What s the best way to build a string of delimited items in Java

Question

While working in a Java app  I recently needed to assemble a comma-delimited list of values to pass to another web service without knowing how many elements there would be in advance  The best I could come up with off the top of my head was something like this   public String appendWithDelimiter  String original  String addition  String delimiter         if   original equals                   return addition        else           return original   delimiter   addition           String parameterString       if   condition   parameterString   appendWithDelimiter  parameterString   elementName          if   anotherCondition   parameterString   appendWithDelimiter  parameterString   anotherElementName            I realize this isn t particularly efficient  since there are strings being created all over the place  but I was going for clarity more than optimization   In Ruby  I can do something like this instead  which feels much more elegant   parameterArray       parameterArray  lt  lt   elementName  if condition  parameterArray  lt  lt   anotherElementName  if anotherCondition  parameterString   parameterArray join         But since Java lacks a join command  I couldn t figure out anything equivalent   So  what s the best way to do this in Java

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Why not write your own join   method   It would take as parameters collection of Strings and a delimiter String   Within the method iterate over the collection and build up your result in a StringBuffer

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Java 8  stringCollection stream   collect Collectors joining

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You can try something like this   StringBuilder sb   new StringBuilder    if  condition    sb append  elementName   append         if  anotherCondition    sb append  anotherElementName   append         String parameterString   sb toString

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Why don t you do in Java the same thing you are doing in ruby  that is creating the delimiter separated string only after you ve added all the pieces to the array   ArrayList lt String gt  parms   new ArrayList lt String gt     if  someCondition  parms add  someString    if  anotherCondition  parms add  someOtherString           String sep       StringBuffer b   new StringBuffer    for  String p  parms        b append sep       b append p       sep    yourDelimiter       You may want to move that for loop in a separate helper method  and also use StringBuilder instead of StringBuffer     Edit  fixed the order of appends

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Apache commons StringUtils class has a join method

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You can generalize it  but there s no join in Java  as you well say   This might work better    public static String join Iterable lt   extends CharSequence gt  s  String delimiter        Iterator lt   extends CharSequence gt  iter   s iterator        if   iter hasNext    return         StringBuilder buffer   new StringBuilder iter next         while  iter hasNext    buffer append delimiter  append iter next         return buffer toString

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Your approach is not too bad  but you should use a StringBuffer instead of using the   sign  The   has the big disadvantage that a new String instance is being created for each single operation  The longer your string gets  the bigger the overhead  So using a StringBuffer should be the fastest way   public StringBuffer appendWithDelimiter  StringBuffer original  String addition  String delimiter             if   original    null                     StringBuffer buffer   new StringBuffer                    buffer append addition                   return buffer            else                   buffer append delimiter                   buffer append addition                   return original                After you have finished creating your string simply call toString   on the returned StringBuffer

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I would use Google Collections   There is a nice Join facility  http   google-collections googlecode com svn trunk javadoc index html com google common base Join html  But if I wanted to write it on my own   package util   import java util ArrayList  import java util Iterable  import java util Collections  import java util Iterator   public class Utils          accept a collection of objects  since all objects have toString       public static String join String delimiter  Iterable lt   extends Object gt  objs            if  objs isEmpty                  return                       Iterator lt   extends Object gt  iter   objs iterator            StringBuilder buffer   new StringBuilder            buffer append iter next             while  iter hasNext                  buffer append delimiter  append iter next                       return buffer toString                  for convenience     public static String join String delimiter  Object    objs            ArrayList lt Object gt  list   new ArrayList lt Object gt             Collections addAll list  objs           return join delimiter  list             I think it works better with an object collection  since now you don t have to convert your objects to strings before you join them

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So a couple of things you might do to get the feel that it seems like you re looking for   1  Extend List class - and add the join method to it   The join method would simply do the work of concatenating and adding the delimiter  which could be a param to the join method   2  It looks like Java 7 is going to be adding extension methods to java - which allows you just to attach a specific method on to a class   so you could write that join method and add it as an extension method to List or even to Collection   Solution 1 is probably the only realistic one  now  though since Java 7 isn t out yet    But it should work just fine   To use both of these  you d just add all your items to the List or Collection as usual  and then call the new custom method to  join  them

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Your approach is not too bad  but you should use a StringBuffer instead of using the   sign  The   has the big disadvantage that a new String instance is being created for each single operation  The longer your string gets  the bigger the overhead  So using a StringBuffer should be the fastest way   public StringBuffer appendWithDelimiter  StringBuffer original  String addition  String delimiter             if   original    null                     StringBuffer buffer   new StringBuffer                    buffer append addition                   return buffer            else                   buffer append delimiter                   buffer append addition                   return original                After you have finished creating your string simply call toString   on the returned StringBuffer

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For those who are in a Spring context their StringUtils class is useful as well   There are many useful shortcuts like    collectionToCommaDelimitedString Collection coll  collectionToDelimitedString Collection coll  String delim  arrayToDelimitedString Object   arr  String delim    and many others    This can be helpful if you are not already using Java 8 and you are already in a Spring context   I prefer it against the Apache Commons  although very good as well  for the Collection support which is easier like this      Encoding Set lt String gt  to String delimited  String asString   org springframework util StringUtils collectionToDelimitedString codes            Decoding String delimited to Set Set lt String gt  collection   org springframework util StringUtils commaDelimitedListToSet asString

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You could write a little join-style utility method that works on java util Lists  public static String join List lt String gt  list  String delim         StringBuilder sb   new StringBuilder         String loopDelim            for String s   list             sb append loopDelim           sb append s                        loopDelim   delim             return sb toString        Then use it like so       List lt String gt  list   new ArrayList lt String gt          if  condition          list add  elementName        if  anotherCondition   list add  anotherElementName         join list

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You can generalize it  but there s no join in Java  as you well say   This might work better    public static String join Iterable lt   extends CharSequence gt  s  String delimiter        Iterator lt   extends CharSequence gt  iter   s iterator        if   iter hasNext    return         StringBuilder buffer   new StringBuilder iter next         while  iter hasNext    buffer append delimiter  append iter next         return buffer toString

