How does one convert a HashMap to a List in Java

Question

In Java  how does one get the values of a HashMap returned as a List

User · Answer

If you only want it to iterate over your HashMap, no need for a list:

HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put (1, "Mark");
map.put (2, "Tarryn");
for (String s : map.values()) {
    System.out.println(s);
}

Of course, if you want to modify your map structurally (i.e. more than only changing the value for an existing key) while iterating, then you better use the "copy to ArrayList" method, since otherwise you'll get a ConcurrentModificationException. Or export as an array:

HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put (1, "Mark");
map.put (2, "Tarryn");
for (String s : map.values().toArray(new String[]{})) {
    System.out.println(s);
}

User · Answer

Collection Interface has 3 views    keySet values entrySet   Other have answered to to convert Hashmap into two lists of key and value  Its perfectly correct     My addition  How to convert  key-value pair   aka entrySet into list          Map m new HashMap              m put 3   dev2              m put 4   dev3           List lt Entry gt  entryList   new ArrayList lt Entry gt  m entrySet            for  Entry s   entryList            System out println s             ArrayList has this constructor

User · Answer

Solution using Java 8 and Stream Api   private static  lt K  V gt   List lt V gt  createListFromMapEntries  Map lt K  V gt  map           return map values   stream   collect Collectors toList             Usage      public static void main  String   args                Map lt Integer  String gt  map   new HashMap lt  gt             map put 1   one            map put 2   two            map put 3   three             List lt String gt  result   createListFromMapEntries map           result forEach System out    println

User · Answer

Assuming you have   HashMap lt Key  Value gt  map     Assigned or populated somehow    For a list of values   List lt Value gt  values   new ArrayList lt Value gt  map values       For a list of keys   List lt Key gt  keys   new ArrayList lt Key gt  map keySet       Note that the order of the keys and values will be unreliable with a HashMap  use a LinkedHashMap if you need to preserve one-to-one correspondence of key and value positions in their respective lists

User · Answer

Basically you should not mess the question with answer  because it is confusing    Then you could specify what convert mean and pick one of this solution  List lt Integer gt  keyList   Collections list Collections enumeration map keySet       List lt String gt  valueList   Collections list Collections enumeration map values

User · Answer

HashMap lt Integer  String gt  map   new HashMap lt Integer  String gt     map put  1   Mark    map put  2   Tarryn    List lt String gt  list   new ArrayList lt String gt  map values     for  String s   list        System out println s

User · Answer

If you wanna maintain the same order in your list  say  your Map looks like   map put 1   msg1   map put 2   msg2   map put 3   msg3     and you want your list looks like    msg1    msg2    msg3        same order as the map   you will have to iterate through the Map      sort your map based on key  otherwise you will get IndexOutofBoundException Map lt String  String gt  treeMap   new TreeMap lt String  String gt  map   List lt String gt  list   new List lt String gt     for  treeMap Entry lt Integer  String gt  entry   treeMap entrySet          list add entry getKey    entry getValue

[java] How does one convert a HashMap to a List in Java?

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