How do I efficiently iterate over each entry in a Java Map

Question

If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it   what is the most efficient way of going through the map   Will the ordering of elements depend on the specific map implementation that I have for the interface

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Most compact with Java 8   map entrySet   forEach System out  println

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I like to concat a counter  then save the final value of the counter   int counter   0  HashMap lt String  String gt  m   new HashMap lt String  String gt     for int i   0 i lt items length i      m put  firstname  i  items get i  getFirstName     counter   i     m put  recordCount  String valueOf counter      Then when you want to retrieve   int recordCount   Integer parseInf m get  recordCount     for int i  0  i lt recordCount i      System out println  First Name      m get  firstname  i

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Using  Java 7   Map lt String String gt  sampleMap   new HashMap lt  gt     for  sampleMap Entry lt String String gt  entry   sampleMap entrySet          String key   entry getKey        String value   entry getValue            your Code as per the Business Justification             Using Java 8   Map lt String String gt  sampleMap   new HashMap lt  gt      sampleMap forEach  k  v  - gt  System out println  Key is        k     Value is        v

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Since Java 10  you can use local variable inference  a k a   var   to make a lot of the already available answers less bloated  For example   for  var entry   map entrySet          System out println entry getKey             entry getValue

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Functional Oprations             Map lt String  String gt  mapString   new HashMap lt  gt                 mapString entrySet   stream   map  entry  - gt                    String mapKey   entry getKey                    return entry                 forEach  entry  - gt                    String mapValue   entry getValue                                   Intrator             Map lt String  String gt  mapString   new HashMap lt  gt                 for  Iterator lt Map Entry lt String  String gt  gt  it   mapString entrySet   iterator    it hasNext                       Map Entry lt String  String gt  entry   it next                    String mapKey   entry getKey                    String mapValue   entry getValue                                 Simple for loop             Map lt String  String gt  mapString   new HashMap lt  gt                 for  Map Entry lt String  String gt  entry   mapString entrySet                      String mapKey   entry getKey                    String mapValue   entry getValue

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Use Java 8   map entrySet   forEach entry - gt  System out println entry getValue

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If your reason for iterating trough the Map  is to do an operation on the value and write to a resulting Map  I recommend using the transform-methods in the Google Guava Maps class   import com google common collect Maps    After you have added the Maps to your imports  you can use Maps transformValues and Maps transformEntries on your maps  like this   public void transformMap        Map lt String  Integer gt  map   new HashMap lt  gt         map put  a   2       map put  b   4        Map lt String  Integer gt  result   Maps transformValues map  num - gt  num   2       result forEach  key  val  - gt  print key  Integer toString val            key a value 4        key b value 8      Map lt String  String gt  result2   Maps transformEntries map   key  value  - gt  value         key             result2 forEach this  print          key a value 2 a         key b value 4 b     private void print String key  String val       System out println  key     key     value     val

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Yes  the order depends on the specific Map implementation    ScArcher2 has the more elegant Java 1 5 syntax  In 1 4  I would do something like this   Iterator entries   myMap entrySet   iterator    while  entries hasNext        Entry thisEntry    Entry  entries next      Object key   thisEntry getKey      Object value   thisEntry getValue

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It doesn t quite answer the OP s question  but might be useful to others who find this page   If you only need the values and not the keys  you can do this   Map lt Ktype  Vtype gt  myMap          for  Vtype v  myMap values        System out println  value      v       Ktype  Vtype are pseudocode

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If you want to iterate through the map in the order that the elements were added  use LinkedHashMap as opposed to just Map   This approach has worked for me in the past   LinkedHashMap lt String Integer gt  test new LinkedHashMap     test put  foo  69   test put  bar  1337    for int i 0 i lt test size   i         System out println test get test keySet   toArray   i         Output   69 1337

