Fastest way to iterate over all the chars in a String

Question

In Java  what would the fastest way to iterate over all the chars in a String  this   String str    a really  really long string   for  int i   0  n   str length    i  lt  n  i          char c   str charAt i       Or this   char   chars   str toCharArray    for  int i   0  n   chars length  i  lt  n  i          char c   chars i       EDIT    What I d like to know is if the cost of repeatedly calling the charAt method during a long iteration ends up being either less than or greater than the cost of performing a single call to toCharArray at the beginning and then directly accessing the array during the iteration   It d be great if someone could provide a robust benchmark for different string lengths  having in mind JIT warm-up time  JVM start-up time  etc  and not just the difference between two calls to System currentTimeMillis

User · Answer

The second one causes a new char array to be created  and all chars from the String to be copied to this new char array  so I would guess that the first one is faster  and less memory-hungry

User · Answer

FIRST UPDATE  Before you try this ever in a production environment  not advised   read this first  http   www javaspecialists eu archive Issue237 html Starting from Java 9  the solution as described won t work anymore  because now Java will store strings as byte   by default  SECOND UPDATE  As of 2016-10-25  on my AMDx64 8core and source 1 8  there is no difference between using  charAt  and field access  It appears that the jvm is sufficiently optimized to inline and streamline any  string charAt n   calls  THIRD UPDATE  As of 2020-09-07  on my Ryzen 1950-X 16 core and source 1 14   charAt1  is 9 times slower than field access and  charAt2  is 4 times slower than field access  Field access is back as the clear winner  Note than the program will need to use byte   access for Java 9  version jvms  It all depends on the length of the String being inspected  If  as the question says  it is for long strings  the fastest way to inspect the string is to use reflection to access the backing char   of the string  A fully randomized benchmark with JDK 8  win32 and win64  on an 64 AMD Phenom II 4 core 955   3 2 GHZ  in both client mode and server mode  with 9 different techniques  see below   shows that using String charAt n  is the fastest for small strings and that using reflection to access the String backing array is almost twice as fast for large strings  THE EXPERIMENT  9 different optimization techniques are tried   All string contents are randomized  The test are done for string sizes in multiples of two starting with 0 1 2 4 8 16 etc   The tests are done 1 000 times per string size  The tests are shuffled into random order each time  In other words  the tests are done in random order every time they are done  over 1000 times over   The entire test suite is done forwards  and backwards  to show the effect of JVM warmup on optimization and times   The entire suite is done twice  once in -client mode and the other in -server mode    CONCLUSIONS -client mode  32 bit  For strings 1 to 256 characters in length  calling string charAt i  wins with an average processing of 13 4 million to 588 million characters per second  Also  it is overall 5 5  faster  client  and 13 9   server  like this      for  int i   0  i  lt  data length    i              if  data charAt i   lt                      doThrow                     than like this with a local final length variable      final int len   data length        for  int i   0  i  lt  len  i              if  data charAt i   lt                      doThrow                     For long strings  512 to 256K characters length  using reflection to access the String s backing array is fastest  This technique is almost twice as fast as String charAt i   178  faster   The average speed over this range was 1 111 billion characters per second  The Field must be obtained ahead of time and then it can be re-used in the library on different strings  Interestingly  unlike the code above  with Field access  it is 9  faster to have a local final length variable than to use  chars length  in the loop check  Here is how Field access can be setup as fastest     final Field field   String class getDeclaredField  quot value quot       field setAccessible true       try          final char   chars    char    field get data          final int len   chars length         for  int i   0  i  lt  len  i                 if  chars i   lt                         doThrow                                 return len       catch  Exception ex           throw new RuntimeException ex         Special comments on -server mode Field access starting winning after 32 character length strings in server mode on a 64 bit Java