somber wind
Hi,
I am a java beginner and just reached generics in my course
I am making some notes about generics, and wanted to know if my understanding is correct here. I need to be sure I am not making wrong notes
Also, why would java behave like this? is this a feature , and is this behaviour of generics any helpful to us?
Also what does it mean by invariance?
cursive edgeBOT
<@&987246399047479336> please have a look, thanks.
mental bramble
so by invariance it means just that a List<String> is not assignable to a List<Object>
hearty sail
Generics in java are invariant because it's the only logical option (except declaration site variance). The reason you can't assign a Box<Dog> to Box<Animal> is that then you'd be able to put a Cat in it
mental bramble
even though you could assign a String to an Object
List<String> strings = ...;
List<Object> objects = strings;
objects.add(123);
strings.get(0); // oh no
Arrays are actually covariant (can put String[] as Object[]), but this means you get runtime errors when putting other values in (ArrayStoreException)
It was a necessary feature before java got generics, but nowadays it's mostly useless
somber wind
so by invariance it means just that a `List<String>` is not assignable to a `Lis...
so you mean to say polymorphism in parameter types of generic classes will not work ?
mental bramble
so you mean to say polymorphism in parameter types of generic classes will not w...
yeah
there are ways around it
like List<? extends Object>
but those are a bit unobvious until you've used "normal" generics for a bit
somber wind
but why would you not require polymorphism there ?
mental bramble
but why would you not require polymorphism there ?
what do you mean by "not require"
"a list of strings cannot be a list of objects even though a string is an object"
somber wind
okay I meant to say why would polymorphims in type parameter would be invalid? is it helpful ?
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