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This is a two part question   How to iterate over the entries of a Map -  ScArcher2 has answered that perfectly   What is the order of iteration - if you are just using Map  then strictly speaking  there are no ordering guarantees   So you shouldn t really rely on the ordering given by any implementation   However  the SortedMap interface extends Map and provides exactly what you are looking for - implementations will aways give a consistent sort order   NavigableMap is another useful extension - this is a SortedMap with additional methods for finding entries by their ordered position in the key set   So potentially this can remove the need for iterating in the first place - you might be able to find the specific entry you are after using the higherEntry  lowerEntry  ceilingEntry  or floorEntry methods   The descendingMap method even gives you an explicit method of reversing the traversal order

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Lambda Expression Java 8   In Java 1 8  Java 8  this has become lot easier by using forEach method from Aggregate operations Stream operations  that looks similar to iterators from Iterable Interface    Just copy paste below statement to your code and rename the HashMap variable from hm to your HashMap variable to print out key-value pair   HashMap lt Integer Integer gt  hm   new HashMap lt Integer  Integer gt               Logic to put the Key Value pair in your HashMap hm         Print the key value pair in one line   hm forEach  k  v  - gt  System out println  key      k     value     v        Just copy and paste above line to your code    Below is the sample code that I tried using Lambda Expression  This stuff is so cool  Must try   HashMap lt Integer  Integer gt  hm   new HashMap lt Integer  Integer gt         Random rand   new Random 47       int i   0      while i  lt  5            i            int key   rand nextInt 20           int value   rand nextInt 50           System out println  Inserting key      key     Value      value           Integer imap   hm put key  value           if  imap    null                System out println  Inserted              else               System out println  Replaced with     imap                                       hm forEach  k  v  - gt  System out println  key      k     value     v     Output   Inserting key  18 Value  5 Inserted Inserting key  13 Value  11 Inserted Inserting key  1 Value  29 Inserted Inserting key  8 Value  0 Inserted Inserting key  2 Value  7 Inserted key  1 value 29 key  18 value 5 key  2 value 7 key  8 value 0 key  13 value 11   Also one can use Spliterator for the same   Spliterator sit   hm entrySet   spliterator      UPDATE    Including documentation links to Oracle Docs  For more on Lambda go to this link and must read Aggregate Operations and for Spliterator go to this link

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Try this with Java 1 4   for  Iterator entries   myMap entrySet   iterator    entries hasNext         Entry entry    Entry  entries next       System out println entry getKey           entry getValue

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The correct way to do this is to use the accepted answer as it is the most efficient  I find the following code looks a bit cleaner   for  String key  map keySet         System out println key         map get key

