Simple way to repeat a string

Question

I m looking for a simple commons method or operator that allows me to repeat some string n times  I know I could write this using a for loop  but I wish to avoid for loops whenever necessary and a simple direct method should exist somewhere  String str    quot abc quot   String repeated   str repeat 3    repeated equals  quot abcabcabc quot     Related to  repeat string javascript Create NSString by repeating another string a given number of times Edited I try to avoid for loops when they are not completely necessary because   They add to the number of lines of code even if they are tucked away in another function   Someone reading my code has to figure out what I am doing in that for loop  Even if it is commented and has meaningful variables names  they still have to make sure it is not doing anything  quot clever quot    Programmers love to put clever things in for loops  even if I write it to  quot only do what it is intended to do quot   that does not preclude someone coming along and adding some additional clever  quot fix quot    They are very often easy to get wrong  For loops involving indexes tend to generate off by one bugs   For loops often reuse the same variables  increasing the chance of really hard to find scoping bugs   For loops increase the number of places a bug hunter has to look

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I really enjoy this question  There is a lot of knowledge and styles  So I can t leave it without show my rock and roll            String string   repeat  1234567890   4       System out println string       System out println                 repeatWithoutCopySample string  100000       System out println string     This take time  try it without printing     System out println string length               The core of the task       SuppressWarnings  AssignmentToMethodParameter   public static char   repeat char   sample  int times        char   r   new char sample length   times       while  --times  gt  -1            System arraycopy sample  0  r  times   sample length  sample length             return r            Java classic style      public static String repeat String sample  int times        return new String repeat sample toCharArray    times              Java extreme memory style       SuppressWarnings  UseSpecificCatch   public static void repeatWithoutCopySample String sample  int times        try           Field valueStringField   String class getDeclaredField  value            valueStringField setAccessible true           valueStringField set sample  repeat  char    valueStringField get sample   times          catch  Exception ex            throw new RuntimeException ex             Do you like it

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String  repeat       repeat  7       Seven period-with-space pairs                   New in Java 11 is the method String  repeat that does exactly what you asked for   String str    abc   String repeated   str repeat 3   repeated equals  abcabcabc      Its Javadoc says          Returns a string whose value is the concatenation of this    string repeated   code count  times      lt p gt     If this string is empty or count is zero then the empty    string is returned         param count number of times to repeat        return A string composed of this string repeated      code count  times or the empty string if this    string is empty or count is zero        throws IllegalArgumentException if the   code count  is    negative         since 11

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Consolidated for quick reference   public class StringRepeat       Java 11 has built-in method - str repeat 3      Apache - StringUtils repeat 3      Google - Strings repeat    n      System arraycopy  static String repeat StringBuilderAppend String str  int n         if  str    null    str isEmpty            return str       StringBuilder sb   new StringBuilder        for  int i   0  i  lt  n  i              sb append str             return sb toString       static String repeat ArraysFill String str  int n        String   strs   new String n       Arrays fill strs  str       return Arrays toString strs  replaceAll                        static String repeat Recursion String str  int n        if  n  lt   0          return         else         return str   repeat Recursion str  n - 1      static String repeat format1 String str  int n        return String format String format     ds   n        replace      str      static String repeat format2 String str  int n        return new String new char n   replace   0   str      static String repeat format3 String str  int n        return String format   0    n    d   0  replace  0   str      static String repeat join String str  int n        return String join     Collections nCopies n  str       static String repeat stream String str  int n        return Stream generate    - gt  str  limit n  collect Collectors joining        public static void main String   args        System out println repeat StringBuilderAppend  Mani   3        System out println repeat ArraysFill  Mani   3        System out println repeat Recursion  Mani   3        System out println repeat format1  Mani   3        System out println repeat format2  Mani   3        System out println repeat format3  Mani   3        System out println repeat join  Mani   3        System out println repeat stream  Mani   3

