Question

For a map like:

Map<Integer, Integer> map = ...;
map.put(1, 1);
map.put(2, 2);
map.put(3, 3);
map.put(4, 4);

Is this code…

for (Integer i : map.keySet()) System.out.println(i);
for (Integer i : map.values()) System.out.println(i);

…guaranteed print the same same sequence twice?

If not, are there any guarantees in for example java.util.HashMap?

Answer 1

No, there is no guarantee, although in practice it will happen (there’s no good reason for the map to use a different iterator for the keys and values).

If you want to guarantee iteration order, iterate the entrySet():

for (Map.Entry<Integer,Integer> entry : map.entrySet())
    // ...

Since you ask about HashMap, note also that any changes to the map will potentially change iteration order, as a result of the mapbeing rehashed.