mjhjmtu

The naive code I have is:

class $
 public static void main(String _) 
 final List<Integer> ints = new ArrayList<>();
 IntStream.iterate(0, i -> i++).limit(5).forEach(val -> ints.add(val));
 System.out.println(ints);
 

where my expectation was to see the following in the console:

[0, 1, 2, 3, 4] 

but the actual is:

[0, 0, 0, 0, 0]

Probably something very simple but what am I missing?

You need to return the incremented value. You postfix incremented a local variable and returned the non-incremented value. Use ++i not i++

final List<Integer> ints = new ArrayList<>();
IntStream.iterate(0, i -> ++i).limit(5).forEach(val -> ints.add(val));
System.out.println(ints);

i++ has the value of i before it's incremented. You need to use the prefix operator.

IntStream.iterate(0, i -> ++i).limit(5).forEach(val -> ints.add(val));

Actually, don't do that. There's no reason to mutate i, it's simply thrown away. Use the side effect free version. That's the whole idea behind functional programming, after all: avoiding side effects.

IntStream.iterate(0, i -> i + 1).limit(5).forEach(val -> ints.add(val));

It would also be better to collect the stream into a list rather than adding values with forEach. It directly expresses the intention to create a list and again avoids side effects.

final List<Integer> ints = IntStream.iterate(0, i -> i + 1).limit(5)
 .boxed().collect(Collectors.toList());

You are using a postfix i++, instead of a prefix ++i while passing on the value to forEach. Changing to the following shall provide you the expected output :

IntStream.iterate(0, i -> ++i).limit(5).forEach(ints::add);

Aside, an alternate way of iterating and combining limit with Java9+ is using IntStream.iterate with an IntPredicate as :

IntStream.iterate(0, i -> i < 5, i -> i + 1).forEach(ints::add);

You could use a print to see what was happend:

final List<Integer> ints = new ArrayList<>();
 IntStream.iterate(0, i -> 
 System.out.println(i);
 return i++;
 ).limit(5)
 .forEach(val -> ints.add(val));
 System.out.println(ints);

In that case, the value of i always will be 0, because the increment occurs after the value is returned, the correct way is

final List<Integer> ints = new ArrayList<>();
 IntStream.iterate(0, i -> 
 System.out.println(i);
 return ++i;
 ).limit(5)
 .forEach(val -> ints.add(val));
 System.out.println(ints);