Tests for several classes implementing a generic interface

Dear All,

I'd like to write a unit test for classes implementing the same
generic interface, Something<T>.
Each class implements Something<T> with a specific class, e.g.
class IntegerThing implements Something<Integer> {...}
class StringThing implements Something<String> {...}

interface Something<T> {
  T createThing();
  void doSomething(T one, T other);
  Set<T> getThings();
}

public class IntegerThing implements Something<Integer> {
  private int counter;
    public Integer createThing() {
    return counter++;
  }
  public void doSomething(Integer one, Integer other) {
    // ...
  }
  public Set<Integer> getThings() {
    return new HashSet<Integer>();
  }
}

Then, I'd like to have a test where I only need to change the line
that instantiates the object under test. I've come up with:
public class ThingTest<T> {
  private Something<T> underTest = (Something<T>) new IntegerThing();

  @Test
  public void test() {
    T thingA = underTest.createThing();
    T thingB = underTest.createThing();
    underTest.doSomething(thingA, thingB);
    Set<T> result = underTest.getThings();
    // assert whatever...
  }
}

I don't want to create a whole parallel tree of ThingTest<Integer>,
ThingTest<Special>; this would be used to test each class just one, to
verify puzzle solutions from students. To check each solution, I'd
just replace "new IntegerThing()" with whatever class they used.

Now, this works fine, but gives me a warning: unchecked cast from
IntegerThing to Something<T>. Is there a way to avoid this? At compile
time, it is known that IntegerThing implements Something<Integer>; is
there a way to get the compiler figure out that T is Integer in this
case?

Thanks,
Kofa