Converting a Map to a List

Question

Is there anyone who knows how to convert a Map to a List

I found here something like this :

List<Value> list = new ArrayList<Value>(map.values());

But this is going to store just the value of the Map to the List

What I want to do is : copy the Key & the Value to the List

So, do you know how to achieve this ?

Maps and Lists are different data structures, how would you want to combine the map keys and values into the list? — beny23, Aug 22 '11 at 12:00
Could you elaborate on what you mean by "copy the key and the value to the list"? — aioobe, Aug 22 '11 at 12:00
why do you want to do this? They are different data structures and are used for different purposes. — luketorjussen, Aug 22 '11 at 12:01
I have a `Map` and a `List` so I want to do like the example showed in the post but by copying both (key & value) — Wassim AZIRAR, Aug 22 '11 at 12:02

score 9 · Accepted Answer · answered Aug 22 '11 at 12:00

9

This will give you a List of the Map entries:

List<Map.Entry<Key, Value>> list = 
    new ArrayList<Map.Entry<Key, Value>>(map.entrySet());

FYI, entries have a getKey() and a getValue() method.

answered Aug 22 '11 at 12:00

Bohemian

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score 3 · Answer 2 · answered Aug 22 '11 at 12:09

One way you can do is Create a List with adding all the keys like :

List list = new ArrayList(map.keySet());

Then add all the values like :

 list.addAll(map.values);

And then probably you have to access with index like: if map size is 10 , you know that you have 20 elements in the list. So you have to write a logic to access the key-value from the list with proper calculation of index like: size/2 something like that. I am not sure if that helps what your requirement is.

dacwe · Answer 3 · 2011-08-22T12:05:54.000

Try storing the Map.Entrys of the map:

new ArrayList<Entry<Key, Value>>(map.entrySet());

Example:

public static void main(String[] args) {

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

    map.put("Hello", 0);
    map.put("World!", 1);

    ArrayList<Entry<String, Integer>> list =
        new ArrayList<Entry<String, Integer>>(map.entrySet());

    System.out.println(list.get(0).getKey() + " -> " + list.get(0).getValue());
}

score 2 · Answer 4 · answered Aug 22 '11 at 12:05

Both @Bohemian and @dacwe are right. I'd say moreover: in most cases you do not have to create your own list. Just use map.entrySet(). It returns Set, but Set is just a Collection that allows iterating over its elements. Iterating is enough in 95% of cases.

Converting a Map to a List

4 Answers4