When using a HashMap are values and keys guaranteed to be in the same order when iterating?

Question

When I iterate over the values or keys are they going to correlate? Will the second key map to the second value?

Answer 1

No, not necessarily. You should really use the entrySet().iterator() for this purpose. With this iterator, you will be walking through all Map.Entry objects in the Map and can access each key and associated value.

Answer 2

to use the entrySet that @Cuchullain mentioned:

Map<String, String> map = new HashMap<String, String>();

// populate hashmap

for (Map.Entry<String, String> entry : map.entrySet()) {
  String key = entry.getKey();
  String value = entry.getValue();
  // your code here
}

Answer 3

You want to use this, LinkedHashMap, for predicable iteration order

Answer 4

public class Test {
 public static void main(String[] args) {
  HashMap <String,String> hashmap = new HashMap<String,String>();
  hashmap.put("one", "1");
  hashmap.put("two", "2");
  hashmap.put("three", "3");
  hashmap.put("four", "4");
  hashmap.put("five", "5");
  hashmap.put("six", "6");

  Iterator <String> keyIterator   = hashmap.keySet().iterator();
  Iterator <String> valueIterator = hashmap.values().iterator();

  while(keyIterator.hasNext()) {
   System.out.println("key: "+keyIterator.next());
  }

  while(valueIterator.hasNext()) {
   System.out.println("value: "+valueIterator.next());
  }
 }
}

key: two
key: five
key: one
key: three
key: four
key: six
value: 2
value: 5
value: 1
value: 3
value: 4
value: 6

Answer 5

Both values() and keySet() delegate to the entrySet() iterator so they will be returned in the same order. But like Alex says it is much better to use the entrySet() iterator directly.

Answer 6

I agree with pmac72. Don't assume that you'll get ordered values or keys from an unordered collection. If it works time to time it is just pure hazard. If you want order to be preserved, use a LinkedHashMap or a TreeMap or commons collections OrderedMap.

Answer 7

The question confused me at first but @Matt cleared it up for me.

Consider using the entrySet() method that returns a set with the key-value pairs on the Map.

Map<Integer, Integer> a = new HashMap<Integer, Integer>(2);
a.put(1, 2);
a.put(2, 3);
for (Map.Entry<Integer, Integer> entry : a.entrySet()) {
    System.out.println(entry.getKey() + " => " + entry.getValue());
}

This outputs:

1 => 2
2 => 3
3 => 3

Answer 8

I second @basszero. While

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

will work, I find using a data structure that does this automatically is nicer. Now, you can just iterate "normally"

Answer 9

HashMap's keySet method returns a Set, which does not guarantee order.
HashMap's values() method returns a Collection, which does not guarantee order.

That said, the question was "are they going to correlate" so technically the answer is maybe, but don't rely on it.