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based on fortran s answer  this is a recusive version that uses a StringBuilder   public static void repeat StringBuilder stringBuilder  String s  int times        if  times  gt  0            repeat stringBuilder append s   s  times - 1            public static String repeat String s  int times        StringBuilder stringBuilder   new StringBuilder s length     times       repeat stringBuilder  s  times       return stringBuilder toString

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If speed is your concern  then you should use as less memory copying as possible  Thus it is required to work with arrays of chars   public static String repeatString String what  int howmany        char   pattern   what toCharArray        char   res   new char howmany   pattern length       int length   pattern length      for  int i   0  i  lt  howmany  i            System arraycopy pattern  0  res  i   length  length       return new String res       To test speed  a similar optimal method using StirngBuilder is like this   public static String repeatStringSB String what  int howmany        StringBuilder out   new StringBuilder what length     howmany       for  int i   0  i  lt  howmany  i            out append what       return out toString        and the code to test it   public static void main String    args        String res      long time       for  int j   0  j  lt  1000  j              res   repeatString  123   100000           res   repeatStringSB  123   100000              time   System nanoTime        res   repeatString  123   1000000       time   System nanoTime   - time      System out println  elapsed repeatString      time        time   System nanoTime        res   repeatStringSB  123   1000000       time   System nanoTime   - time      System out println  elapsed repeatStringSB      time        And here the run results from my system   elapsed repeatString  6006571 elapsed repeatStringSB  9064937   Note that the test for loop is to kick in JIT and have optimal results

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public static String repeat String str  int times        int length   str length        int size   length   times      char   c   new char size       for  int i   0  i  lt  size  i              c i    str charAt i   length             return new String c

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I created a recursive method that do the same thing you want   feel free to use this     public String repeat String str  int count        return count  gt  0    repeat str  count -1    str          i have the same answer on Can I multiply strings in java to repeat sequences

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using only JRE classes  System arraycopy  and trying to minimize the number of temp objects you can write something like   public static String repeat String toRepeat  int times        if  toRepeat    null            toRepeat                  if  times  lt  0            times   0             final int length   toRepeat length        final int total   length   times      final char   src   toRepeat toCharArray        char   dst   new char total        for  int i   0  i  lt  total  i    length            System arraycopy src  0  dst  i  length              return String copyValueOf dst       EDIT  and without loops you can try with   public static String repeat2 String toRepeat  int times        if  toRepeat    null            toRepeat                  if  times  lt  0            times   0             String   copies   new String times       Arrays fill copies  toRepeat       return Arrays toString copies                 replace                         replace                         replaceAll                EDIT 2  using Collections is even shorter   public static String repeat3 String toRepeat  int times        return Collections nCopies times  toRepeat              toString               replace                      replace                      replaceAll                however I still like the first version

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Using recursion  you can do the following  using ternary operators  one line max    public static final String repeat String string  long number        return number    1   string    number   2    0   repeat string   string  number   2    string   repeat string   string   number - 1    2        I know  it s ugly and probably not efficient  but it s one line

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using Dollar is simple as typing    Test public void repeatString         String string    abc       assertThat   string  repeat 3  toString    is  abcabcabc         PS  repeat works also for array  List  Set  etc

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repeated   str   str   str    Sometimes simple is best   Everyone reading the code can see what s happening   And the compiler will do the fancy stuff with StringBuilder behind the scenes for you

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a straightforward one-line solution  requires Java nbsp 8  Collections nCopies  3   abc    stream   collect  Collectors joining

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Simple loop  public static String repeat String string  int times        StringBuilder out   new StringBuilder        while  times--  gt  0            out append string             return out toString

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Java 8 s String join provides a tidy way to do this in conjunction with Collections nCopies      say hello 100 times System out println String join     Collections nCopies 100   hello

