How to convert a Map to List in Java

Question

What is the best way to convert a Map lt key value gt  to a List lt value gt   Just iterate over all values and insert them in a list or am I overlooking something

User · Answer

Map lt String   String  gt  map   new HapshMap lt String   String gt    map add  one   java     map add  two    spring     Set lt Entry lt String String gt  gt  set    map entrySet     List lt Entry lt String   String gt  gt  list   new ArrayList lt Entry lt String   String gt  gt      set    for Entry lt String   String gt  entry   list        System out println entry getKey        System out println entry getValue

User · Answer

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 lt String  String gt  m   new HashMap lt String  String gt     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 lt String gt  list   new ArrayList lt String gt  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 lt String gt  list   new ArrayList lt String gt  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

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Here s the generic method to get values from map   public static  lt T gt  List lt T gt  ValueListFromMap HashMap lt String  T gt  map        List lt T gt  thingList   new ArrayList lt  gt          for  Map Entry lt String  T gt  entry   map entrySet              thingList add entry getValue                return thingList

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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

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HashMap lt Integer  List lt String gt  gt  map   new HashMap lt  gt      List lt String gt  list   new ArrayList lt String gt     list add  Java    list add  Primefaces    list add  JSF    map put 1 list   if map    null       return new ArrayList lt String gt   Collection lt   extends String gt   map values

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List lt Value gt  list   new ArrayList lt Value gt  map values       assuming   Map lt Key Value gt  map

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Using the Java 8 Streams API   List lt Value gt  values   map values   stream   collect Collectors toList

User · Answer

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 lt String gt  list   new ArrayList lt String gt  map values

User · Answer

You can do it like this  List lt Value gt  list   new ArrayList lt Value gt  map values

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you can use this List lt Value gt  list   new ArrayList lt Value gt  map values         or you may use  List lt Value gt  list   new ArrayList lt Value gt     for  Map Entry lt String  String gt  entry   map entrySet      list add entry getValue

User · Answer

If you want to ensure the values in the resultant List lt Value gt  are in the key-ordering of the input Map lt Key  Value gt   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 lt Key Value gt  map  List lt Value gt  list   new ArrayList lt Value gt   new TreeMap lt Key Value gt   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

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Map lt String  Integer gt  map   new HashMap lt String  Integer gt         map put  java   20       map put  C     45        Set  lt Entry lt String  Integer gt  gt  set   map entrySet         List lt Entry lt String  Integer gt  gt  list   new ArrayList lt Entry lt String  Integer gt  gt  set     we can have both key and value pair in list Also can get key and value using Map Entry by iterating over list

User · Answer

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

[java] How to convert a Map to List in Java?

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