A quick and easy way to join array elements with a separator the opposite of split in Java

Question

See Related  NET question   I m looking for a quick and easy way to do exactly the opposite of split so that it will cause    a   b   c   to become  a b c   Iterating through an array requires either adding a condition  if this is not the last element  add the seperator  or using substring to remove the last separator   I m sure there is a certified  efficient way to do it  Apache Commons    How do you prefer doing it in your projects

User · Answer

I prefer Google Collections over Apache StringUtils for this particular problem:

Joiner.on(separator).join(array)

Compared to StringUtils, the Joiner API has a fluent design and is a bit more flexible, e.g. null elements may be skipped or replaced by a placeholder. Also, Joiner has a feature for joining maps with a separator between key and value.

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Using Java 8 you can do this in a very clean way   String join delimiter  elements     This works in three ways   1  directly specifying the elements  String joined1   String join       a    b    c      2  using arrays  String   array   new String      a    b    c     String joined2   String join      array     3  using iterables  List lt String gt  list   Arrays asList array   String joined3   String join      list

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All of these other answers include runtime overhead     like using ArrayList toString   replaceAll      which are very wasteful   I will give you the optimal algorithm with zero overhead  it doesn t look as pretty as the other options  but internally  this is what they are all doing  after piles of other hidden checks  multiple array allocation and other crud    Since you already know you are dealing with strings  you can save a bunch of array allocations by performing everything manually   This isn t pretty  but if you trace the actual method calls made by the other implementations  you ll see it has the least runtime overhead possible   public static String join String separator  String     values      if  values length  0 return      need at least one element     all string operations use a new array  so minimize all calls possible   char   sep   separator toCharArray          determine final size and normalize nulls   int totalSize    values length - 1    sep length    separator size   for  int i   0  i  lt  values length  i          if  values i     null        values i            else       totalSize    values i  length             exact size  no bounds checks or resizes   char   joined   new char totalSize     int pos   0      note  we are iterating all the elements except the last one   for  int i   0  end   values length-1  i  lt  end  i          System arraycopy values i  toCharArray    0         joined  pos  values i  length         pos    values i  length        System arraycopy sep  0  joined  pos  sep length       pos    sep length          now  add the last element       this is why we checked values length    0 off the hop   System arraycopy values values length-1  toCharArray    0      joined  pos  values values length-1  length        return new String joined

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This small function always comes in handy   public static String join String   strings  int startIndex  String separator        StringBuffer sb   new StringBuffer        for  int i startIndex  i  lt  strings length  i              if  i    startIndex  sb append separator           sb append strings i              return sb toString

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it s in StringUtils   http   www java2s com Code JavaAPI org apache commons lang StringUtilsjoinObjectarrayStringseparator htm

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Apache Commons Lang does indeed have a StringUtils join method which will connect String arrays together with a specified separator   For example   String   s   new String     a    b    c    String joined   StringUtils join s             a b c    However  I suspect that  as you mention  there must be some kind of conditional or substring processing in the actual implementation of the above mentioned method   If I were to perform the String joining and didn t have any other reasons to use Commons Lang  I would probably roll my own to reduce the number of dependencies to external libraries

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A fast and simple solution without any 3rd party includes   public static String strJoin String   aArr  String sSep        StringBuilder sbStr   new StringBuilder        for  int i   0  il   aArr length  i  lt  il  i              if  i  gt  0              sbStr append sSep           sbStr append aArr i              return sbStr toString

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This options is fast and clear     public static String join String separator  String    values        StringBuilder sb   new StringBuilder 128       int end   0      for  String s   values          if  s    null            sb append s           end   sb length            sb append separator                     return sb substring 0  end

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The approach that I ve taken has evolved since Java 1 0 to provide readability and maintain reasonable options for backward-compatibility with older Java versions  while also providing method signatures that are drop-in replacements for those from apache commons-lang  For performance reasons  I can see some possible objections to the use of Arrays asList but I prefer helper methods that have sensible defaults without duplicating the one method that performs the actual work  This approach provides appropriate entry points to a reliable method that does not require array list conversions prior to calling   Possible variations for Java version compatibility include substituting StringBuffer  Java 1 0  for StringBuilder  Java 1 5   switching out the Java 1 5 iterator and removing the generic wildcard  Java 1 5  from the Collection  Java 1 2   If you want to take backward compatibility a step or two further  delete the methods that use Collection and move the logic into the array-based method   public static String join String   values        return join values           public static String join String   values  char delimiter        return join Arrays asList values   String valueOf delimiter          To match Apache commons-lang  StringUtils join values  delimiter  public static String join String   values  String delimiter        return join Arrays asList values   delimiter      public static String join Collection lt   gt  values        return join values           public static String join Collection lt   gt  values  char delimiter        return join values  String valueOf delimiter       public static String join Collection lt   gt  values  String delimiter        if  values    null                return new String               StringBuffer strbuf   new StringBuffer         boolean first   true       for  Object value   values                if   first    strbuf append delimiter     else   first   false            strbuf append value toString                return strbuf toString

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With Java 1 8 there is a new StringJoiner class - so no need for Guava or Apache Commons   String str   new StringJoiner      add  a   add  b   add  c   toString      Or using a collection directly with the new stream api   String str   Arrays asList  a    b    c   stream   collect Collectors joining

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public String join String   str  String separator       String retval           for  String s  str   retval   separator   s       return retval replaceFirst separator

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If you re on Android you can TextUtils join delimiter  tokens

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You can use replace and replaceAll with regular expressions   String   strings     a    b    c     String result   Arrays asList strings  toString   replaceAll                    replace               Because Arrays asList   toString   produces    a  b  c    we do a replaceAll to remove the first and last brackets and then  optionally  you can change the      sequence for      your new separator    A stripped version  fewer chars    String   strings     a    b    c     String result         Arrays asList strings   replaceAll                replace                Regular expressions are very powerful  specially String methods  replaceFirst  and  replaceAll   Give them a try

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Even easier you can just use Arrays  so you will get a String with the values of the array separated by a       String concat   Arrays toString myArray     so you will end up with this  concat     a b c    Update  You can then get rid of the brackets using a sub-string as suggested by Jeff  concat   concat substring 1  concat length   -1     so you end up with concat    a b c   if you want to use Kotlin    val concat   myArray joinToString separator           a b c

User · Answer

I m sure there is a certified  efficient way to do it  Apache Commons     yes  apparenty it s  StringUtils join array  separator    http   www java2s com Code JavaAPI org apache commons lang StringUtilsjoinObjectarrayStringseparator htm

[java] A quick and easy way to join array elements with a separator (the opposite of split) in Java

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