Why can't I have 'int' as the type of an ArrayList?

Question

I want to declare an ArrayList of type int.

Why does the following give me an error:

ArrayList<int> list1 = new ArrayList<int>(); 

But the following works:

ArrayList<Integer> list1 = new ArrayList<Integer>(); 

?

Answer 1

ArrayList can only reference types, not primitives. Integer is a class, not a primitive.

When you declare ArrayList<Integer> list1 = new ArrayList<Integer>(), you're creating an ArrayList which will store the Integer type, not the int primitive.

If you want to read about the difference between primitive and reference types, check out http://pages.cs.wisc.edu/~hasti/cs302/examples/primitiveVsRef.html