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Instead of using string concatenation  you should use StringBuilder if your code is not threaded  and StringBuffer if it is

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I personally quite often use  the following simple solution for logging purposes   List lst   Arrays asList  ab    bc    cd    String str   lst toString   replaceAll

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Java 8 Native Type  List lt Integer gt  example  example add 1   example add 2   example add 3       example stream   collect Collectors joining          Java 8 Custom Object   List lt Person gt  person      person stream   map Person  getAge  collect Collectors joining

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public static String join String   strings  char del        StringBuffer sb   new StringBuffer        int len   strings length      boolean appended   false      for  int i   0  i  lt  len  i                  if  appended                        sb append del                     sb append    strings i            appended   true            return sb toString

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Don t know if this really is any better  but at least it s using StringBuilder  which may be slightly more efficient   Down below is a more generic approach if you can build up the list of parameters BEFORE doing any parameter delimiting      Answers real question public String appendWithDelimiters String delimiter  String original  String addition        StringBuilder sb   new StringBuilder original       if sb length    0            sb append delimiter  append addition         else           sb append addition             return sb toString           A more generic case         means a list of indeterminate length of Strings  public String appendWithDelimitersGeneric String delimiter  String    strings        StringBuilder sb   new StringBuilder        for  String string   strings            if sb length    0                sb append delimiter  append string             else               sb append string                        return sb toString       public void testAppendWithDelimiters         String string   appendWithDelimitersGeneric       string1    string2    string3

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And a minimal one  if you don t want to include Apache Commons or Gauva into project dependencies just for the sake of joining strings              param delim   String that should be kept in between the parts     param parts   parts that needs to be joined     return  a String that s formed by joining the parts     private static final String join String delim  String    parts        StringBuilder builder   new StringBuilder        for  int i   0  i  lt  parts length - 1  i              builder append parts i   append delim             if parts length  gt  0           builder append parts parts length - 1              return builder toString

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If you re using Eclipse Collections  you can use makeString   or appendString     makeString   returns a String representation  similar to toString     It has three forms   makeString start  separator  end  makeString separator  defaults start and end to empty strings makeString   defaults the separator to       comma and space    Code example   MutableList lt Integer gt  list   FastList newListWith 1  2  3   assertEquals   1 2 3    list makeString                  assertEquals  1 2 3   list makeString        assertEquals  1  2  3   list makeString     assertEquals list toString    list makeString                     appendString   is similar to makeString    but it appends to an Appendable  like StringBuilder  and is void  It has the same three forms  with an additional first argument  the Appendable   MutableList lt Integer gt  list   FastList newListWith 1  2  3   Appendable appendable   new StringBuilder    list appendString appendable                  assertEquals   1 2 3    appendable toString       If you can t convert your collection to an Eclipse Collections type  just adapt it with the relevant adapter   List lt Object gt  list        ListAdapter adapt list  makeString         Note  I am a committer for Eclipse collections

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I would use Google Collections   There is a nice Join facility  http   google-collections googlecode com svn trunk javadoc index html com google common base Join html  But if I wanted to write it on my own   package util   import java util ArrayList  import java util Iterable  import java util Collections  import java util Iterator   public class Utils          accept a collection of objects  since all objects have toString       public static String join String delimiter  Iterable lt   extends Object gt  objs            if  objs isEmpty                  return                       Iterator lt   extends Object gt  iter   objs iterator            StringBuilder buffer   new StringBuilder            buffer append iter next             while  iter hasNext                  buffer append delimiter  append iter next                       return buffer toString                  for convenience     public static String join String delimiter  Object    objs            ArrayList lt Object gt  list   new ArrayList lt Object gt             Collections addAll list  objs           return join delimiter  list             I think it works better with an object collection  since now you don t have to convert your objects to strings before you join them

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using Dollar is simple as typing   String joined     aCollection  join         NB  it works also for Array and other data types  Implementation  Internally it uses a very neat trick    Override public String join String separator        Separator sep   new Separator separator       StringBuilder sb   new StringBuilder         for  T item   iterable            sb append sep  append item              return sb toString        the class Separator return the empty String only the first time that it is invoked  then it returns the separator   class Separator        private final String separator      private boolean wasCalled       public Separator String separator            this separator   separator          this wasCalled   false              Override     public String toString             if   wasCalled                wasCalled   true              return               else               return separator

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Fix answer Rob Dickerson   It s easier to use   public static String join String delimiter  String    values        StringBuilder stringBuilder   new StringBuilder         for  String value   values                stringBuilder append value           stringBuilder append delimiter              String result   stringBuilder toString         return result isEmpty     result   result substring 0  result length   - 1

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The Google s Guava library has com google common base Joiner class which helps to solve such tasks   Samples    My pets are      Joiner on       join Arrays asList  rabbit    parrot    dog         returns  My pets are  rabbit  parrot  dog   Joiner on   AND    join Arrays asList  field1 1     field2 2    field3 3        returns  field1 1 AND field2 2 AND field3 3   Joiner on      skipNulls   join Arrays asList  London    Moscow   null   New York   null   Paris        returns  London Moscow New York Paris   Joiner on       useForNull  Team held a draw   join Arrays asList  FC Barcelona    FC Bayern   null  null   Chelsea FC    AC Milan        returns  FC Barcelona  FC Bayern  Team held a draw  Team held a draw  Chelsea FC  AC Milan      Here is an article about Guava s string utilities