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To summarize the other answers and combine them with what I know  I found 10 main ways to do this  see below   Also  I wrote some performance tests  see results below   For example  if we want to find the sum of all of the keys and values of a map  we can write    Using iterator and Map Entry  long i   0  Iterator lt Map Entry lt Integer  Integer gt  gt  it   map entrySet   iterator    while  it hasNext          Map Entry lt Integer  Integer gt  pair   it next        i    pair getKey     pair getValue       Using foreach and Map Entry  long i   0  for  Map Entry lt Integer  Integer gt  pair   map entrySet          i    pair getKey     pair getValue       Using forEach from Java 8  final long   i    0   map forEach  k  v  - gt  i 0     k   v    Using keySet and foreach  long i   0  for  Integer key   map keySet          i    key   map get key      Using keySet and iterator  long i   0  Iterator lt Integer gt  itr2   map keySet   iterator    while  itr2 hasNext          Integer key   itr2 next        i    key   map get key      Using for and Map Entry  long i   0  for  Iterator lt Map Entry lt Integer  Integer gt  gt  entries   map entrySet   iterator    entries hasNext            Map Entry lt Integer  Integer gt  entry   entries next        i    entry getKey     entry getValue       Using the Java 8 Stream API  final long   i    0   map entrySet   stream   forEach e - gt  i 0     e getKey     e getValue      Using the Java 8 Stream API parallel  final long   i    0   map entrySet   stream   parallel   forEach e - gt  i 0     e getKey     e getValue      Using IterableMap of Apache Collections  long i   0  MapIterator lt Integer  Integer gt  it   iterableMap mapIterator    while  it hasNext          i    it next     it getValue       Using MutableMap of Eclipse  CS  collections  final long   i    0   mutableMap forEachKeyValue  key  value  - gt        i 0     key   value         Perfomance tests  mode   AverageTime  system   Windows nbsp 8 1 64-bit  Intel i7-4790 3 60 GHz  16 nbsp GB    For a small map  100 elements   score 0 308 is the best  Benchmark                          Mode  Cnt  Score    Error  Units test3 UsingForEachAndJava8         avgt  10   0 308     0 021    s op test10 UsingEclipseMap             avgt  10   0 309     0 009    s op test1 UsingWhileAndMapEntry        avgt  10   0 380     0 014    s op test6 UsingForAndIterator          avgt  10   0 387     0 016    s op test2 UsingForEachAndMapEntry      avgt  10   0 391     0 023    s op test7 UsingJava8StreamApi          avgt  10   0 510     0 014    s op test9 UsingApacheIterableMap       avgt  10   0 524     0 008    s op test4 UsingKeySetAndForEach        avgt  10   0 816     0 026    s op test5 UsingKeySetAndIterator       avgt  10   0 863     0 025    s op test8 UsingJava8StreamApiParallel  avgt  10   5 552     0 185    s op  For a map with 10000 elements   score 37 606 is the best  Benchmark                           Mode   Cnt  Score      Error   Units test10 UsingEclipseMap              avgt   10    37 606      0 790    s op test3 UsingForEachAndJava8          avgt   10    50 368      0 887    s op test6 UsingForAndIterator           avgt   10    50 332      0 507    s op test2 UsingForEachAndMapEntry       avgt   10    51 406      1 032    s op test1 UsingWhileAndMapEntry         avgt   10    52 538      2 431    s op test7 UsingJava8StreamApi           avgt   10    54 464      0 712    s op test4 UsingKeySetAndForEach         avgt   10    79 016     25 345    s op test5 UsingKeySetAndIterator        avgt   10    91 105     10 220    s op test8 UsingJava8StreamApiParallel   avgt   10   112 511      0 365    s op test9 UsingApacheIterableMap        avgt   10   125 714      1 935    s op  For a map with 100000 elements   score 1184 767 is the best  Benchmark                          Mode   Cnt  Score        Error    Units test1 UsingWhileAndMapEntry        avgt   10   1184 767      332 968    s op test10 UsingEclipseMap             avgt   10   1191 735      304 273    s op test2 UsingForEachAndMapEntry      avgt   10   1205 815      366 043    s op test6 UsingForAndIterator          avgt   10   1206 873      367 272    s op test8 UsingJava8StreamApiParallel  avgt   10   1485 895      233 143    s op test5 UsingKeySetAndIterator       avgt   10   1540 281      357 497    s op test4 UsingKeySetAndForEach        avgt   10   1593 342      294 417    s op test3 UsingForEachAndJava8         avgt   10   1666 296      126 443    s op test7 UsingJava8StreamApi          avgt   10   1706 676      436 867    s op test9 UsingApacheIterableMap       avgt   10   3289 866     1445 564    s op    Graphs  performance tests depending on map size     Table  perfomance tests depending on map size             100     600      1100     1600     2100 test10    0 333    1 631    2 752    5 937    8 024 test3     0 309    1 971    4 147    8 147   10 473 test6     0 372    2 190    4 470    8 322   10 531 test1     0 405    2 237    4 616    8 645   10 707 test2     0 376    2 267    4 809    8 403   10 910 test7     0 473    2 448    5 668    9 790   12 125 test9     0 565    2 830    5 952   13 220   16 965 test4     0 808    5 012    8 813   13 939   17 407 test5     0 810    5 104    8 533   14 064   17 422 test8     5 173   12 499   17 351   24 671   30 403   All tests are on GitHub

