Passing in a sub-class to a method but having the

Question

I have an abstract class Vehicle with 2 implemented subclasses RedVehicle and YellowVehicle.

In another class I have a List<Vehicle> containing instances of both subclasses. I want to be able to pass into a method a class type and then use that type to decide which set of objects I want to do something to in the List.

Since Class is generic I should parameterise it with something, however putting the parameter as the parent class Vehicle stops the calling code working since exampleMethod is now expecting a type of Vehicle, not a subclass of RedVehicle or YellowVehicle.

I feel there should be a clean way to do this so what would be the correct way to implement the functionality?

n.b. I don't necessarily have to pass in the Class type, if there are better suggestions I'd be happy to try those.

Calling code:

service.exampleMethod(RedVehicle.class);
service.exampleMethod(YellowVehicle.class);

Fields/Method:

//List of vehicles
//Vehicle has 2 subclasses, RedVehicle and YellowVehicle
private List<Vehicle> vehicles;

//Having <Vehicle> as the Class parameter stops the calling code working
public void exampleMethod(Class<Vehicle> type) 
{
    for(Vehicle v : vehicles)
    {
        if(v.getClass().equals(type))
        {
            //do something
        }
    }
}

Answer 1

Do this instead:

public <T extends Vehicle> void exampleMethod(Class<T> type) 

Answer 2

Why don't you use the visitor pattern?

That way you

• don't need type tokens
• let dynamic dispatch handle the case distinction (instead of if(v.getClass().equals(type)))
• are more flexible (following OCP)

---

In detail:

your abstract class Vehicle gets a method accept(Visitor v), with the subclasses implementing it by calling the appropriate method on v.

public interface Visitor {
  visitRedVehicle(RedVehicle red);
  visitYellowVehicle(YellowVehicle yellow);
}

Using a visitor:

public class Example {

  public void useYellowOnly() {
    exampleMethod(new Visitor() {
        visitRedVehicle(RedVehicle red) {};
        visitYellowVehicle(YellowVehicle yellow) {
             //...action
        });
  }
  public void exampleMethod(Visitor visitor){
      for(Vehicle v : vehicles) {
          v.accept(visitor);
      }  
  }
}