How to achieve conversion from List<SomeParamType> to List<SomeParamType<?>>

Question

While using Spring ApplicationContext and its method getBeansOfType I have run into an issue with generic types. This demonstrates the problem:

class Test
{
  List<Generator<?>> allGenerators = 
      new ArrayList(getBeansOfType(Generator.class).values()); // Raw type warning 
                                                               // for new ArrayList()
  <T> Map<String, T> getBeansOfType(Class<T> klass) {
    return emptyMap();
  }
}

interface Generator<R> {}

I retrieve all beans of a parameterized type from the container. I want to have them as Generator<?> and use custom logic to safely cast to the appropriate type parameter based on a property on each Generator instance.

My problem is that I can't find any way to assign to allGenerators without type safety warnings, even though on a conceptual level I am not committing to any unchecked type parameter. I just want to convert a raw type to a wildcard type.

Is there any idiom which does not require @SuppressWarnings to get there?

Answer 1

Not actually a conversion, but a program that builds a list of the required type without warnings:

public List<Generator<?>> solution() {
    List<Generator<?>> dst = new ArrayList<>();
    for (Generator<?> g : getBeansOfType(Generator.class).values()) {
        dst.add(g);
    }
    return dst;
}

Answer 2

I suppose it's not possible (to convert a raw type to an one with a wildcard parameter).

The reason is that the generic type (and in particular, the wildcard) is something different than a class type and cannot be represented with the tools of class types.