covariance fail

Foo.java

// Assume Cat and Dog are descendants of Animal. Yes I know this is a garbage example of inheritance

public class Foo {
  public void failAppend(List<Cat> catList) {
    Cat cat = new Cat();
    Dog dog = new Dog();
    Aniaml animal = new Cat();
    
    catList.append(cat); // Obviously can do this
    catList.append(dog); // Obviously can't do this
    catList.append(animal); // (Slightly less) obviously can't do this.
    
    failAppend(catList, dog); // Whoops runtime error. We just put a dog into a list statically typed as containing only Cats
  }
  
  public void failAppend(List<Animal> animals, Animal a) {
    animals.append(a); // Duh, obviously we can do this!
  }
}