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There are several ways to iterate a map  Please refer to the following code  When you iterate a map using iterator Interface you must go with Entry lt K V gt  or entrySet    It looks like this      import java util        import java util HashMap      import java util Iterator      import java util Map       public class IteratMapDemo           public static void main String arg                 Map lt String  String gt  mapOne   new HashMap lt String  String gt                 mapOne put  quot 1 quot    quot January quot                mapOne put  quot 2 quot    quot February quot                mapOne put  quot 3 quot    quot March quot                mapOne put  quot 4 quot    quot April quot                mapOne put  quot 5 quot    quot May quot                mapOne put  quot 6 quot    quot June quot                mapOne put  quot 7 quot    quot July quot                mapOne put  quot 8 quot    quot August quot                mapOne put  quot 9 quot    quot September quot                mapOne put  quot 10 quot    quot Octomber quot                mapOne put  quot 11 quot    quot November quot                mapOne put  quot 12 quot    quot December quot                 Iterator it   mapOne entrySet   iterator                while it hasNext                                  Map Entry me    Map Entry  it next                      System out println  quot Get Key through While loop    quot    me getKey                               for Map Entry lt String  String gt  entry mapOne entrySet                       System out println entry getKey      quot   quot    entry getValue                                for  Object key   mapOne keySet                      System out println  quot Key   quot    key toString      quot  Value   quot                                       mapOne get key

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Typical code for iterating over a map is   Map lt String Thing gt  map        for  Map Entry lt String Thing gt  entry   map entrySet          String key   entry getKey        Thing thing   entry getValue                HashMap is the canonical map implementation and doesn t make guarantees  or though it should not change order if no mutating operation are performed on it   SortedMap will return entries based on the natural ordering of the keys  or a Comparator  if provided  LinkedHashMap will either return entries in insertion-order or access-order depending upon how it has been constructed  EnumMap returns entries in natural order of keys    Update  I think this is no longer true   Note  IdentityHashMap entrySet iterator currently has a peculiar implementation which returns the same Map Entry instance for every item in the entrySet  However  every time a new the iterator advances the Map Entry is updated

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The ordering will always depend on the specific map implementation  Using Java  8 you can use either of these  map forEach  k v  - gt    System out println k    quot   quot    v        Or  map entrySet   forEach  e  - gt                System out println e getKey      quot     quot    e getValue                  The result will be the same  same order   The entrySet backed by the map so you are getting the same order  The second one is handy as it allows you to use lambdas  e g  if you want only to print only Integer objects that are greater than 5  map entrySet        stream        filter e- gt  e getValue    gt  5       forEach System out  println    The code below shows iteration through LinkedHashMap and normal HashMap  example   You will see difference in the order  public class HMIteration         public static void main String   args            Map lt Object  Object gt  linkedHashMap   new LinkedHashMap lt  gt             Map lt Object  Object gt  hashMap   new HashMap lt  gt              for  int i 10  i gt  0  i--                linkedHashMap put i  i               hashMap put i  i                      System out println  quot LinkedHashMap  1    quot            linkedHashMap forEach  k v  - gt    System out print k    quot      quot  k hashCode      quot    quot    v    quot    quot                 System out println  quot  nLinkedHashMap  2    quot             linkedHashMap entrySet   forEach  e  - gt                System out print e getKey      quot     quot    e getValue      quot    quot                          System out println  quot  n nHashMap  1    quot            hashMap forEach  k v  - gt    System out print k    quot      quot  k hashCode      quot    quot    v    quot    quot                 System out println  quot  nHashMap  2    quot             hashMap entrySet   forEach  e  - gt                System out print e getKey      quot     quot    e getValue      quot    quot                         Output  LinkedHashMap  1   10    10  10  9    9  9  8    8  8  7    7  7  6    6  6  5    5  5  4    4  4  3    3  3  2    2  2  1    1  1  0    0  0  LinkedHashMap  2   10   10  9   9  8   8  7   7  6   6  5   5  4   4  3   3  2   2  1   1  0   0  HashMap  1   0    0  0  1    1  1  2    2  2  3    3  3  4    4  4  5    5  5  6    6  6  7    7  7  8    8  8  9    9  9  10    10  10  HashMap  2   0   0  1   1  2   2  3   3  4   4  5   5  6   6  7   7  8   8  9   9  10   10

