How to convert a Map to List in Java?
List<Value> list = new ArrayList<Value>(map.values());
assuming:
Map<Key,Value> map;
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
Using the Java 8 Streams API.
List<Value> values = map.values().stream().collect(Collectors.toList());