Generics Java Wildcards and Subtyping

Question

I am reading on Generics in Java atm, and things go a little slow would love some help. I read from Oracles own database:

https://docs.oracle.com/javase/tutorial/java/generics/subtyping.html

At the bottom we can see List<Integer> is a subtype of List<? extends Number> List<? extends Number>

I also came across this stackoverflow question:

Java Generic type : difference between List <? extends Number> and List <T extends Number>

Which says in one answer: this is true:

((List<Integer>)list).add((int) s);

I have verified it, so that's ok. But I don't understand It completely.

What if the Wildcard is the Short class and I add a number higher than 2^15-1(=32767) Shouldn't it give error?

I even tried something like this and it works fine:

import java.util.*;
class CastingWildcard{
    public static void main(String[] args){
        List<? extends Number> list = new ArrayList<Short>();
        int s=32770;
        ((List<Integer>)list).add((int) s);
        System.out.println(list.get(0));
    }
}

To sum up: Why Can I cast List<? extends Number> List<? extends Number> to List<Integer> when the wildcard could be Short, and even Byte, which also extends Number?

Answer 1

You can cast an object to anything you want, but it might fail at runtime. However since generics information isn't present during runtime, your code becomes essentially ((List)list).add(s); . At that point list will take any object, not just a Number . Generics can help you avoid casts and keep type safety during compile time, but during runtime they don't matter anymore.

Answer 2

The cast makes the compiler ignore the fact, that the types may not be assignable.

At runtime the type parameters are unimportant, see type erasure .

The ArrayList internally stores the content in a Object[] array, which means you can add any reference type to the list object, if you "abuse" casting.

You may get a exception when you retrieve a Object though, since there's a cast hidden in the get statement.

Example:

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3));
List<String> list2 = (List) list;
list2.add("Hello World");

Integer i = list.get(0); // works
String s = list2.get(3); // works
s = list2.get(1); // ClassCastException
i = list.get(3); // ClassCastException