How to sort Map values by key in Java

Question

I have a Map that has strings for both keys and values    Data is like following         question1    1     question9    1     question2    4     question5    2      I want to sort the map based on its keys  So  in the end  I will have question1  question2  question3    and so on     Eventually  I am trying to get two strings out of this Map      First String  Questions   in order 1   10  Second String  Answers  in the same order as the question    Right now I have the following   Iterator it   paramMap entrySet   iterator    while  it hasNext          Map Entry pairs    Map Entry  it next        questionAnswers    pairs getKey              This gets me the questions in a string but they are not in order

User · Answer

We can also sort the key by using Arrays sort method   Map lt String  String gt  map   new HashMap lt String  String gt     Object   objArr   new Object map size     for  int i   0  i  lt  map size    i      objArr i    map get i     Arrays sort objArr   for  Object str   objArr    System out println str

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Assuming TreeMap is not good for you  and assuming you can t use generics    List sortedKeys new ArrayList yourMap keySet     Collections sort sortedKeys      Do what you need with sortedKeys

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This code can sort a key-value map in both orders i e  ascending and descending    lt K  V extends Comparable lt V gt  gt  Map lt K  V gt  sortByValues       final Map lt K  V gt  map  int ascending         Comparator lt K gt  valueComparator    new Comparator lt K gt                       private int ascending          public int compare K k1  K k2                int compare   map get k2  compareTo map get k1                if  compare    0  return 1              else return ascending compare                    public Comparator lt K gt  setParam int ascending                        this ascending   ascending              return this                  setParam ascending        Map lt K  V gt  sortedByValues   new TreeMap lt K  V gt  valueComparator       sortedByValues putAll map       return sortedByValues      As an example   Map lt Integer Double gt  recommWarrVals   new HashMap lt Integer Double gt     recommWarrVals   sortByValues recommWarrVals  1       Ascending order recommWarrVals   sortByValues recommWarrVals -1       Descending order

User · Answer

Short answer  Use a TreeMap   This is precisely what it s for     If this map is passed to you and you cannot determine the type  then you can do the following   SortedSet lt String gt  keys   new TreeSet lt  gt  map keySet     for  String key   keys        String value   map get key         do something     This will iterate across the map in natural order of the keys     Longer answer  Technically  you can use anything that implements SortedMap  but except for rare cases this amounts to TreeMap  just as using a Map implementation typically amounts to HashMap     For cases where your keys are a complex type that doesn t implement Comparable or you don t want to use the natural order then TreeMap and TreeSet have additional constructors that let you pass in a Comparator      placed inline for the demonstration  but doesn t have to be a lambda expression Comparator lt Foo gt  comparator    Foo o1  Foo o2  - gt                       SortedSet lt Foo gt  keys   new TreeSet lt  gt  comparator   keys addAll map keySet       Remember when using a TreeMap or TreeSet that it will have different performance characteristics than HashMap or HashSet   Roughly speaking operations that find or insert an element will go from O 1  to O Log N     In a HashMap  moving from 1000 items to 10 000 doesn t really affect your time to lookup an element  but for a TreeMap the lookup time will be about 3 times slower  assuming Log2    Moving from 1000 to 100 000 will be about 6 times slower for every element lookup

User · Answer

Provided you cannot use TreeMap  in Java 8 we can make use of toMap   method in Collectorswhich takes following parameters    keymapper  mapping function to produce keys valuemapper  mapping   function to produce values mergeFunction  a merge function  used to resolve collisions between values associated with the same key mapSupplier  a function which returns a new  empty Map into which the results will be inserted    Java 8 Example  Map lt String String gt  sample   new HashMap lt  gt         push some values to map   Map lt String  String gt  newMapSortedByKey   sample entrySet   stream                        sorted Map Entry  lt String String gt comparingByKey   reversed                         collect Collectors toMap Map Entry  getKey  Map Entry  getValue   e1  e2  - gt  e1  LinkedHashMap  new    Map lt String  String gt  newMapSortedByValue   sample entrySet   stream                            sorted Map Entry  lt String String gt comparingByValue   reversed                             collect Collectors toMap Map Entry  getKey  Map Entry  getValue   e1 e2  - gt  e1  LinkedHashMap  new      We can modify the example to use custom comparator and to sort based on keys as   Map lt String  String gt  newMapSortedByKey   sample entrySet   stream                    sorted  e1 e2  - gt  e1 getKey   compareTo e2 getKey                      collect Collectors toMap Map Entry  getKey  Map Entry  getValue   e1 e2  - gt  e1  LinkedHashMap  new

