foreach loop by primitive or by boxed class in Java

Question

Currently I am reading a book "Java Generics and Collections" and I have found an interesting code:

List<Integer> ints = Arrays.asList(1,2,3);
int s = 0;
for (int n : ints) { s += n; }
assert s == 6;

Here foreach loop iterate over primitives:

for (int n : ints) { ... }

However, Eclipse IDE suggests me to iterate over boxed type:

for (Integer n : ints) { ... }

Are there any benefits of using one or another style of iterating?

Answer 1

Boxing/unboxing has a cost at runtime.

• if you are going to unbox the Integer once within the body of your loop, like you do, it won't make much of a difference.
• if you unbox the Integer many times in the body of your loop, you would be better off unboxing it once with for(int i : ints)
• if you are going to use an Integer in the body of your loop, then using int would lead to an unnecessary unbox/box operation.

For example:

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

for (Integer i : ints) { //better not to unbox
  if (someCondition(i)) subList.add(i);
}

for (int i : ints) { //better to unbox once
  int a = i;
  int b = 2 * i;
  sum += a + b - i;
}