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?

Solution 1

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

assuming:

Map<Key,Value> map;

Solution 2

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

Solution 3

Using the Java 8 Streams API.

List<Value> values = map.values().stream().collect(Collectors.toList());

Solution 4

map.entrySet() gives you a collection of Map.Entry objects containing both key and value. you can then transform this into any collection object you like, such as new ArrayList(map.entrySet());

Solution 5

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

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