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I would use Google Collections   There is a nice Join facility  http   google-collections googlecode com svn trunk javadoc index html com google common base Join html  But if I wanted to write it on my own   package util   import java util ArrayList  import java util Iterable  import java util Collections  import java util Iterator   public class Utils          accept a collection of objects  since all objects have toString       public static String join String delimiter  Iterable lt   extends Object gt  objs            if  objs isEmpty                  return                       Iterator lt   extends Object gt  iter   objs iterator            StringBuilder buffer   new StringBuilder            buffer append iter next             while  iter hasNext                  buffer append delimiter  append iter next                       return buffer toString                  for convenience     public static String join String delimiter  Object    objs            ArrayList lt Object gt  list   new ArrayList lt Object gt             Collections addAll list  objs           return join delimiter  list             I think it works better with an object collection  since now you don t have to convert your objects to strings before you join them

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You can try something like this   StringBuilder sb   new StringBuilder    if  condition    sb append  elementName   append         if  anotherCondition    sb append  anotherElementName   append         String parameterString   sb toString

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And a minimal one  if you don t want to include Apache Commons or Gauva into project dependencies just for the sake of joining strings              param delim   String that should be kept in between the parts     param parts   parts that needs to be joined     return  a String that s formed by joining the parts     private static final String join String delim  String    parts        StringBuilder builder   new StringBuilder        for  int i   0  i  lt  parts length - 1  i              builder append parts i   append delim             if parts length  gt  0           builder append parts parts length - 1              return builder toString

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Slight improvement  speed  of version from izb   public static String join String   strings  char del        StringBuilder sb   new StringBuilder        int len   strings length       if len  gt  1                len -  1       else              return strings 0              for  int i   0  i  lt  len  i                 sb append strings i   append del              sb append strings i         return sb toString

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You could write a little join-style utility method that works on java util Lists  public static String join List lt String gt  list  String delim         StringBuilder sb   new StringBuilder         String loopDelim            for String s   list             sb append loopDelim           sb append s                        loopDelim   delim             return sb toString        Then use it like so       List lt String gt  list   new ArrayList lt String gt          if  condition          list add  elementName        if  anotherCondition   list add  anotherElementName         join list

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using Dollar is simple as typing   String joined     aCollection  join         NB  it works also for Array and other data types  Implementation  Internally it uses a very neat trick    Override public String join String separator        Separator sep   new Separator separator       StringBuilder sb   new StringBuilder         for  T item   iterable            sb append sep  append item              return sb toString        the class Separator return the empty String only the first time that it is invoked  then it returns the separator   class Separator        private final String separator      private boolean wasCalled       public Separator String separator            this separator   separator          this wasCalled   false              Override     public String toString             if   wasCalled                wasCalled   true              return               else               return separator

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The Google s Guava library has com google common base Joiner class which helps to solve such tasks   Samples    My pets are      Joiner on       join Arrays asList  rabbit    parrot    dog         returns  My pets are  rabbit  parrot  dog   Joiner on   AND    join Arrays asList  field1 1     field2 2    field3 3        returns  field1 1 AND field2 2 AND field3 3   Joiner on      skipNulls   join Arrays asList  London    Moscow   null   New York   null   Paris        returns  London Moscow New York Paris   Joiner on       useForNull  Team held a draw   join Arrays asList  FC Barcelona    FC Bayern   null  null   Chelsea FC    AC Milan        returns  FC Barcelona  FC Bayern  Team held a draw  Team held a draw  Chelsea FC  AC Milan      Here is an article about Guava s string utilities

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Don t know if this really is any better  but at least it s using StringBuilder  which may be slightly more efficient   Down below is a more generic approach if you can build up the list of parameters BEFORE doing any parameter delimiting      Answers real question public String appendWithDelimiters String delimiter  String original  String addition        StringBuilder sb   new StringBuilder original       if sb length    0            sb append delimiter  append addition         else           sb append addition             return sb toString           A more generic case         means a list of indeterminate length of Strings  public String appendWithDelimitersGeneric String delimiter  String    strings        StringBuilder sb   new StringBuilder        for  String string   strings            if sb length    0                sb append delimiter  append string             else               sb append string                        return sb toString       public void testAppendWithDelimiters         String string   appendWithDelimitersGeneric       string1    string2    string3

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Slight improvement  speed  of version from izb   public static String join String   strings  char del        StringBuilder sb   new StringBuilder        int len   strings length       if len  gt  1                len -  1       else              return strings 0              for  int i   0  i  lt  len  i                 sb append strings i   append del              sb append strings i         return sb toString

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Why don t you do in Java the same thing you are doing in ruby  that is creating the delimiter separated string only after you ve added all the pieces to the array   ArrayList lt String gt  parms   new ArrayList lt String gt     if  someCondition  parms add  someString    if  anotherCondition  parms add  someOtherString           String sep       StringBuffer b   new StringBuffer    for  String p  parms        b append sep       b append p       sep    yourDelimiter       You may want to move that for loop in a separate helper method  and also use StringBuilder instead of StringBuffer     Edit  fixed the order of appends

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I personally quite often use  the following simple solution for logging purposes   List lst   Arrays asList  ab    bc    cd    String str   lst toString   replaceAll

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Instead of using string concatenation  you should use StringBuilder if your code is not threaded  and StringBuffer if it is

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In Java 8 you can use String join     List lt String gt  list   Arrays asList  foo    bar    baz    String joined   String join   and    list       foo and bar and baz    Also have a look at this answer for a Stream API example

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Instead of using string concatenation  you should use StringBuilder if your code is not threaded  and StringBuffer if it is

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public static String join String   strings  char del        StringBuffer sb   new StringBuffer        int len   strings length      boolean appended   false      for  int i   0  i  lt  len  i                  if  appended                        sb append del                     sb append    strings i            appended   true            return sb toString

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Pre Java 8   Apache s commons lang is your friend here - it provides a join method very similar to the one you refer to in Ruby    StringUtils join java lang Iterable char     Java 8   Java 8 provides joining out of the box via StringJoiner and String join    The snippets below show how you can use them   StringJoiner  StringJoiner joiner   new StringJoiner       joiner add  01   add  02   add  03    String joinedString   joiner toString        01 02 03      String join CharSequence delimiter  CharSequence    elements    String joinedString   String join   -     04    05    06        04 - 05 - 06      String join CharSequence delimiter  Iterable lt   extends CharSequence gt  elements   List lt String gt  strings   new LinkedList lt  gt     strings add  Java   strings add  is    strings add  cool    String message   String join      strings     message returned is   Java is cool

