Best way to concatenate List of String objects

Question

What is the best way to concatenate a list of String objects  I am thinking of doing this way   List lt String gt  sList   new ArrayList lt String gt         add elements  if  sList    null        String listString   sList toString        listString   listString subString 1  listString length   - 1       I somehow found this to be neater than using the StringBuilder StringBuffer approach   Any thoughts comments

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Rather than depending on ArrayList toString   implementation  you could write a one-liner  if you are using java 8   String result   sList stream                         reduce     String  concat     If you prefer using StringBuffer instead of String since String  concat has a runtime of O n 2   you could convert every String to StringBuffer first   StringBuffer result   sList stream                               map StringBuffer  new                              reduce new StringBuffer      StringBuffer  append

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A variation on codefin s answer  public static String concatStringsWSep Iterable lt String gt  strings  String separator        StringBuilder sb   new StringBuilder        String sep           for String s  strings            sb append sep  append s           sep   separator            return sb toString

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Using the Functional Java library  import these   import static fj pre Monoid stringMonoid  import static fj data List list  import fj data List        then you can do this   List lt String gt  ss   list  foo    bar    baz    String s   stringMonoid join ss           Or  the generic way  if you don t have a list of Strings   public static  lt A gt  String showList List lt A gt  l  Show lt A gt  s      return stringMonoid join l map s showS

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Use one of the the StringUtils join methods in Apache Commons Lang   import org apache commons lang3 StringUtils   String result   StringUtils join list           If you are fortunate enough to be using Java 8  then it s even easier   just use String join  String result   String join       list

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Assuming it s faster to just move a pointer   set a byte to null  or however Java implements StringBuilder setLength   rather than check a condition each time through the loop to see when to append the delimiter  you could use this method   public static String Intersperse  Collection lt    collection  String delimiter        StringBuilder sb   new StringBuilder         for  Object item   collection                if  item    null  continue          sb append  item  append  delimiter             sb setLength  sb length    - delimiter length          return sb toString

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if you have json in your dependencies you can use new JSONArray list  toString

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Next variation on Peter Lawrey s answer without initialization of a new string every loop turn  String concatList List lt String gt  sList  String separator        Iterator lt String gt  iter   sList iterator        StringBuilder sb   new StringBuilder         while  iter hasNext                  sb append iter next    append  iter hasNext     separator                  return sb toString

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Depending on the need for performance and amount of elements to be added  this might be an ok solution  If the amount of elements are high  the Arraylists reallocation of memory might be a bit slower than StringBuilder

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In java 8 you can also use a reducer  something like   public static String join List lt String gt  strings  String joinStr        return strings stream   reduce      prev  cur  - gt  prev     cur   joinStr

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Using Java 8   String str   list stream   collect Collectors joining      or even   String str   String join     list

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Your approach is dependent on Java s ArrayList toString   implementation   While the implementation is documented in the Java API and very unlikely to change  there s a chance it could  It s far more reliable to implement this yourself  loops  StringBuilders  recursion whatever you like better    Sure this approach may seem  neater  or more  too sweet  or  money  but it is  in my opinion  a worse approach

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ArrayList inherits its toString  -method from AbstractCollection  ie   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                  Building the string yourself will be far more efficient     If you really want to aggregate the strings beforehand in some sort of List  you should provide your own method to efficiently join them  e g  like this   static String join Collection lt   gt  items  String sep        if items size      0          return          String   strings   new String items size         int length   sep length      items size   - 1        int idx   0      for Object item   items            String str   item toString            strings idx      str          length    str length               char   chars   new char length       int pos   0       for String str   strings            str getChars 0  str length    chars  pos           pos    str length             if pos  lt  length                sep getChars 0  sep length    chars  pos               pos    sep length                         return new String chars

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Have you seen this Coding Horror blog entry   The Sad Tragedy of Micro-Optimization Theater  I am not shure whether or not it is  neater   but from a performance-standpoint it probably won t matter much

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This is the most elegant and clean way I ve found so far   list stream   collect Collectors joining delimiter

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It seems to me that the StringBuilder will be quick and efficient   The basic form would look something like this   public static String concatStrings List lt String gt  strings        StringBuilder sb   new StringBuilder        for String s  strings                sb append s             return sb toString               If that s too simplistic  and it probably is   you can use a similar approach and add a separator like this       public static String concatStringsWSep List lt String gt  strings  String separator        StringBuilder sb   new StringBuilder        for int i   0  i  lt  strings size    i                  sb append strings get i            if i  lt  strings size   - 1              sb append separator             return sb toString                       I agree with the others who have responded to this question when they say that you should not rely on the toString   method of Java s ArrayList

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I somehow found this to be neater than   using the StringBuilder StringBuffer   approach    I guess it depends on what approach you took     The AbstractCollection toString   method simply iterates over all the elements and appends them to a StringBuilder   So your method may be saving a few lines of code but at the cost of extra String manipulation   Whether that tradeoff is a good one is up to you

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Guava is a pretty neat library from Google   Joiner joiner   Joiner on        joiner join sList

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I prefer String join list  in Java 8

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If you are developing for Android  there is TextUtils join provided by the SDK

[java] Best way to concatenate List of String objects?

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