Generics problem (? extends Object)

[Swapnil Trivedi]

Consider the following code:

List<? extends Object> alist = new Vector<String>();
alist.add("USA"); // compile time error

we know that this is giving compiler error because at compile time we don't know what the type is(like ? stands for what)hence at compile time if we try to add anything it will give error..

but my doubt is what is the use of new Vector<String>(); ??
Isn't new Vector<String>() is the runtime type of <? extends Object>

[Krishnamoorthy Sethuraman]

new Vector<String>() is not the runtime type of <? extends Object> but it
is the runtime type of alist at the point of error. We don't know how it will change through the course of program. It's runtime type can keep on changing down the line if we make another assignment. since the runtime type cannot be determined at compile time, generics is limited only to the compile type checking. Hence, the add method on aList is checked for its type on compile type rather than runtime type. This is my guess.

[Lukas Stephienn]

At runtime generics doesnt exist. It is only for the compiler

[Swapnil Trivedi]

Lucius can you explain me in detail. I got your point that Generics exists only at compile time..So, At compile time the <String> on right side of the expression should have some use, Shouldn't it??

And if we can't add anything into <? extends Object> then what is the use of it.