Ismail Marmoush Ismail Marmoush - 4 months ago 8
Java Question

Why we can't do List<Parent> mylist = ArrayList<child>();

Why we can't do

List<Parent> mylist = ArrayList<child>();

Answer

Suppose we could. Then this program would have to be fine:

ArrayList<Banana> bananas = new ArrayList<Banana>();
List<Fruit> fruit = bananas;
fruit.add(new Apple());

Banana banana = bananas.get(0);

That's clearly not type safe - you've ended up with an apple in the collection of bananas.

What you can do is:

List<? extends Fruit> fruit = new ArrayList<Banana>();

this is safe, because the compiler won't then let you try to add to the list of fruit. It knows that it's a list of some kind of fruit, so you could write:

Fruit firstFruit = fruit.get(0);

but it doesn't know what exact kind of fruit it's a list of, and make sure you can't do the wrong thing.

See the Java generics FAQ another explanation.