How to convert a map to list in Java?

Question

What is the best way to convert a Map<key,value> to a list<value>? Just iterate over all values and insert them in a list or am I overlooking something?

Answer 1

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

assuming:

Map<Key,Value> map;

Answer 2

a list of what ?

Assuming map is your instance of Map

map.values() will return a Collection containing all of the map's values

map.keys() will return a Set containing all of the map's keys

Answer 3

The issue here is that Map has two values (a key and value), while a List only has one value (an element).

Therefore, the best that can be done is to either get a List of the keys or the values. (Unless we make a wrapper to hold on to the key/value pair).

Say we have a Map:

Map<String, String> m = new HashMap<String, String>();
m.put("Hello", "World");
m.put("Apple", "3.14");
m.put("Another", "Element");

The keys as a List can be obtained by creating a new ArrayList from a Set returned by the Map.keySet method:

List<String> list = new ArrayList<String>(m.keySet());

While the values as a List can be obtained creating a new ArrayList from a Collection returned by the Map.values method:

List<String> list = new ArrayList<String>(m.values());

The result of getting the List of keys:

Apple
Another
Hello

The result of getting the List of values:

3.14
Element
World

Answer 4

I guess you want to convert the values contained in the Map to a list? Easiest is to call the values() method of the Map interface. This will return the Collection of value objects contained in the map. Note that this collection is backed by the Map object and any changes to the Map object will reflect here. So if you want a separate copy not bound to your map object, simply create a new List object like an ArrayList passing the value collection as below.

ArrayList list = new ArrayList(map.values());