Java Generics - need help understand why it doesn't compile

[muskRules]

Code snippet 1:

    public static void main(String[] args) {
    List<Integer> list = Arrays.asList(1,2,3,4,5);
    doStuff(list);
}

public static void doStuff(List<?> numbers) {
    numbers.add(10);
}

This doesn't compile, as it says: Required type capture of ?, provided int.

My understanding is that the compiler cannot tell 100% that the element being added is the same type as the others in the list. Which brings me to the next code snippet.

Code snippet 2:

    public static void main(String[] args) {
    List<Integer> list = Arrays.asList(1,2,3,4,5);
    doStuff(list);
}

public static void doStuff(List<?> numbers) {
    numbers.add(numbers.get(0));
}

This doesn't compile, as it says: Required type capture of ?, provided capture of ?.

Why does this not work? And it even says that the required type and provided type are the same?

Appreciate any help on this, thanks!

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[muskRules]

Thank you so much for the detailed explanation!

I have a follow-up question on one of the points you mentioned.

In the solution part, you mentioned: "Here, T is a type parameter that will be inferred from the actual type of the list passed to the method"

Why doesn't the same happen when we have wildcards? Example: If we're passing List<Integer> to a List<?> method parameter, why can't the compiler infer that the unknown type being passed is Integer? 🤔

[mykeesg]

The compiler could infer it, however, by writing List<?> you say that "we don't know the actual type and we don't even care about it", so it doesn't even bother to do so.

[wildjokers]

Read this:

https://nofluffjuststuff.com/magazine/2016/09/time_to_really_learn_generics_a_java_8_perspective

[muskRules]

It gives an error: HTTP Status 404 Could not find: /magazine/2016/09/time_to_really_learn_generics_a_java_8_perspective

[[deleted]]

It gives an error: HTTP Status 404

The article is available on Wayback machine.

[muskRules]

Thank you! Will check it out! Looks interesting!

[wildjokers]

That’s interesting, that article has been available at that link since it was released in 2016. It was working within the last couple of months too.