Best way to convert an ArrayList to a string

Question

I have an ArrayList that I want to output completely as a String  Essentially I want to output it in order using the toString of each element separated by tabs  Is there any fast way to do this  You could loop through it  or remove each element  and concatenate it to a String but I think this will be very slow

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It s an O n  algorithm either way  unless you did some multi-threaded solution where you broke the list into multiple sublists  but I don t think that is what you are asking for    Just use a StringBuilder as below   StringBuilder sb   new StringBuilder     for  Object obj   list      sb append obj toString       sb append   t       String finalString   sb toString      The StringBuilder will be a lot faster than string concatenation because you won t be re-instantiating a String object on each concatenation

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Android has a TextUtil class you can use http   developer android com reference android text TextUtils html  String implode   TextUtils join   t   list

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The below code may help you   List list   new ArrayList    list add  1    list add  2    list add  3    String str   list toString    System out println  Step-1       str   str   str replaceAll                  System out println  Step-2       str     Output   Step-1    1  2  3  Step-2   1  2  3

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Download the Apache Commons Lang and use the method    StringUtils join list    StringUtils join list           2nd param is the separator    You can implement it by yourself  of course  but their code is fully tested and is probably the best possible implementation   I am a big fan of the Apache Commons library and I also think it s a great addition to the Java Standard Library

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An elegant way to deal with trailing separation characters is to use Class Separator  StringBuilder buf   new StringBuilder    Separator sep   new Separator   t    for  String each  list  buf append sep  append each   String s   buf toString      The toString method of Class Separator returns the separater  except for the first call  Thus we print the list without trailing  or in this case  leading separators

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Loop through it and call toString   There isn t a magic way  and if there were  what do you think it would be doing under the covers other than looping through it   About the only micro-optimization would be to use StringBuilder instead of String  and even that isn t a huge win - concatenating strings turns into StringBuilder under the covers  but at least if you write it that way you can see what s going on   StringBuilder out   new StringBuilder    for  Object o   list      out append o toString       out append   t      return out toString

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Would the following be any good   List lt String gt  streamValues   new ArrayList lt  gt     Arrays deepToString streamValues toArray

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If you re using Eclipse Collections  you can use the makeString   method   ArrayList lt String gt  list   new ArrayList lt String gt     list add  one    list add  two    list add  three     Assert assertEquals       one ttwo tthree       ArrayListAdapter adapt list  makeString   t       If you can convert your ArrayList to a FastList  you can get rid of the adapter   Assert assertEquals       one ttwo tthree       FastList newListWith  one    two    three   makeString   t       Note  I am a committer for Eclipse Collections

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You can use a Regex for this  This is as concise as it gets  System out println yourArrayList toString   replaceAll                    t

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Changing List to a readable and meaningful String is really a common question that every one may encounter    Case 1   If you have  apache s StringUtils in your class path  as from rogerdpack and Ravi Wallau    import org apache commons lang3 StringUtils  String str   StringUtils join myList     Case 2   If you only want to use ways from JDK 7    import java util Arrays  String str   Arrays toString myList toArray        Just never build wheels by yourself  dont use loop for this one-line task

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In Java 8 it s simple  See example for list of integers   String result   Arrays asList 1 2 3  stream   map Object  toString  reduce  t  u  - gt  t     t    u  orElse        Or multiline version  which is simpler to read    String result   Arrays asList 1 2 3  stream        map Object  toString       reduce  t  u  - gt  t     t    u       orElse        Update - a shorter version  String result   Arrays asList 1 2 3  stream                    map Object  toString                   collect Collectors joining   t

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For this simple use case  you can simply join the strings with comma  If you use Java 8   String csv   String join   t   yourArray     otherwise commons-lang has a join   method   String csv   org apache commons lang3 StringUtils join yourArray    t

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If you were looking for a quick one-liner  as of Java 5 you can do this   myList toString   replaceAll                replaceAll        t     Additionally  if your purpose is just to print out the contents and are less concerned about the   t   you can simply do this   myList toString     which returns a string like      str1  str2  str3    If you have an Array  not ArrayList  then you can accomplish the same like this    Arrays toString myList  replaceAll                replaceAll        t

