java - Generics and Inheritence? -


this question has answer here:

public static void main(string... args) {     list<child> list1 = new arraylist<child>();     method2(list1); }  public static void method2(list<parent> list1) { }

i below compilation error

the method method2(list) undefined ...

above issue can solved modifying list<parent> list1 list<? extends parent> list1.

but if try add child object below 

public static void method2(list<? extends parent> list1) {     child child1 = new child();     list1.add(child1); }

it gives compilation error again

the method add(capture#1-of ? extends parent) in type list not applicable arguments (child)

so question if list<child> can passed parameter list<? extends parent> list1 why can't add child object under list<? extends parent>?

this common misunderstanding. fact child extends parent not make list<child> extend list<parent>. sounds unintuitive in cases one, there is. java tutorial:

given 2 concrete types , b (for example, number , integer), myclass< a> has no relationship myclass< b>, regardless of whether or not , b related. common parent of myclass , myclass object.

read this details.

as adding list, short answer is: imagine have class child2 extends parent, now, list receiving parameter @ method2(list<? extends parent> list1) either list<child1> or list<child2>. thus, given second case possible, adding child1 object not type safe.

now, fact can't add not mean can't other useful stuff, getting size, etc.


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