Which way should be preferred over while creating generic arraylist?

Question

I was reading about Generics. And got into a situation:

List<Object> t1 = new ArrayList<>();

or 

List<?> t2 = new ArrayList<>();

What is the difference in this 2 statements? Which one should be preferred?

now create a list of Strings and try assigning it to t1 and t2. — Patrick Parker, May 11 '18 at 03:06
What's the point of using generics in this case? Might as well just write `List t = new ArrayList();`. — Robby Cornelissen, May 11 '18 at 03:08
@shmosel It gives error `The method add(capture#1-of ?) in the type List is not applicable for the arguments (String)` — Nicky, May 11 '18 at 03:13
@shmosel If that's the case, then why they mention about unbounded wildcards?? How can i use unbounded wildcards? — Nicky, May 11 '18 at 03:14
@shmosel I agree. Better advice would be: don't create heterogeneous lists. — Robby Cornelissen, May 11 '18 at 03:24

score 1 · Answer 1 · answered May 11 '18 at 03:09

In List<Object> t1=new ArrayList<>();, you are creating a List object with generic type Object. The empty diamond symbol on the right is inferred to be <Object>.

On the bottom, with List<?>t2=new ArrayList<>(); This gives a compiler error:

Unexpected token: ?

This is because the Java compiler cannot infer what the generic type of your List is. If you had not declared it with t2=new ArrayList<>(); and left the compiler to infer the type, you could have instantiated it with any generic (t2=new ArrayList<YourObjectTypeHere>()).

So if you had to pick one, DONT do the second one as that gives a compile error.

Which way should be preferred over while creating generic arraylist?

1 Answers1