How does one convert a HashMap to a List in Java?

Question

In Java, how does one get the values of a HashMap returned as a List?

Answer 1

HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put (1, "Mark");
map.put (2, "Tarryn");
List<String> list = new ArrayList<String>(map.values());
for (String s : list) {
    System.out.println(s);
}

Answer 2

Assuming you have:

HashMap<Key, Value> map; // Assigned or populated somehow.

For a list of values:

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

For a list of keys:

List<Key> keys = new ArrayList<Key>(map.keySet());

Note that the order of the keys and values will be unreliable with a HashMap; use a LinkedHashMap if you need to preserve one-to-one correspondence of key and value positions in their respective lists.

Answer 3

If you wanna maintain the same order in your list, say: your Map looks like:

map.put(1, "msg1")
map.put(2, "msg2")
map.put(3, "msg3")

and you want your list looks like

["msg1", "msg2", "msg3"]   // same order as the map

you will have to iterate through the Map:

// sort your map based on key, otherwise you will get IndexOutofBoundException
Map<String, String> treeMap = new TreeMap<String, String>(map)

List<String> list = new List<String>();
for (treeMap.Entry<Integer, String> entry : treeMap.entrySet()) {
    list.add(entry.getKey(), entry.getValue());
}  

Answer 4

If you only want it to iterate over your HashMap, no need for a list:

HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put (1, "Mark");
map.put (2, "Tarryn");
for (String s : map.values()) {
    System.out.println(s);
}

Of course, if you want to modify your map structurally (i.e. more than only changing the value for an existing key) while iterating, then you better use the "copy to ArrayList" method, since otherwise you'll get a ConcurrentModificationException. Or export as an array:

HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put (1, "Mark");
map.put (2, "Tarryn");
for (String s : map.values().toArray(new String[]{})) {
    System.out.println(s);
}

Answer 5

Solution using Java 8 and Stream Api:

private static <K, V>  List<V> createListFromMapEntries (Map<K, V> map){
        return map.values().stream().collect(Collectors.toList());
    }

Usage:

  public static void main (String[] args)
    {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "one");
        map.put(2, "two");
        map.put(3, "three");

        List<String> result = createListFromMapEntries(map);
        result.forEach(System.out :: println);
    }

Answer 6

Basically you should not mess the question with answer, because it is confusing.

Then you could specify what convert mean and pick one of this solution

List<Integer> keyList = Collections.list(Collections.enumeration(map.keySet()));

List<String> valueList = Collections.list(Collections.enumeration(map.values()));