mo ago

then it uses whatever access modifier the property itself has ie public in this case

TheUnquiet•3mo ago

c#
        public void VoegSchip(Schip schip)
        {
            try
            {
                if (!Schepen.ContainsKey(schip.Naam))
                {
                    Schepen.Add(schip.Naam, schip);
                } else
                {
                    throw new DomeinException("Vloot bevat al schip");
                }
            } catch(Exception e) { throw new DomeinException("Vloot : VoegSchip", e); }
        }

c#
        public void VoegSchip(Schip schip)
        {
            try
            {
                if (!Schepen.ContainsKey(schip.Naam))
                {
                    Schepen.Add(schip.Naam, schip);
                } else
                {
                    throw new DomeinException("Vloot bevat al schip");
                }
            } catch(Exception e) { throw new DomeinException("Vloot : VoegSchip", e); }
        }

this is my current function that adds a ship

Pobiega•3mo ago

Man coding in any language other than english is so cursed 😄

TheUnquiet•3mo ago

should I just change the whole thing? I'll rewrite it hold please

Pobiega•3mo ago

no I see whats going on. kinda what is VoegSchip thou? but this doesnt respect the rules you have regarding a ship only existing in one fleet, obviously you just check that THIS fleet doesnt already contain it

TheUnquiet•3mo ago

c#
        public void AddBoat(Boat boat)
        {
            try
            {
                if (!Boats.ContainsKey(ship.Name))
                {
                    Boats.Add(ship.Name, ship);
                } else
                {
                    throw new DomeinException("This ship belongs to this fleet");
                }
            } catch(Exception e) { throw new DomeinException("Fleet : AddBoat", e); }
        }

c#
        public void AddBoat(Boat boat)
        {
            try
            {
                if (!Boats.ContainsKey(ship.Name))
                {
                    Boats.Add(ship.Name, ship);
                } else
                {
                    throw new DomeinException("This ship belongs to this fleet");
                }
            } catch(Exception e) { throw new DomeinException("Fleet : AddBoat", e); }
        }

does this help a little?

Pobiega•3mo ago

Not a big fan of the throw inside the try

TheUnquiet•3mo ago

alright, I'll change that should I just use this instead of what I've had before?

Pobiega•3mo ago

well, currently you can't really do the requirement of not having the same ship in multiple fleets

TheUnquiet•3mo ago

I'll just use your function then, it makes more sense to me

Pobiega•3mo ago

it makes sense to me too the alternative is accessing all other fleets and checking their lists, which isnt very nice make sure you hide access to the dictionary/list thou (also, why is it a dictionary?)

TheUnquiet•3mo ago

because the program needs to be able to find ships easily

Pobiega•3mo ago

renaming a ship is now much harder. Will you very often do name -> ship lookups on a fleet basis?

TheUnquiet•3mo ago

no need for that, we don't need to rename any ships we only need to be able to delete, add and find ships in a fleet also change the fleet of a ship (I'll need to change this function then)

Pobiega•3mo ago

remember, since we are doing double book-keeping, we need to be clever about how you let the code edit these things for example, boat.Fleet = myNewFleet; can be very dangerous here, unless we override the setter and write logic in there to update the old AND new fleets too

TheUnquiet•3mo ago

the current function i have for ChangeFleet is the following:

c#
        public void ChangeFleet(string SourceFleetName, string DestFleetName, string ShipName)
        {
            try
            {
                FLeet src = FindFleet(vlootNaamSrc);
                Fleet dest = FindFleet(vlootNaamDest);
                Ship ship = src.FindFleet(schipNaam);

                if (src != null && dest != null && ship != null)
                {
                    src.DeleteShip(schipNaam);
                    dest.AddShip(schip);
                }
            } catch (Exception e) { throw new DomeinException("Company : ChangeFleet", e); }
        }

c#
        public void ChangeFleet(string SourceFleetName, string DestFleetName, string ShipName)
        {
            try
            {
                FLeet src = FindFleet(vlootNaamSrc);
                Fleet dest = FindFleet(vlootNaamDest);
                Ship ship = src.FindFleet(schipNaam);

                if (src != null && dest != null && ship != null)
                {
                    src.DeleteShip(schipNaam);
                    dest.AddShip(schip);
                }
            } catch (Exception e) { throw new DomeinException("Company : ChangeFleet", e); }
        }

can't I just add something like ship.Fleet = dest?

Pobiega•3mo ago

yeah, you can and you dont even need to do that because dest.AddShip does that

TheUnquiet•3mo ago

really?

Pobiega•3mo ago

src.DeleteShip should however be modified to set it to null and also rename to RemoveShip the ship still exists

TheUnquiet•3mo ago

what do you mean?

Pobiega•3mo ago

try it and see.

TheUnquiet•3mo ago

alright, thank you! It says that a ship with that key has already been added it's not removing the ship (i think) is that right?

Pobiega•3mo ago

remember, double book-keeping you need to remove it from both places ie, both links

TheUnquiet•3mo ago

so from the source Fleet and the Destanation Fleet?

Pobiega•3mo ago

no from the source fleet and the boat and that should be done by fleet.RemoveShip

TheUnquiet•3mo ago

set the Fleet field to null and the to the new Fleet?

Pobiega•3mo ago

ie, when you remove the boat, break both links yes removing a ship means to remove from the list AND to set the fleet prop to null

TheUnquiet•3mo ago

This is RemoveShip :

c#
        public void `RemoveShip(string ShipName)
        {
            try
            {
                if (Ships.ContainsKey(ShipName))
                {
                    Ships.Remove(ShipName);
                    Ship[ShipName].Fleet = null;
                }
            } catch (Exception e) { throw new DomeinException("Fleet : RemoveShip", e); }
        }

c#
        public void `RemoveShip(string ShipName)
        {
            try
            {
                if (Ships.ContainsKey(ShipName))
                {
                    Ships.Remove(ShipName);
                    Ship[ShipName].Fleet = null;
                }
            } catch (Exception e) { throw new DomeinException("Fleet : RemoveShip", e); }
        }

Pobiega•3mo ago

that aint gonna work look at your order of operations

TheUnquiet•3mo ago

I see it the ship isn't in the dict anymore

Pobiega•3mo ago

yeah so turn them around

TheUnquiet•3mo ago

thank you so much! you're amazing

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Advice on making sure an object cannot be added to another list