The simplest way to comma-delimit a list

Question

What is the clearest way to comma-delimit a list in Java   I know several ways of doing it  but I m wondering what the best way is  where  best  means clearest and or shortest  not the most efficient   I have a list and I want to loop over it  printing each value   I want to print a comma between each item  but not after the last one  nor before the first one    List -- gt  Item     Item     List -- gt    Item       Item   Sample solution 1   boolean isFirst   true  for  Item i   list      if  isFirst        System out print i             no comma     isFirst   false      else       System out print      i        comma         Sample solution 2 - create a sublist   if  list size   gt 0      System out print list get 0         no comma   List theRest   list subList 1  list size       for  Item i   theRest        System out print      i        comma         Sample solution 3     Iterator lt Item gt  i   list iterator      if  i hasNext          System out print i next         while  i hasNext          System out print      i next           These treat the first item specially  one could instead treat the last one specially   Incidentally  here is how List toString is implemented  it s inherited from AbstractCollection   in Java 1 6   public String toString         Iterator lt E gt  i   iterator        if    i hasNext            return            StringBuilder sb   new StringBuilder        sb append           for                E e   i next            sb append e    this     this Collection     e           if    i hasNext                return sb append      toString            sb append                  It exits the loop early to avoid the comma after the last item   BTW  this is the first time I recall seeing   this Collection    here s code to provoke it   List l   new LinkedList    l add l   System out println l     I welcome any solution  even if they use unexpected libraries  regexp    and also solutions in languages other than Java  e g  I think Python Ruby have an intersperse function - how is that implemented     Clarification  by libraries  I mean the standard Java libraries   For other libraries  I consider them with other languages  and interested to know how they re implemented   EDIT toolkit mentioned a similar question  Last iteration of enhanced for loop in java  And another  Does the last element in a loop deserve a separate treatment

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If you can use Groovy  which runs on the JVM    def list     a    b    c    d   println list join

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public static String join  List lt String gt  list  String separator      String listToString          if  list    null    list isEmpty         return listToString         for  String element   list       listToString    element   separator         listToString   listToString substring 0  separator length        return listToString

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I like this solution   String delim     -    string        for  String item   myCollection      string    delim   item   string   string substring delim length       I assume it can make use of StringBuilder too

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In my opinion  this is the simplest to read and understand   StringBuilder sb   new StringBuilder    for String string   strings        sb append string  append         sb setLength sb length   - 1   String result   sb toString

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StringBuffer sb   new StringBuffer     for  int i   0  i  lt  myList size    i           if  i  gt  0                 sb append                   sb append myList get i

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I usually do this    StringBuffer sb   new StringBuffer    Iterator it   myList iterator    if  it hasNext      sb append it next   toString       while  it hasNext      sb append      append it next   toString         Though I think I ll to a this check from now on as per the Java implementation

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I somewhat like this approach  which I found on a blog some time ago  Unfortunately I don t remember the blog s name URL   You can create a utility helper class that looks like this   private class Delimiter       private final String delimiter      private boolean first   true       public Delimiter String delimiter                this delimiter   delimiter              Override     public String toString                 if  first                first   false              return                        return delimiter            Using the helper class is simple as this   StringBuilder sb   new StringBuilder    Delimiter delimiter   new Delimiter         for  String item   list        sb append delimiter       sb append item

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I didn t compile it    but should work  or be close to working    public static  lt T gt  String toString final List lt T gt  list                                     final String delim        final StringBuilder builder       builder   new StringBuilder         for final T item   list                builder append item           builder append delim                 kill the last delim     builder setLength builder length   - delim length          return  builder toString

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String delimiter        StringBuilder sb   new StringBuilder    for  Item i   list        sb append delimiter  append i     sb toString   replaceFirst delimiter

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StringBuffer result   new StringBuffer    for Iterator it list iterator  it hasNext          if  result length   gt 0      result append          result append it next         Update  As Dave Webb mentioned in the comments this may not produce correct results if the first items in the list are empty strings

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I usually use something similar to version 3  It works well c c   bash      P

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If you use the Spring Framework you can do it with StringUtils   public static String arrayToDelimitedString Object   arr  public static String arrayToDelimitedString Object   arr  String delim  public static String collectionToCommaDelimitedString Collection coll  public static String collectionToCommaDelimitedString Collection coll  String delim

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Method  String join List lt Object gt  collection  String delimiter       StringBuilder stringBuilder   new StringBuilder        int size   collection size        for  Object value   collection            size --          if size  gt  0               stringBuilder append value  append delimiter                        return stringBuilder toString        Usage  Given the array of  1 2 3   join myArray          1 2 3

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Java 8 provides several new ways to do this    The join method on String  A joining collector for streams from the Collectors class  The StringJoiner class    Example      Util method for strings and other char sequences List lt String gt  strs   Arrays asList  1    2    3    String listStr1   String join      strs       For any type using streams and collectors List lt Object gt  objs   Arrays asList 1  2  3   String listStr2   objs stream        map Object  toString       collect joining                      Using the new StringJoiner class StringJoiner joiner   new StringJoiner                  joiner setEmptyValue   lt empty gt     for  Integer i   objs      joiner add i toString       String listStr3   joiner toString      The approach using streams assumes import static java util stream Collectors joining

