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
        }
    }
}

score 76 · Accepted Answer · answered Oct 24 '12 at 19:45

76

Do this instead:

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

answered Oct 24 '12 at 19:45

mprivat

21,582
4
54
64

I knew it would be something simple. Is there a special name for having the generic type before the return type/a link to a general source would be nice. I'll try and tomorrow and accept if it works thanks. – Peanut Oct 24 '12 at 21:19
1

have a look at oracle's generics introduction or angelika langer's java generics FAQ. +1 for mprivat's direct solution. – DaveFar Oct 24 '12 at 21:57
1

@DaveBall Thanks for the angelika langer suggesetion, I'm aware of generics as a concept and have used them but have never looked at them in depth, this looks like a great in-depth discussion of them. The link for anyone else interested: http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html – Peanut Oct 25 '12 at 00:49
A little elaboration of this answer can be found [here](http://stackoverflow.com/a/9421315/1276636) – Sufian Oct 18 '14 at 13:14
@Peanut They are called generic methods – veritas Sep 24 '20 at 18:37
I understand the Class part in the argument to support type Vehicle. What I now understand is that if a method uses a generic in its argument, it needs to be added to the method syntax structure before the return type.of the method too – veritas Sep 24 '20 at 18:53

DaveFar · Answer 2 · 2012-10-24T21:54:57.223

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);
      }  
  }
}

score 1 · Answer 3 · answered Apr 08 '23 at 20:55

1

I have found this syntax to be working as expected:

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

answered Apr 08 '23 at 20:55

not2savvy

2,902
3
22
37

score 0 · Answer 4 · answered Mar 03 '21 at 17:33

The accepted answer works and got me where I wanted to go. I thought I would add this just to make it clearer to anyone who might need it.

In this case RevisedExposure is a sub-class of Exposure. I need to call GetMetadata() with a list of either of these, which results in the same result set.

private async Task<List<Metadata>> GetMetadata<T>(List<T> exposures) where T : Exposure

Now I can call this method from two places with different versions of the list like this.

var metadata = await GetExposureMetadata(revisions);

or

var metadata = await GetExposureMetadata(exposures);

works great!

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

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