machine on my AMD 64 machine  That was not seen until 512 characters length in client mode  Also worth noting I think  when I was running JDK 8  32 bit build  in server mode  the overall performance was 7  slower for both large and small strings  This was with build 121 Dec 2013 of JDK 8 early release  So  for now  it seems that 32 bit server mode is slower than 32 bit client mode  That being said     it seems the only server mode that is worth invoking is on a 64 bit machine  Otherwise it actually hampers performance  For 32 bit build running in -server mode on an AMD64  I can say this   String charAt i  is the clear winner overall  Although between sizes 8 to 512 characters there were winners among  new   reuse  and  field   String charAt i  is 45  faster in client mode Field access is twice as fast for large Strings in client mode   Also worth saying  String chars    Stream and the parallel version  are a bust  Way slower than any other way  The Streams API is a rather slow way to perform general string operations  Wish List Java String could have predicate accepting optimized methods such as contains predicate   forEach consumer   forEachWithIndex consumer   Thus  without the need for the user to know the length or repeat calls to String methods  these could help parsing libraries beep-beep beep speedup  Keep dreaming    Happy Strings   SH The test used the following 9 methods of testing the string for the presence of whitespace   quot charAt1 quot  -- CHECK THE STRING CONTENTS THE USUAL WAY  int charAtMethod1 final String data        final int len   data length        for  int i   0  i  lt  len  i              if  data charAt i   lt                      doThrow                        return len      quot charAt2 quot  -- SAME AS ABOVE BUT USE String length   INSTEAD OF MAKING A FINAL LOCAL int FOR THE LENGTh int charAtMethod2 final String data        for  int i   0  i  lt  data length    i              if  data charAt i   lt                      doThrow                        return data length        quot stream quot  -- USE THE NEW JAVA-8 String s IntStream AND PASS IT A PREDICATE TO DO THE CHECKING int streamMethod final String data  final IntPredicate predicate        if  data chars   anyMatch predicate             doThrow              return data length        quot streamPara quot  -- SAME AS ABOVE  BUT OH-LA-LA - GO PARALLEL       avoid this at all costs int streamParallelMethod final String data  IntPredicate predicate        if  data chars   parallel   anyMatch predicate             doThrow              return data length        quot reuse quot  -- REFILL A REUSABLE char   WITH THE STRINGS CONTENTS int reuseBuffMethod final char   reusable  final String data        final int len   data length        data getChars 0  len  reusable  0       for  int i   0  i  lt  len  i              if  reusable i   lt                      doThrow                        return len      quot new1 quot  -- OBTAIN A NEW COPY OF THE char   FROM THE STRING int newMethod1 final String data        final int len   data length        final char   copy   data toCharArray        for  int i   0  i  lt  len  i              if  copy i   lt                      doThrow                        return len      quot new2 quot  -- SAME AS ABOVE  BUT USE  quot FOR-EACH quot  int newMethod2 final String data        for  final char c   data toCharArray              if  c  lt                      doThrow                        return data length        quot field1 quot  -- FANCY   OBTAIN FIELD FOR ACCESS TO THE STRING S INTERNAL char   int fieldMethod1 final Field field  final String data        try           final char   chars    char    field get data           final int len   chars length          for  int i   0  i  lt  len  i                  if  chars i   lt                          doThrow                                    return len        catch  Exception ex            throw new RuntimeException ex             quot field2 quot  -- SAME AS ABOVE  BUT USE  quot FOR-EACH quot  int fieldMethod2 final Field field  final String data        final char   chars      try           chars    char    field get data         catch  Exception ex            throw new RuntimeException ex             for  final char c   chars            if  c  lt                      doThrow                        return chars length     COMPOSITE RESULTS FOR CLIENT -client MODE  forwards and backwards tests combined  Note  that the -client mode with Java 32 bit and -server mode with Java 64 bit are the same as below on my AMD64 machine  Size     WINNER  charAt1 charAt2  stream streamPar   reuse    new1    new2  field1  field2 1        charAt    77 0     72 0   462 0     584 0   127 5    89 5    86 0   159 5   165 0 2        charAt    38 0     36 5   284 0   32712 5    57 5    48 3    50 