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If you are using Spring MVC then you can try following steps   import org springframework util StringUtils   List lt String gt  groupIds   new List lt String gt      groupIds add  a        groupIds add  b        groupIds add  c     String csv   StringUtils arrayToCommaDelimitedString groupIds toArray       It will result to a b c

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With Java 5 variable args  so you don t have to stuff all your strings into a collection or array explicitly   import junit framework Assert  import org junit Test   public class StringUtil       public static String join String delim  String    strings                StringBuilder builder   new StringBuilder             if  strings    null                        for  String str   strings                                if  builder length    gt  0                                        builder append delim  append                                         builder append str                                              return builder toString               Test     public void joinTest                 Assert assertEquals     StringUtil join      null            Assert assertEquals     StringUtil join                    Assert assertEquals     StringUtil join      new String 0             Assert assertEquals  test   StringUtil join       test             Assert assertEquals  foo  bar   StringUtil join       foo    bar             Assert assertEquals  foo  bar  x   StringUtil join       foo    bar    x

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public static String join String   strings  char del        StringBuffer sb   new StringBuffer        int len   strings length      boolean appended   false      for  int i   0  i  lt  len  i                  if  appended                        sb append del                     sb append    strings i            appended   true            return sb toString

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You can generalize it  but there s no join in Java  as you well say   This might work better    public static String join Iterable lt   extends CharSequence gt  s  String delimiter        Iterator lt   extends CharSequence gt  iter   s iterator        if   iter hasNext    return         StringBuilder buffer   new StringBuilder iter next         while  iter hasNext    buffer append delimiter  append iter next         return buffer toString

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So basically something like this   public static String appendWithDelimiter String original  String addition  String delimiter     if  original equals            return addition    else       StringBuilder sb   new StringBuilder original length     addition length     delimiter length             sb append original           sb append delimiter           sb append addition           return sb toString

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You can use Java s StringBuilder type for this  There s also StringBuffer  but it contains extra thread safety logic that is often unnecessary

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Pre Java 8   Apache s commons lang is your friend here - it provides a join method very similar to the one you refer to in Ruby    StringUtils join java lang Iterable char     Java 8   Java 8 provides joining out of the box via StringJoiner and String join    The snippets below show how you can use them   StringJoiner  StringJoiner joiner   new StringJoiner       joiner add  01   add  02   add  03    String joinedString   joiner toString        01 02 03      String join CharSequence delimiter  CharSequence    elements    String joinedString   String join   -     04    05    06        04 - 05 - 06      String join CharSequence delimiter  Iterable lt   extends CharSequence gt  elements   List lt String gt  strings   new LinkedList lt  gt     strings add  Java   strings add  is    strings add  cool    String message   String join      strings     message returned is   Java is cool

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You should probably use a StringBuilder with the append method to construct your result  but otherwise this is as good of a solution as Java has to offer

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If you are using Spring MVC then you can try following steps   import org springframework util StringUtils   List lt String gt  groupIds   new List lt String gt      groupIds add  a        groupIds add  b        groupIds add  c     String csv   StringUtils arrayToCommaDelimitedString groupIds toArray       It will result to a b c

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Note  if you have access to Java5      use StringBuilder in preference to StringBuffer      All that has to be replaced is the class name      StringBuffer will work in Java 1 4  though   appendWithDelimiter  StringBuffer buffer  String addition       String delimiter         if   buffer length      0            buffer append addition         else           buffer append delimiter           buffer append addition             StringBuffer parameterBuffer   new StringBuffer    if   condition          appendWithDelimiter parameterBuffer   elementName            if   anotherCondition         appendWithDelimiter parameterBuffer   anotherElementName               Finally  to return a string representation  call toString   when returning  return parameterBuffer toString

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Note  if you have access to Java5      use StringBuilder in preference to StringBuffer      All that has to be replaced is the class name      StringBuffer will work in Java 1 4  though   appendWithDelimiter  StringBuffer buffer  String addition       String delimiter         if   buffer length      0            buffer append addition         else           buffer append delimiter           buffer append addition             StringBuffer parameterBuffer   new StringBuffer    if   condition          appendWithDelimiter parameterBuffer   elementName            if   anotherCondition         appendWithDelimiter parameterBuffer   anotherElementName               Finally  to return a string representation  call toString   when returning  return parameterBuffer toString

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Use StringBuilder and class Separator  StringBuilder buf   new StringBuilder    Separator sep   new Separator        for  String each   list        buf append sep  append each       Separator wraps a delimiter  The delimiter is returned by Separator s toString method  unless on the first call which returns the empty string   Source code for class Separator  public class Separator        private boolean skipFirst      private final String value       public Separator             this                   public Separator String value            this value   value          this skipFirst   true             public void reset             skipFirst   true             public String toString             String sep   skipFirst        value          skipFirst   false          return sep

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Why not write your own join   method   It would take as parameters collection of Strings and a delimiter String   Within the method iterate over the collection and build up your result in a StringBuffer

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Java 8  stringCollection stream   collect Collectors joining

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Instead of using string concatenation  you should use StringBuilder if your code is not threaded  and StringBuffer if it is

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In Java 8 you can use String join     List lt String gt  list   Arrays asList  foo    bar    baz    String joined   String join   and    list       foo and bar and baz    Also have a look at this answer for a Stream API example

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For those who are in a Spring context their StringUtils class is useful as well   There are many useful shortcuts like    collectionToCommaDelimitedString Collection coll  collectionToDelimitedString Collection coll  String delim  arrayToDelimitedString Object   arr  String delim    and many others    This can be helpful if you are not already using Java 8 and you are already in a Spring context   I prefer it against the Apache Commons  although very good as well  for the Collection support which is easier like this      Encoding Set lt String gt  to String delimited  String asString   org springframework util StringUtils collectionToDelimitedString codes            Decoding String delimited to Set Set lt String gt  collection   org springframework util StringUtils commaDelimitedListToSet asString

