HashMap: keySet() and values order

[Dmitriy Davydenko]

HashMap doesn't guarantee the order of the elements.

But if i call
keySet().toArray( new String[0] )
and then (without changing the map)
values().toArray( new String[0] )

is the order of the elements the same in the arrays? Or no guarantees?

[Monika Joshi]

Hi Golem,

The order of the elements would be same in the array.

Just to clarify, if we have a hashmap with values (1, "one"), (2, "two"), (3, "three")
keySet().toArray( new String[0] ) - would generate the keys of the elements in hashmap. - ["1","2","3"]

values().toArray( new String[0] ) - would generate the values of the elements in hashmap.["one", "two", "three"]

Regards,
Minakshi

[Gopi Chella]

I dont think the order of the elements will be same in array. Please check the below example i made

 public static void main(String[] args) {

		Map mp = new HashMap();
		mp.put("India","Asia");
		mp.put("France", "Europe");
		mp.put("Arab", "MiddleEast");
		mp.put("SouthAfrica", "Africa");
		mp.put("Brazil", "N.America");
		mp.put("Banglore", "India");
		mp.put("New york", "USA");
		
		Object[] ob = mp.keySet().toArray();
		Object[] ob1 = mp.values().toArray();
		

		for (int i = 0; i < ob.length; i++)
		{
			System.out.print(ob[i]);
			System.out.print(" -- ");
			System.out.println(ob1[i]);
		}
		System.out.println(" ");
		System.out.println(mp);
	}   

The ouput is :

SouthAfrica -- Africa
India -- Asia
Banglore -- India
New york -- USA
Arab -- MiddleEast
France -- Europe
Brazil -- N.America    

I hope if you need the elements to be Ordered then go for Linked Hash Map and if you want the element to be in sorted manner then go for TreeMap

[Devaka Cooray]

"Golem XIV", please check your private messages regarding an important administrative matter.

[Rob Spoor]

If you do not change the contents of the map then the order will (most likely) be the same. The iteration order isn't random, it's just unspecified.

Minakshi Sood wrote:Just to clarify, if we have a hashmap with values (1, "one"), (2, "two"), (3, "three")
keySet().toArray( new String[0] ) - would generate the keys of the elements in hashmap. - ["1","2","3"]

Not necessarily. As the API says the order is unspecified. It might be 1, 2, 3 but it might also be 2, 1, 3 or anything else.

[Jesper de Jong]

Minakshi Sood wrote:The order of the elements would be same in the array.

Maybe, but don't count on it! There's no guarantee that the order of elements in the keySet() and values() always lines up. Don't write code that relies on that, even if it works on your system - it might not work with a different version of Java or on another operating system.

If you need two lists with the keys and the elements in such a way that they line up, do something like this (assuming keys and values are both strings):

Map<String, String> map = ...;

List<String> keys = new ArrayList<String>();
List<String> values = new ArrayList<String>();

for (Map.Entry<String, String> entry : map) {
	keys.add(entry.getKey());
	values.add(entry.getValue());
}