How to find object of given type in a heterogeneous List

Question

I have a heterogeneous List that can contain any arbitrary type of object. I have a need to find an element of the List that is of a certain type. Looking through the answers of other generics related questions, I'm not finding exactly what I need.

Here's an example of what I'm trying to accomplish:

List <Object> list = new ArrayList <Object>(); 

...

private someMethod() {
   Customer cust = findInList( Customer.class );
   Invoice inv = findInList( Invoice.class );
}

So, how do I define findInList using generics? I gather that type erasure causes issues here and I don't know as much about that as I probably should, but I'd rather not define multiple "find" methods since there could be dozens of different types of objects living in the List.

Answer 1

You can define the method using Class.isInstance() and Class.cast() method. Here is a sample implementation (with extra argument for the list):

static <T> T findInList(List<?> list, Class<T> clazz) {
  for (Object o : list) {
    if (clazz.isInstance(o)) {
      return clazz.cast(o);
    }
  }
  return null;
}

Update: I would recommend against putting multiple types in the Collection. It's usually a sign that either need a custom datatype (e.g. Transaction) or a Tuple value.

Answer 2

You can use Typesafe Heterogeneous Container pattern described by Josh Bloch. Here is an example from Josh's presentation:

class Favorites {
    private Map<Class<?>, Object> favorites =
            new HashMap<Class<?>, Object>();

    public <T> void setFavorite(Class<T> klass, T thing) {
        favorites.put(klass, thing);
    }

    public <T> T getFavorite(Class<T> klass) {
        return klass.cast(favorites.get(klass));
    }

    public static void main(String[] args) {
        Favorites f = new Favorites();
        f.setFavorite(String.class, "Java");
        f.setFavorite(Integer.class, 0xcafebabe);
        String s = f.getFavorite(String.class);
        int i = f.getFavorite(Integer.class);
    }
}

You can easily extend this to support List of favorites per type instead of a single value.