Java generics - ArrayList initialization -

it known arraylist init. should this

arraylist<a> = new arraylist<a>(); arraylist<integer> = new arraylist<number>(); // compile-time error 

so, why java allow these ?

1. arraylist<? extends object> a1 = new arraylist<object>(); 2. arraylist<?> a2 = new arraylist<integer>(); 

then, if correct why doesn't allow these ?

1. a1.add(3); 2. a2.add(3); 

the compiler message : method add(int, capture#1-of ? extends object) in type arraylist not applicable arguments (int)

more general

  1. a1.add(null e);   2. a2.add(? e); 

i read hear you. thanks

the other amusing point :

 arraylist<arraylist<?>> = new arraylist<arraylist<?>>(); // correct  arraylist<?> = new arraylist<?>(); // wrong. know it's reason have  question in mind mentioned above  

you can't assign list<number> reference of type list<integer> because list<number> allows types of numbers other integer. if allowed that, following allowed:

list<number> numbers = new arraylist<number>(); numbers.add(1.1); // add double list<integer> ints = numbers; integer fail = ints.get(0); // classcastexception! 

the type list<integer> making guarantee contains integer. that's why you're allowed integer out of without casting. can see, if compiler allowed list of type such number assigned list<integer> guarantee broken.

assigning list<integer> reference of type such list<?> or list<? extends number> legal because ? means "some unknown subtype of given type" (where type object in case of ? , number in case of ? extends number).

since ? indicates do not know specific type of object list accept, isn't legal add null it. are, however, allowed retrieve object it, purpose of using ? extends x bounded wildcard type. note opposite true ? super x bounded wildcard type... list<? super integer> "a list of unknown type @ least supertype of integer". while don't know type of list (could list<integer>, list<number>, list<object>) know sure whatever is, integer can added it.

finally, new arraylist<?>() isn't legal because when you're creating instance of paramterized class arraylist, have give specific type parameter. use whatever in example (object, foo, doesn't matter) since you'll never able add null since you're assigning directly arraylist<?> reference.