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One option for the foreach loop is   StringBuilder sb   new StringBuilder    for String s list     if  sb length   gt 0  sb append         sb append s

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Java 8 and later  Using StringJoiner class    StringJoiner joiner   new StringJoiner       for  Item item   list        joiner add item toString       return joiner toString      Using Stream  and Collectors   return list stream           map Object  toString          collect Collectors joining       toString      Java 7 and earlier  See also  285523  String delim       for  Item i   list        sb append delim  append i       delim

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public String toString List lt Item gt  items        StringBuilder sb   new StringBuilder            for  Item item   items                sb append item  append                   if  sb length    gt   2                  looks cleaner in C  sb Length -  2          sb setLength sb length   - 2              sb append            return sb toString

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None of the answers uses recursion so far     public class Main        public static String toString List lt String gt  list  char d            int n   list size            if n  0  return             return n  gt  1   Main toString list subList 0  n - 1   d    d                     list get n - 1    list get 0              public static void main String   args            List lt String gt  list   Arrays asList new String    1   2   3             System out println Main toString list

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if  array length gt 0              edited in response Joachim s comment   sb append array i    for  int i 1  i lt array length  i      sb append      append array i      Based on Clearest way to comma-delimit a list  Java    Using this idea  Does the last element in a loop deserve a separate treatment

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Joiner on      join myList    Joiner

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org apache commons lang3 StringUtils join list

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In Python its easy      join  yourlist    In C  there is a static method on the String class   String Join      yourlistofstrings   Sorry  not sure about Java but thought I d pipe up as you asked about other languages  I m sure there would be something similar in Java

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private String betweenComma ArrayList lt String gt  strings        String united           for  String string   strings            united   united         string            return united replaceFirst

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Based on Java s List toString implementation   Iterator i   list iterator    for          sb append i next       if    i hasNext    break    ab append            It uses a grammar like this   List -- gt   Item      Item   By being last-based instead of first-based  it can check for skip-comma with the same test to check for end-of-list    I think this one is very elegant  but I m not sure about clarity

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Because your delimiter is      you could use any of the following   public class StringDelim        public static void removeBrackets String string            System out println string substring 1  string length   - 1               public static void main String    args               Array toString   example         String    arr     Hi     My    name    is    br3nt            String string   Arrays toString arr           removeBrackets string               List toString   example         List lt String gt  list   new ArrayList lt String gt             list add  Hi            list add  My            list add  name            list add  is            list add  br3nt            string   list toString            removeBrackets string               Map values   toString   example         Map lt String  String gt  map   new LinkedHashMap lt String  String gt             map put  1    Hi            map put  2    My            map put  3    name            map put  4    is            map put  5    br3nt            System out println map values   toString             removeBrackets string               Enum toString   example         EnumSet lt Days gt  set   EnumSet allOf Days class           string   set toString            removeBrackets string              public enum Days           MON  Monday            TUE  Tuesday            WED  Wednesday            THU  Thursday            FRI  Friday            SAT  Saturday            SUN  Sunday             private final String day           Days String day   this day   day           public String toString    return this day             If your delimiter is ANYTHING else then this isn t going to work for you

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You can also unconditionally add the delimiter string  and after the loop remove the extra delimiter at the end   Then an  if list is empty then return this string  at the beginning will allow you to avoid the check at the end  as you cannot remove characters from an empty list   So the question really is    Given a loop and an if  what do you think is the clearest way to have these together

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There is a pretty way to achieve this using Java 8   List lt String gt  list   Arrays asList array   String joinedString   String join      list

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Copy paste of my own answer from here   Many of the solutions described here are a bit over the top  IMHO  especially those that rely on external libraries  There is a nice clean  clear idiom for achieving a comma separated list that I have always used  It relies on the conditional     operator   Edit  Original solution correct  but non-optimal according to comments  Trying a second time   int   array    1  2  3   StringBuilder builder   new StringBuilder    for  int i   0    i  lt  array length  i           builder append i    0             append array i      There you go  in 4 lines of code including the declaration of the array and the StringBuilder   2nd Edit  If you are dealing with an Iterator       List lt Integer gt  list   Arrays asList 1  2  3       StringBuilder builder   new StringBuilder        for  Iterator it   list iterator    it hasNext             builder append it next    append it hasNext

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for int i 0  length list size    i lt length  i      result   i  0            list get i

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This is very short  very clear  but gives my sensibilities the creeping horrors   It s also a bit awkward to adapt to different delimiters  especially if a String  not char    for  Item i   list    sb append      append i   if  sb charAt 0        sb deleteCharAt 0     Inspired by  Last iteration of enhanced for loop in java

[java] The simplest way to comma-delimit a list?

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