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Don t know if this really is any better  but at least it s using StringBuilder  which may be slightly more efficient   Down below is a more generic approach if you can build up the list of parameters BEFORE doing any parameter delimiting      Answers real question public String appendWithDelimiters String delimiter  String original  String addition        StringBuilder sb   new StringBuilder original       if sb length    0            sb append delimiter  append addition         else           sb append addition             return sb toString           A more generic case         means a list of indeterminate length of Strings  public String appendWithDelimitersGeneric String delimiter  String    strings        StringBuilder sb   new StringBuilder        for  String string   strings            if sb length    0                sb append delimiter  append string             else               sb append string                        return sb toString       public void testAppendWithDelimiters         String string   appendWithDelimitersGeneric       string1    string2    string3

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You should probably use a StringBuilder with the append method to construct your result  but otherwise this is as good of a solution as Java has to offer

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So a couple of things you might do to get the feel that it seems like you re looking for   1  Extend List class - and add the join method to it   The join method would simply do the work of concatenating and adding the delimiter  which could be a param to the join method   2  It looks like Java 7 is going to be adding extension methods to java - which allows you just to attach a specific method on to a class   so you could write that join method and add it as an extension method to List or even to Collection   Solution 1 is probably the only realistic one  now  though since Java 7 isn t out yet    But it should work just fine   To use both of these  you d just add all your items to the List or Collection as usual  and then call the new custom method to  join  them

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With Java 5 variable args  so you don t have to stuff all your strings into a collection or array explicitly   import junit framework Assert  import org junit Test   public class StringUtil       public static String join String delim  String    strings                StringBuilder builder   new StringBuilder             if  strings    null                        for  String str   strings                                if  builder length    gt  0                                        builder append delim  append                                         builder append str                                              return builder toString               Test     public void joinTest                 Assert assertEquals     StringUtil join      null            Assert assertEquals     StringUtil join                    Assert assertEquals     StringUtil join      new String 0             Assert assertEquals  test   StringUtil join       test             Assert assertEquals  foo  bar   StringUtil join       foo    bar             Assert assertEquals  foo  bar  x   StringUtil join       foo    bar    x

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Note  if you have access to Java5      use StringBuilder in preference to StringBuffer      All that has to be replaced is the class name      StringBuffer will work in Java 1 4  though   appendWithDelimiter  StringBuffer buffer  String addition       String delimiter         if   buffer length      0            buffer append addition         else           buffer append delimiter           buffer append addition             StringBuffer parameterBuffer   new StringBuffer    if   condition          appendWithDelimiter parameterBuffer   elementName            if   anotherCondition         appendWithDelimiter parameterBuffer   anotherElementName               Finally  to return a string representation  call toString   when returning  return parameterBuffer toString

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in Java 8 you can do this like   list stream   map Object  toString           collect Collectors joining delimiter      if list has nulls you can use   list stream   map String  valueOf           collect Collectors joining delimiter     it also supports prefix and suffix   list stream   map String  valueOf           collect Collectors joining delimiter  prefix  suffix

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You can generalize it  but there s no join in Java  as you well say   This might work better    public static String join Iterable lt   extends CharSequence gt  s  String delimiter        Iterator lt   extends CharSequence gt  iter   s iterator        if   iter hasNext    return         StringBuilder buffer   new StringBuilder iter next         while  iter hasNext    buffer append delimiter  append iter next         return buffer toString

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in Java 8 you can do this like   list stream   map Object  toString           collect Collectors joining delimiter      if list has nulls you can use   list stream   map String  valueOf           collect Collectors joining delimiter     it also supports prefix and suffix   list stream   map String  valueOf           collect Collectors joining delimiter  prefix  suffix

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You can use Java s StringBuilder type for this  There s also StringBuffer  but it contains extra thread safety logic that is often unnecessary

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You re making this a little more complicated than it has to be  Let s start with the end of your example   String parameterString       if   condition   parameterString   appendWithDelimiter  parameterString   elementName          if   anotherCondition   parameterString   appendWithDelimiter  parameterString   anotherElementName            With the small change of using a StringBuilder instead of a String  this becomes   StringBuilder parameterString   new StringBuilder    if  condition  parameterString append  elementName   append       if  anotherCondition  parameterString append  anotherElementName   append             When you re done  I assume you have to check a few other conditions as well   just make sure you remove the tailing comma with a command like this   if  parameterString length    gt  0       parameterString deleteCharAt parameterString length   - 1     And finally  get the string you want with  parameterString toString      You could also replace the     in the second call to append with a generic delimiter string that can be set to anything  If you have a list of things you know you need to append  non-conditionally   you could put this code inside a method that takes a list of strings

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Use an approach based on java lang StringBuilder     A mutable sequence of characters      Like you mentioned  all those string concatenations are creating Strings all over   StringBuilder won t do that   Why StringBuilder instead of StringBuffer   From the StringBuilder javadoc      Where possible  it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations

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So basically something like this   public static String appendWithDelimiter String original  String addition  String delimiter     if  original equals            return addition    else       StringBuilder sb   new StringBuilder original length     addition length     delimiter length             sb append original           sb append delimiter           sb append addition           return sb toString

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Pre Java 8   Apache s commons lang is your friend here - it provides a join method very similar to the one you refer to in Ruby    StringUtils join java lang Iterable char     Java 8   Java 8 provides joining out of the box via StringJoiner and String join    The snippets below show how you can use them   StringJoiner  StringJoiner joiner   new StringJoiner       joiner add  01   add  02   add  03    String joinedString   joiner toString        01 02 03      String join CharSequence delimiter  CharSequence    elements    String joinedString   String join   -     04    05    06        04 - 05 - 06      String join CharSequence delimiter  Iterable lt   extends CharSequence gt  elements   List lt String gt  strings   new LinkedList lt  gt     strings add  Java   strings add  is    strings add  cool    String message   String join      strings     message returned is   Java is cool

