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javalistdictionarycollections
Javaa
Javaa

Reputation: 8269

How do I convert a Map to List in Java?

How do I convert a Map<key,value> to a List<value>? Should I iterate over all map values and insert them into a list?

Upvotes: 823

Views: 1095158

Answers (13)

Hakan Anlamaz
Hakan Anlamaz

Reputation: 61

public List<Object> convertMapToList(Map<Object, Object> map){
    return new ArrayList<>(map.values());
}

Upvotes: 2

siva prasad
siva prasad

Reputation: 49

 Map<String, String > map = new HapshMap<String, String>;
 map.add("one","java");
 map.add("two", "spring");
 Set<Entry<String, String>> set = map.entrySet();
 List<Entry<String, String>> list = new ArrayList<Entry<String, String>>    (set);
 for(Entry<String, String> entry : list) {
   System.out.println(entry.getKey());
   System.out.println(entry.getValue());
 }

Upvotes: 4

Stefan Haberl
Stefan Haberl

Reputation: 10559

If you want an immutable copy of the values:

List<Value> list = List.copyOf(map.values())

Upvotes: 1

saurabhgoyal795
saurabhgoyal795

Reputation: 1285

// you can use this
List<Value> list = new ArrayList<Value>(map.values());

// or you may use 
List<Value> list = new ArrayList<Value>();
for (Map.Entry<String, String> entry : map.entrySet())
{
list.add(entry.getValue());    
}

Upvotes: 5

Hiral Pancholi
Hiral Pancholi

Reputation: 62

Here's the generic method to get values from map.

public static <T> List<T> ValueListFromMap(HashMap<String, T> map) {
    List<T> thingList = new ArrayList<>();

    for (Map.Entry<String, T> entry : map.entrySet()) {
        thingList.add(entry.getValue());
    }

    return thingList;
}

Upvotes: 1

Matej Kormuth
Matej Kormuth

Reputation: 2285

Using the Java 8 Streams API.

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

Upvotes: 72

user11
user11

Reputation: 197

    Map<String, Integer> map = new HashMap<String, Integer>();
    map.put("java", 20);
    map.put("C++", 45);

    Set <Entry<String, Integer>> set = map.entrySet();

    List<Entry<String, Integer>> list = new ArrayList<Entry<String, Integer>>(set);

we can have both key and value pair in list.Also can get key and value using Map.Entry by iterating over list.

Upvotes: 8

M0les
M0les

Reputation: 111

If you want to ensure the values in the resultant List<Value> are in the key-ordering of the input Map<Key, Value>, you need to "go via" SortedMap somehow.

Either start with a concrete SortedMap implementation (Such as TreeMap) or insert your input Map into a SortedMap before converting that to List. e.g.:

Map<Key,Value> map;
List<Value> list = new ArrayList<Value>( new TreeMap<Key Value>( map ));

Otherwise you'll get whatever native ordering the Map implementation provides, which can often be something other than the natural key ordering (Try Hashtable or ConcurrentHashMap, for variety).

Upvotes: 6

Diego Amicabile
Diego Amicabile

Reputation: 589

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.

Upvotes: 26

java dude
java dude

Reputation: 301

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());

Upvotes: 30

maneesh
maneesh

Reputation: 1112

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

Upvotes: 15

coobird
coobird

Reputation: 161022

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

Upvotes: 145

cletus
cletus

Reputation: 625307

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

assuming:

Map<Key,Value> map;

Upvotes: 1534

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