Question

I have the following code:

public <T extends SomeObject> long doSomething(T someObject){
    List<? extends SomeObject> l = new LinkedList<>();
    l.add(someObject);
}

this causes a compilation error – telling me that there is no suitable methods found: add(T),
why is that?

If l accept things that extends SomeObject shouldn’t it accept someObject as it bounds to extend SomeObject?

Answer 1

List<? extends SomeObject> l

What do you mean by that? Of course it will generate an error.

Take this example :SomeObject is Fruit, you have 2 derived classes Apple and Orange

Your list what will it contain? Apples or Oranges? The compiler cannot tell. So it generates error.

If you replace List<? extends SomeObject> l with List<SomeObject> l. Then this will work because Apple and Orange are both Fruit.