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

"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());
 } "

Answer 2

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).

Answer 3

// 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());    
}

Answer 4

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 5

HashMap<Integer, List<String>> map = new HashMap<>(); 
List<String> list = new ArrayList<String>();
list.add("Java");
list.add("Primefaces");
list.add("JSF");
map.put(1,list);
if(map != null){
    return new ArrayList<String>((Collection<? extends String>) map.values());
}

Answer 6

    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.