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

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