Why can't I assign an instance with another type to a parameterized variable?

Question

Why can't I do this:

LinkedList<Fruit> myFruits = new LinkedList<Apple>();

Error message:

Type mismatch: cannot convert from LinkedList<Apple> to LinkedList<Fruit>

Where is the difference to the following?

Fruit fruit = new Apple();

possible duplicate of [Is \`List\` a subclass of \`List\`? Why aren't Java's generics implicitly polymorphic?](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicit) — jlordo, Jan 03 '13 at 18:27

score 10 · Accepted Answer · answered Jan 03 '13 at 15:09

Consider what you can do with a LinkedList<Fruit> - and think about what you'd want this code to do:

LinkedList<Apple> apples = new LinkedList<Apple>();
LinkedList<Fruit> fruits = apples;
fruits.add(new Banana());

Apple apple = apples.getFirst(); // Safe at compile time, but it's a Banana!

The conversion is the only place it makes sense for this to fail at compile-time. Now what you can write is:

LinkedList<? extends Fruit> myFruits = new LinkedList<Apple>();

... and then the compiler won't let you add anything to the list, as it doesn't know what the real element type is. Likewise you coudl write:

LinkedList<? super Apple> apples = new LinkedList<Fruit>();

Now you can add apples to the list, but you can't get apples out of the list, as again you don't know what the type would be.

Thanks for your answer. I've just found http://stackoverflow.com/a/6828257/562769 which also helped. — Martin Thoma, Jan 03 '13 at 15:39

score 1 · Answer 2 · edited May 23 '17 at 11:43

1

Polymorphism simply doesn't apply to generic types.

LinkedList<Fruit> is not same as LinkedList<Apple> even though Fruit is a super class of Apple.

Refer to this Answer for the reason.

edited May 23 '17 at 11:43

Community

1
1

answered Jan 03 '13 at 15:10

PermGenError

45,977
8
87
106

score 0 · Answer 3 · answered Jan 03 '13 at 15:09

Because then you could add an Orange to myFruits and that shouldn't work as the actual list is a list of Apple

For example (If you could make this);

List<Apple> myApples = new LinkedList<Apple>();
List<Fruit> myFruits = new LinkedList<Apple>();
myFruits.add(new Orange());

now myApples got an Orange in it

score 0 · Answer 4 · answered Jan 03 '13 at 15:17

A simple assignement is allowed because the reference is copied and the original is left untouched.

Apple apple = new Apple();
Fruit fruit = apple;
fruit = new Banana(); // apple is not touched and is still an Apple

whereas (AtomicReference to be a simple collection)

AtomicReference<Apple> apple = new AtomicReference<>(new Apple());
AtomicReference<Fruit> fruit = (AtomicReference) apple; // warning but compiles.
fruit.set(new Banana()); // this alters apple as well, making it invalid!
Apple apple2 = apple.get(); // throws ClassCastException.

Why can't I assign an instance with another type to a parameterized variable?

4 Answers4