Java Collection <Object> or Collection <?>

Question

I am trying to use List instead of List

 List<?> list = new ArrayList<Integer>(); ... list.add(1); //compile error 

What am I doing wrong and how does the new value differ from Integer? Maybe I'm using List in my project?

Answer 1

List<?> Means a list of some type, but we don’t know which type. You can put objects of the desired type on the list, but since you do not know the type, you really cannot put anything on such a list (other than null ).

There is no way around this other than declaring your variable as a List<Integer> .

Answer 2

 List<Integer> list = new ArrayList<Integer>(); 

The general type must always be the same.

Answer 3

 List<Integer> list = new ArrayList<Integer>(); ... list.add(1); 

Answer 4

 List<?> list = new ArrayList<Integer>(); 

? means some type, you must specify the type as a whole. therefore the correct syntax is:

 List<Integer> list = new ArrayList<Integer>();