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List lt String gt  list   new ArrayList lt String gt     Map lt String  String gt  map   new HashMap lt String  String gt     for  String str   map keySet       list add str     Collections sort list   for  String str   list     System out println str

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Using Java 8   Map lt String  Integer gt  sortedMap   unsortMap entrySet   stream                sorted Map Entry comparingByKey                 collect Collectors toMap Map Entry  getKey  Map Entry  getValue                       oldValue  newValue  - gt  oldValue  LinkedHashMap  new

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Just in case you don t wanna use a TreeMap public static Map lt Integer  Integer gt  sortByKey Map lt Integer  Integer gt  map            List lt Map Entry lt Integer  Integer gt  gt  list   new ArrayList lt  gt  map entrySet             list sort Comparator comparingInt Map Entry  getKey            Map lt Integer  Integer gt  sortedMap   new LinkedHashMap lt  gt             list forEach e - gt  sortedMap put e getKey    e getValue              return sortedMap         Also  in-case you wanted to sort your map on the basis of values just change Map Entry  getKey to Map Entry  getValue

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In Java 8 you can also use  stream   sorted     myMap keySet   stream   sorted   forEach key - gt            String value   myMap get key            System out println  key      key           System out println  value      value

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In Java 8  To sort a Map lt K  V gt  by key  putting keys into a List lt K gt    List lt K gt  result   map keySet   stream   sorted   collect Collectors toList       To sort a Map lt K  V gt  by key  putting entries into a List lt Map Entry lt K  V gt  gt    List lt Map Entry lt K  V gt  gt  result       map entrySet           stream           sorted Map Entry comparingByKey            collect Collectors toList       Last but not least  to sort strings in a locale-sensitive manner - use a Collator  comparator  class   Collator collator   Collator getInstance Locale US   collator setStrength Collator PRIMARY      case insensitive collator  List lt Map Entry lt String  String gt  gt  result       map entrySet           stream           sorted Map Entry comparingByKey collator           collect Collectors toList

User · Answer

Just use TreeMap  new TreeMap lt String  String gt  unsortMap     Be aware that the TreeMap is sorted according to the natural ordering of its  keys

User · Answer

Use a TreeMap

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Using the TreeMap you can sort the map   Map lt String  String gt  map   new HashMap lt  gt             Map lt String  String gt  treeMap   new TreeMap lt  gt  map   for  String str   treeMap keySet          System out println str

User · Answer

If you already have a map and would like to sort it on keys  simply use    Map lt String  String gt  treeMap   new TreeMap lt String  String gt  yourMap     A complete working example    import java util HashMap  import java util Set  import java util Map  import java util TreeMap  import java util Iterator   class SortOnKey    public static void main String   args       HashMap lt String String gt  hm   new HashMap lt String String gt        hm put  3   three       hm put  1   one       hm put  4   four       hm put  2   two       printMap hm      Map lt String  String gt  treeMap   new TreeMap lt String  String gt  hm      printMap treeMap      main  public static void printMap Map lt String String gt  map        Set s   map entrySet        Iterator it   s iterator        while   it hasNext              Map Entry entry    Map Entry  it next           String key    String  entry getKey           String value    String  entry getValue           System out println key       gt      value          while     System out println                                 printMap     class

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A good solution is provided here   We have a HashMap that stores values in unspecified order   We define an auxiliary TreeMap and we copy all data from HashMap into TreeMap using the putAll method   The resulting entries in the TreeMap are in the key-order

[java] How to sort Map values by key in Java?

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