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You should probably use a StringBuilder with the append method to construct your result  but otherwise this is as good of a solution as Java has to offer

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Apache commons StringUtils class has a join method

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Apache commons StringUtils class has a join method

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Use an approach based on java lang StringBuilder     A mutable sequence of characters      Like you mentioned  all those string concatenations are creating Strings all over   StringBuilder won t do that   Why StringBuilder instead of StringBuffer   From the StringBuilder javadoc      Where possible  it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations

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Don t know if this really is any better  but at least it s using StringBuilder  which may be slightly more efficient   Down below is a more generic approach if you can build up the list of parameters BEFORE doing any parameter delimiting      Answers real question public String appendWithDelimiters String delimiter  String original  String addition        StringBuilder sb   new StringBuilder original       if sb length    0            sb append delimiter  append addition         else           sb append addition             return sb toString           A more generic case         means a list of indeterminate length of Strings  public String appendWithDelimitersGeneric String delimiter  String    strings        StringBuilder sb   new StringBuilder        for  String string   strings            if sb length    0                sb append delimiter  append string             else               sb append string                        return sb toString       public void testAppendWithDelimiters         String string   appendWithDelimitersGeneric       string1    string2    string3

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Pre Java 8   Apache s commons lang is your friend here - it provides a join method very similar to the one you refer to in Ruby    StringUtils join java lang Iterable char     Java 8   Java 8 provides joining out of the box via StringJoiner and String join    The snippets below show how you can use them   StringJoiner  StringJoiner joiner   new StringJoiner       joiner add  01   add  02   add  03    String joinedString   joiner toString        01 02 03      String join CharSequence delimiter  CharSequence    elements    String joinedString   String join   -     04    05    06        04 - 05 - 06      String join CharSequence delimiter  Iterable lt   extends CharSequence gt  elements   List lt String gt  strings   new LinkedList lt  gt     strings add  Java   strings add  is    strings add  cool    String message   String join      strings     message returned is   Java is cool

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Your approach is not too bad  but you should use a StringBuffer instead of using the   sign  The   has the big disadvantage that a new String instance is being created for each single operation  The longer your string gets  the bigger the overhead  So using a StringBuffer should be the fastest way   public StringBuffer appendWithDelimiter  StringBuffer original  String addition  String delimiter             if   original    null                     StringBuffer buffer   new StringBuffer                    buffer append addition                   return buffer            else                   buffer append delimiter                   buffer append addition                   return original                After you have finished creating your string simply call toString   on the returned StringBuffer

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If you want to apply comma in a List of object s properties  This is the way i found most useful    here getName   is a string property of a class i have been trying to add     to   String message   listName stream   map list -  list getName    collect Collectors joining

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In the case of Android  the StringUtils class from commons isn t available  so for this I used  android text TextUtils join CharSequence delimiter  Iterable tokens    http   developer android com reference android text TextUtils html

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Your approach is not too bad  but you should use a StringBuffer instead of using the   sign  The   has the big disadvantage that a new String instance is being created for each single operation  The longer your string gets  the bigger the overhead  So using a StringBuffer should be the fastest way   public StringBuffer appendWithDelimiter  StringBuffer original  String addition  String delimiter             if   original    null                     StringBuffer buffer   new StringBuffer                    buffer append addition                   return buffer            else                   buffer append delimiter                   buffer append addition                   return original                After you have finished creating your string simply call toString   on the returned StringBuffer

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With Java 5 variable args  so you don t have to stuff all your strings into a collection or array explicitly   import junit framework Assert  import org junit Test   public class StringUtil       public static String join String delim  String    strings                StringBuilder builder   new StringBuilder             if  strings    null                        for  String str   strings                                if  builder length    gt  0                                        builder append delim  append                                         builder append str                                              return builder toString               Test     public void joinTest                 Assert assertEquals     StringUtil join      null            Assert assertEquals     StringUtil join                    Assert assertEquals     StringUtil join      new String 0             Assert assertEquals  test   StringUtil join       test             Assert assertEquals  foo  bar   StringUtil join       foo    bar             Assert assertEquals  foo  bar  x   StringUtil join       foo    bar    x

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In the case of Android  the StringUtils class from commons isn t available  so for this I used  android text TextUtils join CharSequence delimiter  Iterable tokens    http   developer android com reference android text TextUtils html

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You can try something like this   StringBuilder sb   new StringBuilder    if  condition    sb append  elementName   append         if  anotherCondition    sb append  anotherElementName   append         String parameterString   sb toString

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Note  if you have access to Java5      use StringBuilder in preference to StringBuffer      All that has to be replaced is the class name      StringBuffer will work in Java 1 4  though   appendWithDelimiter  StringBuffer buffer  String addition       String delimiter         if   buffer length      0            buffer append addition         else           buffer append delimiter           buffer append addition             StringBuffer parameterBuffer   new StringBuffer    if   condition          appendWithDelimiter parameterBuffer   elementName            if   anotherCondition         appendWithDelimiter parameterBuffer   anotherElementName               Finally  to return a string representation  call toString   when returning  return parameterBuffer toString

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You could write a little join-style utility method that works on java util Lists  public static String join List lt String gt  list  String delim         StringBuilder sb   new StringBuilder         String loopDelim            for String s   list             sb append loopDelim           sb append s                        loopDelim   delim             return sb toString        Then use it like so       List lt String gt  list   new ArrayList lt String gt          if  condition          list add  elementName        if  anotherCondition   list add  anotherElementName         join list

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You should probably use a StringBuilder with the append method to construct your result  but otherwise this is as good of a solution as Java has to offer

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Why not write your own join   method   It would take as parameters collection of Strings and a delimiter String   Within the method iterate over the collection and build up your result in a StringBuffer