3    89 0    91 5 4        charAt    19 5     18 5   458 6    3169 0    33 0    26 8    27 5    54 1    52 6 8        charAt     9 8      9 9   100 5    1370 9    17 3    14 4    15 0    26 9    26 4 16       charAt     6 1      6 5    73 4     857 0     8 4     8 2     8 3    13 6    13 5 32       charAt     3 9      3 7    54 8     428 9     5 0     4 9     4 7     7 0     7 2 64       charAt     2 7      2 6    48 2     232 9     3 0     3 2     3 3     3 9     4 0 128      charAt     2 1      1 9    43 7     138 8     2 1     2 6     2 6     2 4     2 6 256      charAt     1 9      1 6    42 4      90 6     1 7     2 1     2 1     1 7     1 8 512      field1     1 7      1 4    40 6      60 5     1 4     1 9     1 9     1 3     1 4 1 024    field1     1 6      1 4    40 0      45 6     1 2     1 9     2 1     1 0     1 2 2 048    field1     1 6      1 3    40 0      36 2     1 2     1 8     1 7     0 9     1 1 4 096    field1     1 6      1 3    39 7      32 6     1 2     1 8     1 7     0 9     1 0 8 192    field1     1 6      1 3    39 6      30 5     1 2     1 8     1 7     0 9     1 0 16 384   field1     1 6      1 3    39 8      28 4     1 2     1 8     1 7     0 8     1 0 32 768   field1     1 6      1 3    40 0      26 7     1 3     1 8     1 7     0 8     1 0 65 536   field1     1 6      1 3    39 8      26 3     1 3     1 8     1 7     0 8     1 0 131 072  field1     1 6      1 3    40 1      25 4     1 4     1 9     1 8     0 8     1 0 262 144  field1     1 6      1 3    39 6      25 2     1 5     1 9     1 9     0 8     1 0  COMPOSITE RESULTS FOR SERVER -server MODE  forwards and backwards tests combined  Note  this is the test for Java 32 bit running in server mode on an AMD64  The server mode for Java 64 bit was the same as Java 32 bit in client mode except that Field access starting winning after 32 characters size  Size     WINNER  charAt1 charAt2  stream streamPar   reuse    new1    new2  field1  field2 1        charAt     74 5    95 5   524 5     783 0    90 5   102 5    90 5   135 0   151 5 2        charAt     48 5    53 0   305 0   30851 3    59 3    57 5    52 0    88 5    91 8 4        charAt     28 8    32 1   132 8    2465 1    37 6    33 9    32 3    49 0    47 0 8          new2     18 0    18 6    63 4    1541 3    18 5    17 9    17 6    25 4    25 8 16         new2     14 0    14 7   129 4    1034 7    12 5    16 2    12 0    16 0    16 6 32         new2      7 8     9 1    19 3     431 5     8 1     7 0     6 7     7 9     8 7 64        reuse      6 1     7 5    11 7     204 7     3 5     3 9     4 3     4 2     4 1 128       reuse      6 8     6 8     9 0     101 0     2 6     3 0     3 0     2 6     2 7 256      field2      6 2     6 5     6 9      57 2     2 4     2 7     2 9     2 3     2 3 512       reuse      4 3     4 9     5 8      28 2     2 0     2 6     2 6     2 1     2 1 1 024    charAt      2 0     1 8     5 3      17 6     2 1     2 5     3 5     2 0     2 0 2 048    charAt      1 9     1 7     5 2      11 9     2 2     3 0     2 6     2 0     2 0 4 096    charAt      1 9     1 7     5 1       8 7     2 1     2 6     2 6     1 9     1 9 8 192    charAt      1 9     1 7     5 1       7 6     2 2     2 5     2 6     1 9     1 9 16 384   charAt      1 9     1 7     5 1       6 9     2 2     2 5     2 5     1 9     1 9 32 768   charAt      1 9     1 7     5 1       6 1     2 2     2 5     2 5     1 9     1 9 65 536   charAt      1 9     1 7     5 1       5 5     2 2     2 4     2 4     1 9     1 9 131 072  charAt      1 9     1 7     5 1       5 4     2 3     2 5     2 5     1 9     1 9 262 144  charAt      1 9     1 7     5 1       5 1     2 3     2 5     2 5     1 9     1 9  FULL RUNNABLE PROGRAM CODE  to test on Java 7 and earlier  remove the two streams tests  import java lang reflect Field  import java util ArrayList  import java util Collections  import java util List  import java util Random  import java util function IntPredicate           author Saint Hill  lt http   stackoverflow com users 1584255 saint-hill gt      public final class TestStrings           we will not test strings longer than 512KM     final int MAX STRING SIZE   1024   256          for each string size  we will do all the tests        this many times     final int TRIES PER STRING SIZE   1000       public static void main String   args  throws Exception           new TestStrings   run               void run   throws Exception               double the length of the data until it reaches MAX chars long            0 1 2 4 8 16 32 64 128 256              final List lt Integer gt  sizes   new ArrayList lt  gt             for  int n   0  n  lt   MAX STRING SIZE  n    n    0   1   n   2                 sizes add n                         CREATE RANDOM  FOR SHUFFLING ORDER OF TESTS          final Random random   