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Iterator iterator   map entrySet   iterator        while  iterator hasNext              Map Entry element    Map Entry it next            LOGGER debug  Key      element getKey             LOGGER debug  value      element getValue

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Map lt String  String gt  map    for  Map Entry lt String  String gt  entry   map entrySet          MapKey   entry getKey        MapValue   entry getValue

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Map lt String  String gt  map       for  Map Entry lt String  String gt  entry   map entrySet          System out println entry getKey           entry getValue

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You can do it using generics   Map lt Integer  Integer gt  map   new HashMap lt Integer  Integer gt     Iterator lt Map Entry lt Integer  Integer gt  gt  entries   map entrySet   iterator    while  entries hasNext          Map Entry lt Integer  Integer gt  entry   entries next        System out println  Key       entry getKey        Value       entry getValue

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In Map one can Iteration over keys and or values and or both  e g   entrySet   depends on one s interested in  Like   Iterate through the keys - gt  keySet   of the map   Map lt String  Object gt  map          for  String key   map keySet             your Business logic         Iterate through the values - gt  values   of the map   for  Object value   map values             your Business logic         Iterate through the both - gt  entrySet   of the map   for  Map Entry lt String  Object gt  entry   map entrySet           String key   entry getKey         Object value   entry getValue           your Business logic          Moreover  there are 3 different ways to iterate through a HashMap  They are as below    1  for  Map Entry entry   hm entrySet          System out print  quot key val   quot        System out println entry getKey      quot   quot    entry getValue          2  Iterator iter   hm keySet   iterator    while iter hasNext          Integer key    Integer iter next        String val    String hm get key       System out println  quot key val   quot    key    quot   quot    val        3  Iterator it   hm entrySet   iterator    while  it hasNext          Map Entry entry    Map Entry  it next        Integer key    Integer entry getKey        String val    String entry getValue        System out println  quot key val   quot    key    quot   quot    val

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public class abcd      public static void main String   args               Map lt Integer  String gt  testMap   new HashMap lt Integer  String gt             testMap put 10   a            testMap put 20   b            testMap put 30   c            testMap put 40   d            for  Integer key testMap keySet                  String value testMap get key               System out println value                       OR  public class abcd       public static void main String   args               Map lt Integer  String gt  testMap   new HashMap lt Integer  String gt             testMap put 10   a            testMap put 20   b            testMap put 30   c            testMap put 40   d            for  Entry lt Integer  String gt  entry   testMap entrySet                  Integer key entry getKey                String value entry getValue

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Java 8  We have got forEach method that accepts a lambda expression  We have also got stream APIs  Consider a map   Map lt String String gt  sample   new HashMap lt  gt     sample put  A   Apple    sample put  B    Ball      Iterate over keys   sample keySet   forEach  k  - gt  System out println k      Iterate over values   sample values   forEach  v  - gt  System out println v      Iterate over entries  Using forEach and Streams    sample forEach  k v  - gt  System out println k         v     sample entrySet   stream   forEach  entry  - gt                Object currentKey   entry getKey                Object currentValue   entry getValue                System out println currentKey         currentValue                 The advantage with streams is they can be parallelized easily in case we want to  We simply need to use parallelStream   in place of stream   above   forEachOrdered vs forEach with streams   The forEach does not follow encounter order  if defined  and is inherently non-deterministic in nature where as the forEachOrdered does  So forEach does not guarantee that the order would be kept  Also check this for more