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here is the latest Stringutils java StringUtils java      public static String repeat String str  int repeat           Performance tuned for 2 0  JDK1 4       if  str    null            return null            if  repeat  lt   0            return EMPTY            int inputLength   str length        if  repeat    1    inputLength    0            return str            if  inputLength    1  amp  amp  repeat  lt   PAD LIMIT            return repeat str charAt 0   repeat              int outputLength   inputLength   repeat      switch  inputLength            case 1               return repeat str charAt 0   repeat           case 2               char ch0   str charAt 0               char ch1   str charAt 1               char   output2   new char outputLength               for  int i   repeat   2 - 2  i  gt   0  i--  i--                    output2 i    ch0                  output2 i   1    ch1                            return new String output2           default               StringBuilder buf   new StringBuilder outputLength               for  int i   0  i  lt  repeat  i                      buf append str                             return buf toString                  it doesn t even need to be this big  can be made into this  and can be copied and pasted  into a utility class in your project       public static String repeat String str  int num        int len   num   str length        StringBuilder sb   new StringBuilder len       for  int i   0  i  lt  times  i              sb append str             return sb toString            So e5  I think the best way to do this would be to simply use the above mentioned code or any of the answers here  but commons lang is just too big if it s a small project

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Here s a way to do it using only standard String functions and no explicit loops      create a string made up of  n  copies of  s repeated   String format String format     ds   n        replace     s

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Despite your desire not to use loops  I think you should use a loop   String repeatString String s  int repetitions        if repetitions  lt  0  throw SomeException         else if s    null  return null       StringBuilder stringBuilder   new StringBuilder s length     repetitions        for int i   0  i  lt  repetitions  i            stringBuilder append s        return stringBuilder toString        Your reasons for not using a for loop are not good ones   In response to your criticisms    Whatever solution you use will almost certainly be longer than this   Using a pre-built function only tucks it under more covers  Someone reading your code will have to figure out what you re doing in that non-for-loop   Given that a for-loop is the idiomatic way to do this  it would be much easier to figure out if you did it with a for loop   Yes someone might add something clever  but by avoiding a for loop you are doing something clever   That s like shooting yourself in the foot intentionally to avoid shooting yourself in the foot by accident  Off-by-one errors are also mind-numbingly easy to catch with a single test   Given that you should be testing your code  an off-by-one error should be easy to fix and catch   And it s worth noting  the code above does not contain an off-by-one error   For loops are equally easy to get right  So don t reuse variables   That s not the for-loop s fault  Again  so does whatever solution you use   And as I noted before  a bug hunter will probably be expecting you to do this with a for loop  so they ll have an easier time finding it if you use a for loop

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for the sake of readability and portability   public String repeat String str  int count       if count  lt   0   return          return new String new char count   replace   0   str

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If you are using Java  lt   7  this is as  quot concise quot  as it gets     create a string made up of n copies of string s String format  quot  0 quot    n    quot d quot   0  replace  quot 0 quot   s    In Java 8 and above there is a more readable way     create a string made up of n copies of string s String join  quot  quot   Collections nCopies n  s     Finally  for Java 11 and above  there is a new repeat  int count  method specifically for this purpose link   quot abc quot  repeat 12    Alternatively  if your project uses java libraries there are more options  For Apache Commons  StringUtils repeat  quot abc quot   12    For Google Guava  Strings repeat  quot abc quot   12

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I wanted a function to create a comma-delimited list of question marks for JDBC purposes  and found this post  So  I decided to take two variants and see which one performed better  After 1 million iterations  the garden-variety StringBuilder took 2 seconds  fun1   and the cryptic supposedly more optimal version  fun2  took 30 seconds  What s the point of being cryptic again   private static String fun1 int size        StringBuilder sb   new StringBuilder size   2       for  int i   0  i  lt  size  i              sb append                  return sb substring 1      private static String fun2 int size        return new String new char size   replaceAll   0         substring 1

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If you are worried about performance  just use a StringBuilder inside the loop and do a  toString   on exit of the Loop  Heck  write your own Util Class and reuse it  5 Lines of code max

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Commons Lang StringUtils repeat    Usage   String str    abc   String repeated   StringUtils repeat str  3    repeated equals  abcabcabc

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Here is the shortest version  Java 1 5  required    repeated   new String new char n   replace   0   s     Where n is the number of times you want to repeat the string and s is the string to repeat   No imports or libraries needed