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Use an approach based on java lang StringBuilder     A mutable sequence of characters      Like you mentioned  all those string concatenations are creating Strings all over   StringBuilder won t do that   Why StringBuilder instead of StringBuffer   From the StringBuilder javadoc      Where possible  it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations

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You could write a little join-style utility method that works on java util Lists  public static String join List lt String gt  list  String delim         StringBuilder sb   new StringBuilder         String loopDelim            for String s   list             sb append loopDelim           sb append s                        loopDelim   delim             return sb toString        Then use it like so       List lt String gt  list   new ArrayList lt String gt          if  condition          list add  elementName        if  anotherCondition   list add  anotherElementName         join list

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Java 8 Native Type  List lt Integer gt  example  example add 1   example add 2   example add 3       example stream   collect Collectors joining          Java 8 Custom Object   List lt Person gt  person      person stream   map Person  getAge  collect Collectors joining

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Apache commons StringUtils class has a join method

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You re making this a little more complicated than it has to be  Let s start with the end of your example   String parameterString       if   condition   parameterString   appendWithDelimiter  parameterString   elementName          if   anotherCondition   parameterString   appendWithDelimiter  parameterString   anotherElementName            With the small change of using a StringBuilder instead of a String  this becomes   StringBuilder parameterString   new StringBuilder    if  condition  parameterString append  elementName   append       if  anotherCondition  parameterString append  anotherElementName   append             When you re done  I assume you have to check a few other conditions as well   just make sure you remove the tailing comma with a command like this   if  parameterString length    gt  0       parameterString deleteCharAt parameterString length   - 1     And finally  get the string you want with  parameterString toString      You could also replace the     in the second call to append with a generic delimiter string that can be set to anything  If you have a list of things you know you need to append  non-conditionally   you could put this code inside a method that takes a list of strings

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Use an approach based on java lang StringBuilder     A mutable sequence of characters      Like you mentioned  all those string concatenations are creating Strings all over   StringBuilder won t do that   Why StringBuilder instead of StringBuffer   From the StringBuilder javadoc      Where possible  it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations

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If you want to apply comma in a List of object s properties  This is the way i found most useful    here getName   is a string property of a class i have been trying to add     to   String message   listName stream   map list -  list getName    collect Collectors joining

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With Java 5 variable args  so you don t have to stuff all your strings into a collection or array explicitly   import junit framework Assert  import org junit Test   public class StringUtil       public static String join String delim  String    strings                StringBuilder builder   new StringBuilder             if  strings    null                        for  String str   strings                                if  builder length    gt  0                                        builder append delim  append                                         builder append str                                              return builder toString               Test     public void joinTest                 Assert assertEquals     StringUtil join      null            Assert assertEquals     StringUtil join                    Assert assertEquals     StringUtil join      new String 0             Assert assertEquals  test   StringUtil join       test             Assert assertEquals  foo  bar   StringUtil join       foo    bar             Assert assertEquals  foo  bar  x   StringUtil join       foo    bar    x

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So a couple of things you might do to get the feel that it seems like you re looking for   1  Extend List class - and add the join method to it   The join method would simply do the work of concatenating and adding the delimiter  which could be a param to the join method   2  It looks like Java 7 is going to be adding extension methods to java - which allows you just to attach a specific method on to a class   so you could write that join method and add it as an extension method to List or even to Collection   Solution 1 is probably the only realistic one  now  though since Java 7 isn t out yet    But it should work just fine   To use both of these  you d just add all your items to the List or Collection as usual  and then call the new custom method to  join  them

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Fix answer Rob Dickerson   It s easier to use   public static String join String delimiter  String    values        StringBuilder stringBuilder   new StringBuilder         for  String value   values                stringBuilder append value           stringBuilder append delimiter              String result   stringBuilder toString         return result isEmpty     result   result substring 0  result length   - 1

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So a couple of things you might do to get the feel that it seems like you re looking for   1  Extend List class - and add the join method to it   The join method would simply do the work of concatenating and adding the delimiter  which could be a param to the join method   2  It looks like Java 7 is going to be adding extension methods to java - which allows you just to attach a specific method on to a class   so you could write that join method and add it as an extension method to List or even to Collection   Solution 1 is probably the only realistic one  now  though since Java 7 isn t out yet    But it should work just fine   To use both of these  you d just add all your items to the List or Collection as usual  and then call the new custom method to  join  them

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You can use Java s StringBuilder type for this  There s also StringBuffer  but it contains extra thread safety logic that is often unnecessary

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You re making this a little more complicated than it has to be  Let s start with the end of your example   String parameterString       if   condition   parameterString   appendWithDelimiter  parameterString   elementName          if   anotherCondition   parameterString   appendWithDelimiter  parameterString   anotherElementName            With the small change of using a StringBuilder instead of a String  this becomes   StringBuilder parameterString   new StringBuilder    if  condition  parameterString append  elementName   append       if  anotherCondition  parameterString append  anotherElementName   append             When you re done  I assume you have to check a few other conditions as well   just make sure you remove the tailing comma with a command like this   if  parameterString length    gt  0       parameterString deleteCharAt parameterString length   - 1     And finally  get the string you want with  parameterString toString      You could also replace the     in the second call to append with a generic delimiter string that can be set to anything  If you have a list of things you know you need to append  non-conditionally   you could put this code inside a method that takes a list of strings

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Why don t you do in Java the same thing you are doing in ruby  that is creating the delimiter separated string only after you ve added all the pieces to the array   ArrayList lt String gt  parms   new ArrayList lt String gt     if  someCondition  parms add  someString    if  anotherCondition  parms add  someOtherString           String sep       StringBuffer b   new StringBuffer    for  String p  parms        b append sep       b append p       sep    yourDelimiter       You may want to move that for loop in a separate helper method  and also use StringBuilder instead of StringBuffer     Edit  fixed the order of appends