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With Eclipse Collections  you would use the forEachKeyValue method on the MapIterable interface  which is inherited by the MutableMap and ImmutableMap interfaces and their implementations   MutableBag lt String gt  result   Bags mutable empty    MutableMap lt Integer  String gt  map   Maps mutable of 1   One   2   Two   3   Three    map forEachKeyValue  key  value  - gt  result add key   value    Assert assertEquals Bags mutable of  1One    2Two    3Three    result     Using an anonymous inner class  you can write the code as follows   final MutableBag lt String gt  result   Bags mutable empty    MutableMap lt Integer  String gt  map   Maps mutable of 1   One   2   Two   3   Three    map forEachKeyValue new Procedure2 lt Integer  String gt          public void value Integer key  String value                result add key   value             Assert assertEquals Bags mutable of  1One    2Two    3Three    result     Note  I am a committer for Eclipse Collections

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If you have a generic untyped Map you can use   Map map   new HashMap    for  Map Entry entry     Set lt Map Entry gt   map entrySet           System out println entry getKey           entry getValue

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I copied the data of a map to another with this code   HashMap product   HashMap shopping truck get i   HashMap tmp   new HashMap    for  Iterator it   product entrySet   iterator    it hasNext           Map Entry thisEntry    Map Entry  it next        tmp put thisEntry getKey    thisEntry getValue

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Iterating a Map is very easy    for Object key  map keySet        Object value  map get key        Do your stuff     For instance  you have a Map lt String  int gt  data   for Object key  data keySet       int value  data get key

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You can search for the key and with the help of the key you can find the associated value of the map as map has unique key  see what happens when key is duplicate here or here  Demo map    Map lt String  String gt  map   new HashMap      map put  quot name quot    quot Badri Paudel quot      map put  quot age quot    quot 23 quot      map put  quot address quot    quot KTM quot      map put  quot faculty quot    quot BE quot      map put  quot major quot    quot CS quot      map put  quot head quot    quot AVD quot       To get key only  you can use map keySet    like this   for String key   map keySet            System out println key        To get value only   you can use map values    like this        for String value   map values            System out println value        To get both key and its value you still can use map keySet    and get its corresponding value  like this      this prints the key value pair   for  String k   map keySet              System out println k    quot   quot    map get k     quot   quot           map get key  gives the value pointed by that key

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Here is a generic type-safe method which can be called to dump any given Map   import java util Iterator  import java util Map   public class MapUtils       static interface ItemCallback lt K  V gt            void handler K key  V value  Map lt K  V gt  map              public static  lt K  V gt  void forEach Map lt K  V gt  map  ItemCallback lt K  V gt  callback            Iterator lt Map Entry lt K  V gt  gt  it   map entrySet   iterator             while  it hasNext                  Map Entry lt K  V gt  entry   it next                 callback handler entry getKey    entry getValue    map                        public static  lt K  V gt  void printMap Map lt K  V gt  map            forEach map  new ItemCallback lt K  V gt                   Override             public void handler K key  V value  Map lt K  V gt  map                    System out println key           value                                       Example  Here is an example of its use  Notice that the type of the Map is inferred by the method   import java util     public class MapPrinter       public static void main String   args            List lt Map lt      gt  gt  maps   new ArrayList lt Map lt      gt  gt                  private static final long serialVersionUID   1L                                add new LinkedHashMap lt String  Integer gt                          private static final long serialVersionUID   1L                                                put  One   0                           put  Two   1                           put  Three   3                                                              add new LinkedHashMap lt String  Object gt                          private static final long serialVersionUID   1L                                                put  Object   new Object                             put  Integer   new Integer 0                            put  Double   new Double 0 0                                                                                for  Map lt      gt  map   maps                MapUtils printMap map               System out println                        Output  One   0 Two   1 Three   3  Object   java lang Object 15db9742 Integer   0 Double   0 0

