The Java Tutorials : Lesson Generics Question and Answers

Question

Hello,
I've just finished reading the Generics Lesson from the Java Tutorials,
http://java.sun.com/docs/books/tutorial/java/generics/index.html
and was hoping to see the answers in the Question and Answer section at the end of the tutorial, but the link doesn't seem to be working.
Could someone please grade my answers?
The question is (copied from the Java Tutorials):
1. Consider the following classes:
    public class AnimalHouse<E> {
        private E animal;
        public void setAnimal(E x) {
            animal = x;
        public E getAnimal() {
            return animal;
    public class Animal{
    public class Cat extends Animal {
    public class Dog extends Animal {
    }For the following code snippets, identify whether the code:
* fails to compile,
* compiles with a warning,
* generates an error at runtime, or
* none of the above (compiles and runs without problem.)
a. AnimalHouse<Animal> house = new AnimalHouse<Cat>();
b. AnimalHouse<Dog> house = new AnimalHouse<Animal>();
c. AnimalHouse<?> house = new AnimalHouse<Cat>();
house.setAnimal(new Cat());
d. AnimalHouse house = new AnimalHouse();
house.setAnimal(new Dog());
My answers are:
a. fails to compile
b. fails to compile
c. compiles and runs without problem
d. compiles with a warning
I realize that I could re-read the tutorial, but aside from learning about Generics, I'm also trying to gauge how well I absorb new information, the first time I read it, so I would appreciate if someone could tell me if my answers are correct. Explanations for incorrect answers would also be appreciated.
Thanks

Answer 1

Hi,
as far as I understand it, and am able to explain, creating
new AnimalHouse<Cat>();has the knowledge (at compiletime) about the generic type Cat.
But as soon as this is assigned to
AnimalHouse<?> house = new AnimalHouse<Cat>();the variable house, this knowledge is lost for the variable house. So you have house which does not know the underlying generic type used. Then, there are either two options:
- allow adding anything via the set...-method
- do not allow adding/setting anything at all
In the first case, you would create the problem of being able to potentially add instances of types that do not have any relation to the originally used type Cat - which will cause problems (imagine setting a Car instance instead of a Cat instance and lateron expecting to retrieve something that should be a Cat but is a Car...).
So, the only safe option is the second one.
Weird to explain, but maybe there are better explanations around...
Bye.