Generics and Wildcards

[ajeeb]

Hi,
Am trying to understand wildcards (bounded) in java generics. I have method say:

	public static void someMethod(List <? extends Number> list) {
		list.add(0, new Integer(9)); //compile error
	}

While trying to add to the list (a list of "anything that extends the Number class" ), i will get a compile-time error. Why the compiler does not understand that Integer is a sub-type of Number ? Am I missing some important point ?

[emceenugget]

Found this interesting, since I haven't worked with generics in quite some time, and did a little search. Basically, it's not safe to add an Integer to a list of objects that extend Number because we do not know what ? is. That is, ? is a subtype of Number, but we do not know if it is Rectangle or a supertype of Rectangle.

More info can be found in pages 6-7 in this pdf file:
http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf

Anyways, you should just use List<Number>, as all classes that extend Number can be put into the list anyways.

[Steve11235]

Emcee is right about not complicating the type designation. In fact, it is the cause of your compile error.