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I have 2 classes: truck and sedan. When some action occurs, I want to add a sedan to the hashMap in Truck class. The map tells what Sedans are currently in the Truck's cargo. I don't want Truck and Sedan to know about each other so I have made a method in CarManager that Sedan can call which passes the id of the sedan and the id of the Truck that Sedan wants to be added to. Then CarManager will inform Truck that a Sedan wants to be added to the list. The issue is I don't know how CarManager will inform the Truck and what I should have in that addSedan method. I do have a HashMap in CarManager that has a collection of CarEntities. The Truck's addCar method cannot be accessed by CarManager since it isnt in the interface and I dont want to add it in the interface because not all CarEntity will use it. Can anyone help?

public interface CarEntity {
    String getId();
    double getSpeed();
    void move();
}

public class CarManager {
    private HashMap<String, CarEntity> hash = new HashMap<String, CarEntity>();
    public void addSedan(String carId, String truckId) {
    ???
    hash.get(truckId).addCarr(carId); //I don't think this will work
    }

}

public class Truck implements CarEntity { 
    private HashMap<String, CarEntity> cargo = new HashMap<String, CarEntity>();
    public void addCar(String id, CarEntity ce) {
        cargo.put(id,ce);
}

public class Sedan implements CarEntity {
    CarManager.addSedan("Car 1", "Truck 5");
}
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3 Answers 3

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If you may not use casts and instanceof and must use polymorphism, then add two methods to your CarEntity interface :

boolean canBeLoadedWithCars();
void addCar(CarEntity c) throws IllegalStateException;

The trucks can be loaded with cars, and thus implement the first methof by returning true. The other ones return false.

The addCar method of Truck adds the car to their map, whereas the other implementations throw an IllegalStateException because they can't be loaded with cars.

So the addCar method of the manager becomes

CarEntity truck = hashMap.get(truckId);
if (truck.canBeLoadedWithCars() {
    truck.addCar(sedan);
}
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7 Comments

But then I run into the problem that all CarEntities will need to implement those methods when truck only needs it
The implementation of those methods is super-easy. Just one line of code. But you could inherit a default implementation from an AbstractCarEntity class, and only override them in the Truck class.
But isn't it bad design to have some methods in an interface when only one class uses it?
No. It isn't necessarily bad design. You might introduce a Tractor later, which is also able to load cars, and your design wouldn't change at all, which is not the case by using instanceof and casts. That's the beauty of polymorphism.
I see. This was my initial intention but shouldn't the other classes that implement addCar just be blank like void addCar(CarEntity c) {}? What is that one line of code you are referring to?
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I guess one thing that you can do is 

CarEntity t = hash.get(truckId); 
if (t instanceof Truck)
   downcast car entity to truck
   call add car method
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1 Comment

I can't use that. Casting is frowned upon in my case.
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The answer depends on who is doing the action. If the Sedans add themselves to the truck then you should have an addTruck method which adds all of the trucks into the manager. The manager would store the Trucks in a Map.

private Map<String, Truck> trucks = new HashMap<String, Truck>();
public void registerTruck(Truck truck) {
    trucks.put(truck.getId(), truck);
}

Then the addCar() method on the manager would do:

public void addCar(String truckId, CarEntity car) {
    Truck truck = trucks.get(truckId);
    // null handling needed here
    truck.addCar(car);
}

If, instead, the trucks take the cars then you could register the cars instead. If you need to have both be by string id then you'll need to register both cars and trucks and do something like:

private Map<String, Truck> trucks = new HashMap<String, Truck>();
private Map<String, Sedan> sedans = new HashMap<String, Sedan>();

public void registerTruck(Truck truck) {
    trucks.put(truck.getId(), truck);
}
public void registerSedan(Sedan sedan) {
    sedans.put(sedan.getId(), sedan);
}

public void addSedan(String sedanId, String truckId) {
    Sedan sedan = sedans.get(sedanId);
    Truck truck = trucks.get(truckId);
    // null handling needed here
    truck.addCar(sedan);
}

Typically we use Java interfaces to accomplish the decoupling. The Truck class should be able to add a CarEntity to its load without knowing that it is a Sedan. In this case an addCar(CarEntity car) method on Truck sounds fine. The Sedan will never know it is on a Truck and all the truck knows are the methods exposed through the CarEntity interface. In this case maybe the manager goes away.

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5 Comments

I'm still a bit confused. Where is the trucks HashMap located in? In CarManager? Because I trying to to add a Sedan into a HashMap that is in the Truck class, not CarManager.
If you only want to refer to the Sedans and Trucks as String ids in the manager then the manager is going to have to have a mapping from truckId -> truck and sedanId -> Sedan. There still is going to be a collection of CarEntity objects in Truck.
Oh and you don't need the id field on the Truck.addCar(). It should call getId() on the CarEntity.
Sorry, I'm still a little confused. In addSedan(), truck.addCar(sedan); Is that a typo for truck? Shouldn't it be cargo.addCar(sedan); in my case?
You've found your answer but I thought I'd reply anyway. The addSedan() method is in the manager. It locates the truck and sedan instances and then calls truck.addCar(sedan). Inside of the Truck.addCar() method, it does a cargo.put(car.getId(), car).

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