new Random             System out println  quot Rate in nanoseconds per character inspected  quot            System out printf  quot      FORWARDS  tries per size   s        n quot   TRIES PER STRING SIZE            printHeadings TRIES PER STRING SIZE  random            for  int size   sizes                reportResults size  test size  TRIES PER STRING SIZE  random                          reverse order or string sizes         Collections reverse sizes            System out println  quot  quot            System out println  quot Rate in nanoseconds per character inspected  quot            System out printf  quot      BACKWARDS  tries per size   s        n quot   TRIES PER STRING SIZE            printHeadings TRIES PER STRING SIZE  random            for  int size   sizes                reportResults size  test size  TRIES PER STRING SIZE  random                                               METHODS OF CHECKING THE CONTENTS          OF A STRING  ALWAYS CHECKING FOR          WHITESPACE  CHAR  lt                                CHECK THE STRING CONTENTS     int charAtMethod1 final String data            final int len   data length            for  int i   0  i  lt  len  i                  if  data charAt i   lt                          doThrow                                    return len                SAME AS ABOVE BUT USE String length          instead of making a new final local int      int charAtMethod2 final String data            for  int i   0  i  lt  data length    i                  if  data charAt i   lt                          doThrow                                    return data length                  USE new Java-8 String s IntStream        pass it a PREDICATE to do the checking     int streamMethod final String data  final IntPredicate predicate            if  data chars   anyMatch predicate                 doThrow                      return data length                  OH LA LA - GO PARALLEL        int streamParallelMethod final String data  IntPredicate predicate            if  data chars   parallel   anyMatch predicate                 doThrow                      return data length                  Re-fill a resuable char   with the contents        of the String s char       int reuseBuffMethod final char   reusable  final String data            final int len   data length            data getChars 0  len  reusable  0           for  int i   0  i  lt  len  i                  if  reusable i   lt                          doThrow                                    return len                Obtain a new copy of char   from String     int newMethod1 final String data            final int len   data length            final char   copy   data toCharArray            for  int i   0  i  lt  len  i                  if  copy i   lt                          doThrow                                    return len                Obtain a new copy of char   from String        but use FOR-EACH     int newMethod2 final String data            for  final char c   data toCharArray                  if  c  lt                          doThrow                                    return data length                  FANCY         OBTAIN FIELD FOR ACCESS TO THE STRING S        INTERNAL CHAR       int fieldMethod1 final Field field  final String data            try               final char   chars    char    field get data               final int len   chars length              for  int i   0  i  lt  len  i                      if  chars i   lt                              doThrow                                                return len            catch  Exception ex                throw new RuntimeException ex                           same as above but use FOR-EACH     int fieldMethod2 final Field field  final String data            final char   chars          try               chars    char    field get data             catch  Exception ex                throw new RuntimeException ex                     for  final char c   chars                if  c  lt                          doThrow                                    return chars length                               Make a list of tests  We will shuffle a copy of this list repeatedly        while we repeat this test                 param data         return             List lt Jobber gt  makeTests String data  throws Exception              make a list of tests         final List lt Jobber gt  tests   new ArrayList lt Jobber gt              tests add new Jobber  quot charAt1 quot                 int check                     return charAtMethod1 data                                      tests add new Jobber  quot charAt2 quot                 int check                     return charAtMethod2 data                                      tests add new Jobber  quot stream quot                 