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This contains less characters than your question  public static String repeat String s  int n        if s    null            return null            final StringBuilder sb   new StringBuilder s length     n       for int i   0  i  lt  n  i              sb append s             return sb toString

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If you re like me and want to use Google Guava and not Apache Commons  You can use the repeat method in the Guava Strings class   Strings repeat  -   60

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Try this out       public static char   myABCs     a    b    c    public static int numInput  static Scanner in   new Scanner System in    public static void main String   args        System out print  Enter Number of Times to repeat          numInput   in nextInt        repeatArray numInput      public static int repeatArray int y        for  int a   0  a  lt  y  a              for  int b   0  b  lt  myABCs length  b                  System out print myABCs b                                      System out print                 return y

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OOP Solution  Nearly every answer proposes a static function as a solution  but thinking Object-Oriented  for reusability-purposes and clarity  I came up with a Solution via Delegation through the CharSequence-Interface  which also opens up usability on mutable CharSequence-Classes    The following Class can be used either with or without Separator-String CharSequence and each call to  toString    builds the final repeated String  The Input Separator are not only limited to String-Class  but can be every Class which implements CharSequence  e g  StringBuilder  StringBuffer  etc    Source-Code          Helper-Class for Repeating Strings and other CharSequence-Implementations     author Maciej Schuttkowski     public class RepeatingCharSequence implements CharSequence       final int count      CharSequence internalCharSeq           CharSequence separator                      CONSTRUCTOR - RepeatingCharSequence         param input CharSequence to repeat         param count Repeat-Count             public RepeatingCharSequence CharSequence input  int count            if count  lt  0              throw new IllegalArgumentException  Can not repeat String     input     less than 0 times  count   count           if count  gt  0              internalCharSeq   input          this count   count                       CONSTRUCTOR - Strings RepeatingCharSequence         param input CharSequence to repeat         param count Repeat-Count         param separator Separator-Sequence to use             public RepeatingCharSequence CharSequence input  int count  CharSequence separator            this input  count           this separator   separator              Override     public CharSequence subSequence int start  int end            checkBounds start           checkBounds end           int subLen   end - start          if  subLen  lt  0                throw new IndexOutOfBoundsException  Illegal subSequence-Length    subLen                     return  start    0  amp  amp  end    length      this                       toString   substring start  subLen              Override     public int length               We return the total length of our CharSequences with the separator 1 time less than amount of repeats          return count  lt  1   0                      internalCharSeq length   count     separator length    count-1                Override     public char charAt int index            final int internalIndex   internalIndex index             Delegate to Separator-CharSequence or Input-CharSequence depending on internal index          if internalIndex  gt  internalCharSeq length  -1                return separator charAt internalIndex-internalCharSeq length                       return internalCharSeq charAt internalIndex              Override     public String toString             return count  lt  1                        new StringBuilder this  toString               private void checkBounds int index            if index  lt  0    index  gt   length                throw new IndexOutOfBoundsException  Index out of Bounds    index             private int internalIndex int index               We need to add 1 Separator-Length to total length before dividing             as we subtracted one Separator-Length in  length            return index     length   separator length    count             Usage-Example   public static void main String   args          String input    12345         StringBuffer input   new StringBuffer  12345        StringBuilder input   new StringBuilder  123          String separator     lt   gt        StringBuilder separator   new StringBuilder   lt        append   gt         int repeatCount   2       CharSequence repSeq   new RepeatingCharSequence input  repeatCount  separator       String repStr   repSeq toString         System out println  Repeat   repeatCount   tSeparator   separator   tInput   input   tLength   input length         System out println  CharSeq  tLength   repSeq length     tVal   repSeq       System out println  String   tLength   repStr length     tVal   repStr          Here comes the Magic with a StringBuilder as Input  as you can append to the String-Builder       and at the same Time your Repeating-Sequence s toString  -Method returns the updated String        input append  ff        System out println repSeq         Same can be done with the Separator      separator append        append   gt         System out println repSeq       Example-Output   Repeat 2    Separator  lt      Input 123   Length 3 CharSeq     Length 8    Val 123 lt  123 String      Length 8    Val 123 lt  123 123ff lt  123ff 123ff lt      gt 123ff