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In theory  the most efficient way will depend on which implementation of Map  The official way to do this is to call map entrySet    which returns a set of Map Entry  each of which contains a key and a value  entry getKey   and entry getValue      In an idiosyncratic implementation  it might make some difference whether you use map keySet    map entrySet   or something else  But I can t think of a reason why anyone would write it like that  Most likely it makes no difference to performance what you do   And yes  the order will depend on the implementation - as well as  possibly  the order of insertion and other hard-to-control factors    edit  I wrote valueSet   originally but of course entrySet   is actually the answer

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package com test   import java util Collection  import java util HashMap  import java util Iterator  import java util Map  import java util Map Entry  import java util Set   public class Test        public static void main String   args            Map lt String  String gt  map   new HashMap lt String  String gt             map put  ram    ayodhya            map put  krishan    mathura            map put  shiv    kailash             System out println            Keys                      Set lt String gt  keys   map keySet            for  String key   keys                System out println key                      System out println            Values                      Collection lt String gt  values   map values            for  String value   values                System out println value                      System out println        Keys and Values  Using for each loop                   for  Map Entry lt String  String gt  entry   map entrySet                  System out println  Key      entry getKey       t Value                          entry getValue                        System out println        Keys and Values  Using while loop                   Iterator lt Entry lt String  String gt  gt  entries   map entrySet   iterator            while  entries hasNext                  Map Entry lt String  String gt  entry    Map Entry lt String  String gt   entries                      next                System out println  Key      entry getKey       t Value                          entry getValue                        System out                  println     Keys and Values  Using java 8 using lambdas                 map forEach  k  v  - gt  System out                  println  Key      k     t value      v

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This is the easiest way of doing it I believe        For example  this could be a map object    Map lt String  Integer gt  MAP   new Map lt  gt         Do something like put keys value pairs into the map  etc    MAP put  Denver   35   MAP put  Patriots   14       Then  simply use a for each loop like this to iterate    for  Object o   MAP entrySet          Map Entry pair    Map Entry  o         Do whatever with the pair here  i e  pair getKey    or pair getValue

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FYI  you can also use map keySet   and map values   if you re only interested in keys values of the map and not the other

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Yes  as many people agreed this is the best way to iterate over a Map   But there are chances to throw nullpointerexception if the map is null  Don t forget to put null  check in                                                                                                                                                  - - - -                                                                                   for  Map Entry lt String  Object gt  entry   map entrySet          String key   entry getKey        Object value   entry getValue

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With Java 8  you can iterate Map using forEach and lambda expression   map forEach  k  v  - gt  System out println  k         v

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An effective iterative solution over a Map is a for loop from Java 5 through Java 7  Here it is  for  String key   phnMap keySet          System out println  quot Key   quot    key    quot  Value   quot    phnMap get key       From Java 8 you can use a lambda expression to iterate over a Map  It is an enhanced forEach phnMap forEach  k v  - gt  System out println  quot Key   quot    k    quot  Value   quot    v     If you want to write a conditional for lambda you can write it like this  phnMap forEach  k v - gt       System out println  quot Key   quot    k    quot  Value   quot    v       if  quot abc quot  equals k            System out println  quot Hello abc quot

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Map forEach  What about simply using Map  forEach where both the key and the value are passed to your BiConsumer   map forEach  k v - gt       System out println k  - gt   v

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There are several ways to iterate over map   Here is comparison of their performances for a common data set stored in map by storing a million key value pairs in map and will iterate over map   1  Using entrySet   in for each loop  for  Map Entry lt String Integer gt  entry   testMap entrySet          entry getKey        entry getValue        50 milliseconds  2  Using keySet   in for each loop  for  String key   testMap keySet          testMap get key       76 milliseconds  3  Using entrySet   and iterator  Iterator lt Map Entry lt String Integer gt  gt  itr1   testMap entrySet   iterator    while itr1 hasNext          Map Entry lt String Integer gt  entry   itr1 next        entry getKey        entry getValue        50 milliseconds  4  Using keySet   and iterator  Iterator itr2   testMap keySet   iterator    while itr2 hasNext          String key   itr2 next        testMap get key       75 milliseconds  I have referred this link