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So basically something like this   public static String appendWithDelimiter String original  String addition  String delimiter     if  original equals            return addition    else       StringBuilder sb   new StringBuilder original length     addition length     delimiter length             sb append original           sb append delimiter           sb append addition           return sb toString

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Use StringBuilder and class Separator  StringBuilder buf   new StringBuilder    Separator sep   new Separator        for  String each   list        buf append sep  append each       Separator wraps a delimiter  The delimiter is returned by Separator s toString method  unless on the first call which returns the empty string   Source code for class Separator  public class Separator        private boolean skipFirst      private final String value       public Separator             this                   public Separator String value            this value   value          this skipFirst   true             public void reset             skipFirst   true             public String toString             String sep   skipFirst        value          skipFirst   false          return sep

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Use StringBuilder and class Separator  StringBuilder buf   new StringBuilder    Separator sep   new Separator        for  String each   list        buf append sep  append each       Separator wraps a delimiter  The delimiter is returned by Separator s toString method  unless on the first call which returns the empty string   Source code for class Separator  public class Separator        private boolean skipFirst      private final String value       public Separator             this                   public Separator String value            this value   value          this skipFirst   true             public void reset             skipFirst   true             public String toString             String sep   skipFirst        value          skipFirst   false          return sep

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If you re using Eclipse Collections  you can use makeString   or appendString     makeString   returns a String representation  similar to toString     It has three forms   makeString start  separator  end  makeString separator  defaults start and end to empty strings makeString   defaults the separator to       comma and space    Code example   MutableList lt Integer gt  list   FastList newListWith 1  2  3   assertEquals   1 2 3    list makeString                  assertEquals  1 2 3   list makeString        assertEquals  1  2  3   list makeString     assertEquals list toString    list makeString                     appendString   is similar to makeString    but it appends to an Appendable  like StringBuilder  and is void  It has the same three forms  with an additional first argument  the Appendable   MutableList lt Integer gt  list   FastList newListWith 1  2  3   Appendable appendable   new StringBuilder    list appendString appendable                  assertEquals   1 2 3    appendable toString       If you can t convert your collection to an Eclipse Collections type  just adapt it with the relevant adapter   List lt Object gt  list        ListAdapter adapt list  makeString         Note  I am a committer for Eclipse collections

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public static String join String   strings  char del        StringBuffer sb   new StringBuffer        int len   strings length      boolean appended   false      for  int i   0  i  lt  len  i                  if  appended                        sb append del                     sb append    strings i            appended   true            return sb toString

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You re making this a little more complicated than it has to be  Let s start with the end of your example   String parameterString       if   condition   parameterString   appendWithDelimiter  parameterString   elementName          if   anotherCondition   parameterString   appendWithDelimiter  parameterString   anotherElementName            With the small change of using a StringBuilder instead of a String  this becomes   StringBuilder parameterString   new StringBuilder    if  condition  parameterString append  elementName   append       if  anotherCondition  parameterString append  anotherElementName   append             When you re done  I assume you have to check a few other conditions as well   just make sure you remove the tailing comma with a command like this   if  parameterString length    gt  0       parameterString deleteCharAt parameterString length   - 1     And finally  get the string you want with  parameterString toString      You could also replace the     in the second call to append with a generic delimiter string that can be set to anything  If you have a list of things you know you need to append  non-conditionally   you could put this code inside a method that takes a list of strings

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You can try something like this   StringBuilder sb   new StringBuilder    if  condition    sb append  elementName   append         if  anotherCondition    sb append  anotherElementName   append         String parameterString   sb toString

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Why not write your own join   method   It would take as parameters collection of Strings and a delimiter String   Within the method iterate over the collection and build up your result in a StringBuffer

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So basically something like this   public static String appendWithDelimiter String original  String addition  String delimiter     if  original equals            return addition    else       StringBuilder sb   new StringBuilder original length     addition length     delimiter length             sb append original           sb append delimiter           sb append addition           return sb toString

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I would use Google Collections   There is a nice Join facility  http   google-collections googlecode com svn trunk javadoc index html com google common base Join html  But if I wanted to write it on my own   package util   import java util ArrayList  import java util Iterable  import java util Collections  import java util Iterator   public class Utils          accept a collection of objects  since all objects have toString       public static String join String delimiter  Iterable lt   extends Object gt  objs            if  objs isEmpty                  return                       Iterator lt   extends Object gt  iter   objs iterator            StringBuilder buffer   new StringBuilder            buffer append iter next             while  iter hasNext                  buffer append delimiter  append iter next                       return buffer toString                  for convenience     public static String join String delimiter  Object    objs            ArrayList lt Object gt  list   new ArrayList lt Object gt             Collections addAll list  objs           return join delimiter  list             I think it works better with an object collection  since now you don t have to convert your objects to strings before you join them

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Why don t you do in Java the same thing you are doing in ruby  that is creating the delimiter separated string only after you ve added all the pieces to the array   ArrayList lt String gt  parms   new ArrayList lt String gt     if  someCondition  parms add  someString    if  anotherCondition  parms add  someOtherString           String sep       StringBuffer b   new StringBuffer    for  String p  parms        b append sep       b append p       sep    yourDelimiter       You may want to move that for loop in a separate helper method  and also use StringBuilder instead of StringBuffer     Edit  fixed the order of appends

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You can use Java s StringBuilder type for this  There s also StringBuffer  but it contains extra thread safety logic that is often unnecessary

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Use StringBuilder and class Separator  StringBuilder buf   new StringBuilder    Separator sep   new Separator        for  String each   list        buf append sep  append each       Separator wraps a delimiter  The delimiter is returned by Separator s toString method  unless on the first call which returns the empty string   Source code for class Separator  public class Separator        private boolean skipFirst      private final String value       public Separator             this                   public Separator String value            this value   value          this skipFirst   true             public void reset             skipFirst   true             public String toString             String sep   skipFirst        value          skipFirst   false          return sep

[java] What's the best way to build a string of delimited items in Java?

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