final IntPredicate predicate   new IntPredicate                     public boolean test int value                        return value  lt                                                      int check                     return streamMethod data  predicate                                      tests add new Jobber  quot streamPar quot                 final IntPredicate predicate   new IntPredicate                     public boolean test int value                        return value  lt                                                      int check                     return streamParallelMethod data  predicate                                         Reusable char   method         tests add new Jobber  quot reuse quot                 final char   cbuff   new char MAX STRING SIZE                int check                     return reuseBuffMethod cbuff  data                                         New char   from String         tests add new Jobber  quot new1 quot                 int check                     return newMethod1 data                                         New char   from String         tests add new Jobber  quot new2 quot                 int check                     return newMethod2 data                                         Use reflection for field access         tests add new Jobber  quot field1 quot                 final Field field                                 field   String class getDeclaredField  quot value quot                    field setAccessible true                              int check                     return fieldMethod1 field  data                                         Use reflection for field access         tests add new Jobber  quot field2 quot                 final Field field                                 field   String class getDeclaredField  quot value quot                    field setAccessible true                              int check                     return fieldMethod2 field  data                                      return tests                        We use this class to keep track of test results             abstract class Jobber            final String name          long nanos          long chars          long runs           Jobber String name                this name   name                     abstract int check             final double nanosPerChar                 double charsPerRun   chars   runs              long nanosPerRun   nanos   runs              return charsPerRun    0   nanosPerRun   nanosPerRun   charsPerRun                     final void run                 runs                long time   System nanoTime                chars    check                nanos    System nanoTime   - time                          MAKE A TEST STRING OF RANDOM CHARACTERS A-Z     private String makeTestString int testSize  char start  char end            Random r   new Random            char   data   new char testSize           for  int i   0  i  lt  data length  i                  data i     char   start   r nextInt end                      return new String data                 WE DO THIS IF WE FIND AN ILLEGAL CHARACTER IN THE STRING     public void doThrow             throw new RuntimeException  quot Bzzzt -- Illegal Character   quot                          1  get random string of correct length 2  get tests  List lt Jobber gt   3         perform tests repeatedly  shuffling each time             List lt Jobber gt  test int size  int tries  Random random  throws Exception           String data   makeTestString size   A    Z            List lt Jobber gt  tests   makeTests data           List lt Jobber gt  copy   new ArrayList lt  gt  tests           while  tries--  gt  0                Collections shuffle copy  random               for  Jobber ti   copy                    ti run                                       check to make sure all char counts the same         long runs   tests get 0  runs          long count   tests get 0  chars          for  Jobber ti   tests                if  ti runs    runs  amp  amp  ti chars    count                    throw new Exception  quot Char counts should match if all correct algorithms quot                                    return tests             private void printHeadings final int TRIES PER STRING SIZE  final Random random  throws Exception           System out print  quot   Size quot            for  Jobber ti   test 0  TRIES PER STRING SIZE  random                 System out printf  quot  9s quot   ti name                     System out println  quot  quot               private void reportResults int size  List lt Jobber gt  tests            System out printf  quot  6d quot   size           for  Jobber ti   tests                System out printf  quot   9 2f quot   ti nanosPerChar                       System out println  quot  quot