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There are a lot of ways to do this  Below is a few simple steps   Suppose you have one Map like   Map lt String  Integer gt  m   new HashMap lt String  Integer gt       Then you can do something like the below to iterate over map elements                 Using an iterator                  Iterator lt Entry lt String  Integer gt  gt  me   m entrySet   iterator    while me hasNext         Entry lt String  Integer gt  pair   me next        System out println pair getKey           pair getValue                       Using foreach                          for Entry lt String  Integer gt  me   m entrySet         System out println me getKey             me getValue                       Using keySet                               for String s   m keySet         System out println s           m get s                      Using keySet and iterator                   Iterator lt String gt  me   m keySet   iterator    while me hasNext         String key   me next        System out println key           m get key

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Example of using iterator and generics   Iterator lt Map Entry lt String  String gt  gt  entries   myMap entrySet   iterator    while  entries hasNext        Map Entry lt String  String gt  entry   entries next      String key   entry getKey      String value   entry getValue

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Java 8   You can use lambda expressions   myMap entrySet   stream   forEach  entry  - gt        Object currentKey   entry getKey        Object currentValue   entry getValue          For more information  follow this

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If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it  what is the most efficient way of going through the map    If efficiency of looping the keys is a priority for your app  then choose a Map implementation that maintains the keys in your desired order      Will the ordering of elements depend on the specific map implementation that I have for the interface    Yes  absolutely     Some Map implementations promise a certain iteration order  others do not  Different implementations of Map maintain different ordering of the key-value pairs     See this table I created summarizing the various Map implementations bundled with Java 11  Specifically  notice the iteration order column  Click tap to zoom     You can see there are four Map implementations maintaining an order    TreeMap ConcurrentSkipListMap  LinkedHashMap EnumMap   NavigableMap interface  Two of those implement the NavigableMap interface  TreeMap  amp  ConcurrentSkipListMap    The older SortedMap interface is effectively supplanted by the newer NavigableMap interface  But you may find 3rd-party implementations implementing the older interface only   Natural order  If you want a Map that keeps its pairs arranged by the    natural order    of the key  use TreeMap or ConcurrentSkipListMap  The term    natural order    means the class of the keys implements Comparable  The value returned by the compareTo method is used for comparison in sorting   Custom order  If you want to specify a custom sorting routine for your keys to be used in maintaining a sorted order  pass a Comparator implementation appropriate to the class of your keys  Use either TreeMap or ConcurrentSkipListMap  passing your Comparator    Original insertion order  If you want the pairs of your map to be kept in their original order in which you inserted them into the map  use LinkedHashMap    Enum-definition order  If you are using an enum such as DayOfWeek or Month as your keys  use the EnumMap class  Not only is this class highly optimized to use very little memory and run very fast  it maintains your pairs in the order defined by the enum  For DayOfWeek  for example  the key of DayOfWeek MONDAY will be first found when iterated  and the key of DayOfWeek SUNDAY will be last   Other considerations  In choosing a Map implementation  also consider    NULLs  Some implementations forbid accept a NULL as key and or value   Concurrency  If you are manipulating the map across threads  you must use an implementation that supports concurrency  Or wrap the map with Collections  synchronizedMap  less preferable      Both of these considerations are covered in the graphic table above

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In Java 8 you can do it clean and fast using the new lambdas features    Map lt String String gt  map   new HashMap lt  gt      map put  SomeKey    SomeValue     map forEach   k v  - gt   do something with key and value          such as  map forEach   k v  - gt  System out println  Key      k      Value      v      The type of k and v will be inferred by the compiler and there is no need to use Map Entry anymore   Easy-peasy

[java] How do I efficiently iterate over each entry in a Java Map?

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