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So you want to avoid loops   Here you have it   public static String repeat String s  int times        if  times  lt   0  return         else return s   repeat s  times-1        of course I know this is ugly and inefficient  but it doesn t have loops  -p   You want it simpler and prettier  use jython   s   3   Edit  let s optimize it a little bit  -D  public static String repeat String s  int times       if  times  lt   0  return        else if  times   2    0  return repeat s s  times 2      else return s   repeat s s  times 2       Edit2  I ve done a quick and dirty benchmark for the 4 main alternatives  but I don t have time to run it several times to get the means and plot the times for several inputs    So here s the code if anybody wants to try it   public class Repeat       public static void main String   args             int n   Integer parseInt args 0            String s   args 1           int l   s length            long start  end           start   System currentTimeMillis            for  int i   0  i  lt  n  i                  if repeatLog2 s i  length    i l  throw new RuntimeException                      end   System currentTimeMillis            System out println  RecLog2Concat       end-start     ms             start   System currentTimeMillis            for  int i   0  i  lt  n  i                  if repeatR s i  length    i l  throw new RuntimeException                                     end   System currentTimeMillis            System out println  RecLinConcat       end-start     ms             start   System currentTimeMillis            for  int i   0  i  lt  n  i                  if repeatIc s i  length    i l  throw new RuntimeException                      end   System currentTimeMillis            System out println  IterConcat       end-start     ms             start   System currentTimeMillis            for  int i   0  i  lt  n  i                  if repeatSb s i  length    i l  throw new RuntimeException                      end   System currentTimeMillis            System out println  IterStrB       end-start     ms               public static String repeatLog2 String s  int times            if  times  lt   0                return                       else if  times   2    0                return repeatLog2 s s  times 2                     else              return s   repeatLog2 s s  times 2                        public static String repeatR String s  int times            if  times  lt   0                return                       else               return s   repeatR s  times-1                        public static String repeatIc String s  int times            String tmp               for  int i   0  i  lt  times  i                  tmp    s                    return tmp             public static String repeatSb String s  int n            final StringBuilder sb   new StringBuilder            for int i   0  i  lt  n  i                  sb append s                     return sb toString              It takes 2 arguments  the first is the number of iterations  each function run with repeat times arg from 1  n  and the second is the string to repeat   So far  a quick inspection of the times running with different inputs leaves the ranking something like this  better to worse     Iterative StringBuilder append  1x   Recursive concatenation log2 invocations   3x   Recursive concatenation linear invocations   30x   Iterative concatenation linear   45x     I wouldn t ever guessed that the recursive function was faster than the for loop  -o  Have fun ctional xD

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If you only know the length of the output string  and it may be not divisible by the length of the input string   then use this method   static String repeat String s  int length        return s length    gt   length   s substring 0  length    repeat s   s  length       Usage demo   for  int i   0  i  lt  50  i        System out println repeat          i      Don t use with empty s and length   0  since it s impossible to get the desired result in this case

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public static String rep int a String k                       if a lt  0                  return                else              a--                 return k rep a k              You can use this recursive method for you desired goal

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With java-8  you can also use Stream generate   import static java util stream Collectors joining      String repeated   Stream generate    - gt   abc   limit 3  collect joining        abcabcabc    and you can wrap it in a simple utility method if needed   public static String repeat String str  int times       return Stream generate    - gt  str  limit times  collect joining

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Not the shortest  but  i think  the fastest way is to use the StringBuilder             Repeat a String as many times you need             param i - Number of Repeating the String        param s - The String wich you want repeated        return The string n - times          public static String repeate int i  String s        StringBuilder sb   new StringBuilder        for  int j   0  j  lt  i  j          sb append s       return sb toString

[java] Simple way to repeat a string

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