User · Answer

This is just micro-optimisation that you shouldn t worry about   char   chars   str toCharArray      returns you a copy of str character arrays  in JDK  it returns a copy of characters by calling System arrayCopy    Other than that  str charAt   only checks if the index is indeed in bounds and returns a character within the array index   The first one doesn t create additional memory in JVM

User · Answer

Despite  Saint Hill s answer if you consider the time complexity of str toCharArray      the first one is faster even for very large strings  You can run the code below to see it for yourself           char    ch   new char 1 000 000 00       String str   new String ch      to create a large string         --- gt  from here     long currentTime   System nanoTime        for  int i   0  n   str length    i  lt  n  i              char c   str charAt i                --- gt  to here     System out println  str charAt i     System nanoTime  -currentTime  1000000 0     ms                       ch   str toCharArray   itself takes lots of time                   --- gt  from here     currentTime   System nanoTime        ch   str toCharArray        for  int i   0  n   str length    i  lt  n  i              char c   ch i                --- gt  to  here     System out println  ch   str toCharArray     c   ch i      System nanoTime  -currentTime  1000000 0     ms       output   str charAt i  5 492102  ms  ch   str toCharArray     c   ch i   79 400064  ms

User · Answer

Just for curiosity and to compare with Saint Hill s answer   If you need to process heavy data you should not use JVM in client mode  Client mode is not made for optimizations    Let s compare results of  Saint Hill benchmarks using a JVM in Client mode and Server mode   Core2Quad Q6600 G0   2 4GHz JavaSE 1 7 0 40   See also  Real differences between  quot java -server quot  and  quot java -client quot      CLIENT MODE   len        2     111k charAt i    105k cbuff i     62k new i     17k field access     chars ms   len        4     285k charAt i    166k cbuff i    114k new i     43k field access     chars ms   len        6     315k charAt i    230k cbuff i    162k new i     69k field access     chars ms   len        8     333k charAt i    275k cbuff i    181k new i     85k field access     chars ms   len       12     342k charAt i    342k cbuff i    222k new i    117k field access     chars ms   len       16     363k charAt i    347k cbuff i    275k new i    152k field access     chars ms   len       20     363k charAt i    392k cbuff i    289k new i    180k field access     chars ms   len       24     375k charAt i    428k cbuff i    311k new i    205k field access     chars ms   len       28     378k charAt i    474k cbuff i    341k new i    233k field access     chars ms   len       32     376k charAt i    492k cbuff i    340k new i    251k field access     chars ms   len       64     374k charAt i    551k cbuff i    374k new i    367k field access     chars ms   len      128     385k charAt i    624k cbuff i    415k new i    509k field access     chars ms   len      256     390k charAt i    675k cbuff i    436k new i    619k field access     chars ms   len      512     394k charAt i    703k cbuff i    439k new i    695k field access     chars ms   len     1024     395k charAt i    718k cbuff i    462k new i    742k field access     chars ms   len     2048     396k charAt i    725k cbuff i    471k new i    767k field access     chars ms   len     4096     396k charAt i    727k cbuff i    459k new i    780k field access     chars ms   len     8192     397k charAt i    712k cbuff i    446k new i    772k field access     chars ms       SERVER MODE   len        2      86k charAt i     41k cbuff i     46k new i     80k field access     chars ms   len        4     571k charAt i    250k cbuff i     97k new i    222k field access     chars ms   len        6     666k charAt i    333k cbuff i    125k new i    315k field access     chars ms   len        8     800k charAt i    400k cbuff i    181k new i    380k field access     chars ms   len       12     800k charAt i    521k cbuff i    260k new i    545k field access     chars ms   len       16     800k charAt i    592k cbuff i    296k new i    640k field access     chars ms   len       20     800k charAt i    666k cbuff i    408k new i    800k field access     chars ms   len       24     800k charAt i    705k cbuff i    452k new i    800k field access     chars ms   len       28     777k charAt i    736k cbuff i    368k new i    933k field access     chars ms   len       32     800k charAt i    780k cbuff i    571k new i    969k field access     chars ms   len       64     800k charAt i    901k cbuff i    800k new i    1306k field access     chars ms   len      128     1084k charAt i    888k cbuff i    633k new i    1620k field access     chars ms   len      256     1122k charAt i    966k cbuff i    729k new i    1790k field access     chars ms   len      512     1163k charAt i    1007k cbuff i    676k new i    1910k field access     chars ms   len     1024     1179k charAt i    1027k cbuff i    698k new i    1954k field access     chars ms   len     2048     1184k charAt i    1043k cbuff i    732k new i    2007k field access     chars ms   len     4096     1188k charAt i    1049k cbuff i    742k new i    2031k field access     chars ms   len     8192     1157k charAt i    1032k cbuff i    723k new i    2048k field access     chars ms       CONCLUSION   As you can see  server mode is much faster

