Question mark in Java generics

Question

I have read that <?> is exactly the same as <? extends Object>. Then what is the difference between:

1. Set<?>
2. Set<Object>
3. Set

I've tried adding String.class and MyClass.class into all these sets but in the second case it doesn't compile.

Another example I don't get is:

1. Set<Class>
2. Set<Class<?>>

To me it seems they are identical but if I have a method which returns Set<Class<?>>, I can't assign it's return value to variable of type Set<Class>.

I am sorry if this is duplicate, but I have read all the other posts and there are very few examples and I still can't understand it.

Answer 1

with Set<?> , i can use any type i want like : Set<?> s =new HashSet<Intger>() or Set<?> s =new HashSet<String>().Since, i can make ? refer to anything i'm not allowed to put anything other than null in the collection and i will get Object out of the collection.

Set<Object> says i can only assign type Object for ex : Set<Object> s =new HashSet<Object>() but not Set<Object> s =new HashSet<String>().In this case i can add any type because Object is the super base type but i will get Object out of the collection.

Set is the basic raw type , you can add anything and get an Object type out of it.

Set<Class> is different than Set<Class<?>> .The first one says "I am a Set of non specific Class types " while the second one says "I am a Set of some specific Class types but don't know which".