How to pass a derived class to a function accepting the parent type

Question

I have a Sport base class that has a couple variables. I also have a few different classes derived from Sport such as Volleyball, Soccer, etc. that have additional methods and variables. They look somewhat like this.

public class Sport {
    public String commonVar;
    public Sport(String c) {
        this.commonVar = c;
    }
}

And then Volleyball:

public class Volleyball extends Sport {
    public String onlyVolleyball;
    public Volleyball(String c, String thisVar) {
        super(c);
        this.onlyVolleyball = thisVar;
    }
}

Now I want to be passing the derived classes of Sport into a function elsewhere, but making the parameters

public otherFunction(int one, int two, Sport sport) { }

Only treats sport as a Sport object (it will accept the derived classes but I cannot access any of the variables and methods not common to Sport). My question is how do I create a function that I am able to send all types of Sport derivatives to without losing any data?

Answer 1

You can't, but you can check if the object is actually a member of the extending class, then cast it and use it, as follows:

public otherFunction(int one, int two, Sport sport)
{
    if(sport instanceof Volleyball)
    {
        Volleyball volley = (Volleyball) sport;
        //Do Volleyball things
    }
    else
    {
        //Only do sport things.
    }
}

Answer 2

You shouldn't. If that function is to work with the abstraction Sport then all the information it needs should be available in Sport. Else you should really have functions (or strategies/handlers or similar) for each.

You are breaking the Open Closed Principle (the O in SOLID) in object orientation principles.

You CAN do it though, with instanceof checks and then cast:

if (sport instanceof Volleyball) {
  Volleyball v = (Volleyball)sport;
}

Answer 3

Basically, solution to your problem depends on what do you really want to achieve.

At the same time, considering your post, in my opinion you are attempting to mix different responsibilities, which are:

1. Handling Sport instances.
2. Handling derivatives of Sport instances.

Possibly this means that your method does not require Sport instances at all. What does it actually need is access to specific sports. Correct me if I'm wrong.

Therefore most simple solution would be to declare otherFunction like following:

public otherFunction(int one, int two, VolleyBall sport)
public otherFunction(int one, int two, Soccer sport)
public otherFunction(int one, int two, BasketBall sport)
public otherFunction(int one, int two, AnyOtherAwsomeSport sport)

In this way you are clearly telling what kind of sports your function is handling and moreover you have quite clear interface that is easy to understand and maintain.