Passing in a sub-class to a method but having the super class as the parameter?

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)

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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);       }     } }