User · Answer

Looks like niether is faster or slower      public static void main String arguments                  Build a long string         StringBuilder sb   new StringBuilder            for int j   0  j  lt  10000  j                  sb append  a really  really long string                      String str   sb toString            for  int testscount   0  testscount  lt  10  testscount                       Test 1             long start   System currentTimeMillis                for int c   0  c  lt  10000000  c                      for  int i   0  n   str length    i  lt  n  i                          char chr   str charAt i                       doSomethingWithChar chr    To trick JIT optimistaion                                              System out println  1       System currentTimeMillis   - start                   Test 2             start   System currentTimeMillis                char   chars   str toCharArray                for int c   0  c  lt  10000000  c                      for  int i   0  n   chars length  i  lt  n  i                          char chr   chars i                       doSomethingWithChar chr    To trick JIT optimistaion                                             System out println  2       System currentTimeMillis   - start                System out println                            public static void doSomethingWithChar char chr            int newInt   chr  lt  lt  2          For long strings I ll chose the first one  Why copy around long strings  Documentations says      public char   toCharArray     Converts this string to a new character array        Returns    a newly allocated character array whose length is the length of this string and whose contents are initialized to contain the character sequence represented by this string      Edit 1   I ve changed the test to trick JIT optimisation     Edit 2  Repeat test 10 times to let JVM warm up     Edit 3  Conclusions   First of all str toCharArray    copies entire string in memory  It can be memory consuming for long strings  Method String charAt    looks up char in char array inside String class checking index before  It looks like for short enough Strings first method  i e  chatAt method  is a bit slower due to this index check  But if the String is long enough  copying whole char array gets slower  and the first method is faster  The longer the string is  the slower toCharArray performs  Try to change limit in for int j   0  j  lt  10000  j    loop to see it  If we let JVM warm up code runs faster  but proportions are the same   After all it s just micro-optimisation

User · Answer

The first one using str charAt should be faster   If you dig inside the source code of String class  we can see that charAt is implemented as follows   public char charAt int index        if   index  lt  0      index  gt   count             throw new StringIndexOutOfBoundsException index             return value index   offset       Here  all it does is index an array and return the value   Now  if we see the implementation of toCharArray  we will find the below   public char   toCharArray         char result     new char count       getChars 0  count  result  0       return result     public void getChars int srcBegin  int srcEnd  char dst    int dstBegin        if  srcBegin  lt  0            throw new StringIndexOutOfBoundsException srcBegin             if  srcEnd  gt  count            throw new StringIndexOutOfBoundsException srcEnd             if  srcBegin  gt  srcEnd            throw new StringIndexOutOfBoundsException srcEnd - srcBegin             System arraycopy value  offset   srcBegin  dst  dstBegin           srcEnd - srcBegin       As you see  it is doing a System arraycopy which is definitely going to be a tad slower than not doing it

User · Answer

String toCharArray   creates new char array  means allocation of memory of string length  then copies original char array of string using System arraycopy   and then returns this copy to caller   String charAt   returns character at position i from original copy  that s why String charAt   will be faster than String toCharArray    Although  String toCharArray   returns copy and not char from original String array  where String charAt   returns character from original char array  Code below returns value at the specified index of this string   public char charAt int index        if   index  lt  0      index  gt   value length             throw new StringIndexOutOfBoundsException index             return value index       code below returns a newly allocated character array whose length is the length of this string  public char   toCharArray            Cannot use Arrays copyOf because of class initialization order issues     char result     new char value length       System arraycopy value  0  result  0  value length       return result

[java] Fastest way to iterate over all the chars in a String

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