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This is quite an old conversation by now and apache commons are now using a StringBuilder internally  http   commons apache org lang api src-html org apache commons lang StringUtils html line 3045  This will as we know improve performance  but if performance is critical then the method used might be somewhat inefficient  Whereas the interface is flexible and will allow for consistent behaviour across different Collection types it is somewhat inefficient for Lists  which is the type of Collection in the original question    I base this in that we are incurring some overhead which we would avoid by simply iterating through the elements in a traditional for loop  Instead there are some additional things happening behind the scenes checking for concurrent modifications  method calls etc  The enhanced for loop will on the other hand result in the same overhead since the iterator is used on the Iterable object  the List

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If you don t want the last  t after the last element  you have to use the index to check  but remember that this only  works   i e  is O n   when lists implements the RandomAccess   List lt String gt  list   new ArrayList lt String gt     list add  one    list add  two    list add  three     StringBuilder sb   new StringBuilder list size     apprAvg      every apprAvg  gt  1 is better than none for  int i   0  i  lt  list size    i          sb append list get i        if  i  lt  list size   - 1            sb append   t            System out println sb toString

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Java 8 introduces a String join separator  list  method  see Vitalii Federenko s answer  Before Java 8  using a loop to iterate over the ArrayList was the only option  DO NOT use this code  continue reading to the bottom of this answer to see why it is not desirable  and which code should be used instead  ArrayList lt String gt  list   new ArrayList lt String gt     list add  quot one quot    list add  quot two quot    list add  quot three quot     String listString    quot  quot    for  String s   list        listString    s    quot  t quot      System out println listString    In fact  a string concatenation is going to be just fine  as the javac compiler will optimize the string concatenation as a series of append operations on a StringBuilder anyway  Here s a part of the disassembly of the bytecode from the for loop from the above program     61   new  13    class java lang StringBuilder    64   dup    65   invokespecial    14    Method java lang StringBuilder  quot  lt init gt  quot    V    68   aload 2    69   invokevirtual    15    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     72   aload   4    74   invokevirtual    15    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     77   ldc  16    String  t    79   invokevirtual    15    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     82   invokevirtual    17    Method java lang StringBuilder toString   Ljava lang String   As can be seen  the compiler optimizes that loop by using a StringBuilder  so performance shouldn t be a big concern   OK  on second glance  the StringBuilder is being instantiated on each iteration of the loop  so it may not be the most efficient bytecode  Instantiating and using an explicit StringBuilder would probably yield better performance   In fact  I think that having any sort of output  be it to disk or to the screen  will be at least an order of a magnitude slower than having to worry about the performance of string concatenations  Edit  As pointed out in the comments  the above compiler optimization is indeed creating a new instance of StringBuilder on each iteration   Which I have noted previously   The most optimized technique to use will be the response by Paul Tomblin  as it only instantiates a single StringBuilder object outside of the for loop  Rewriting to the above code to  ArrayList lt String gt  list   new ArrayList lt String gt     list add  quot one quot    list add  quot two quot    list add  quot three quot     StringBuilder sb   new StringBuilder    for  String s   list        sb append s       sb append  quot  t quot       System out println sb toString      Will only instantiate the StringBuilder once outside of the loop  and only make the two calls to the append method inside the loop  as evidenced in this bytecode  which shows the instantiation of StringBuilder and the loop         Instantiation of the StringBuilder outside loop     33   new  8    class java lang StringBuilder    36   dup    37   invokespecial    9    Method java lang StringBuilder  quot  lt init gt  quot    V    40   astore 2         snip a few lines for initializing the loop        Loading the StringBuilder inside the loop  then append     66   aload 2    67   aload   4    69   invokevirtual    14    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     72   pop    73   aload 2    74   ldc  15    String  t    76   invokevirtual    14    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     79   pop  So  indeed the hand optimization should be better performing  as the inside of the for loop is shorter and there is no need to instantiate a StringBuilder on each iteration

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I see quite a few examples which depend on additional resources  but it seems like this would be the simplest solution   which is what I used in my own project  which is basically just converting from an ArrayList to an Array and then to a List       List lt Account gt  accounts   new ArrayList lt  gt         public String accountList               Account   listingArray   accounts toArray new Account accounts size            String listingString   Arrays toString listingArray         return listingString

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In one Line    From  12 0 1 78 12  to 12 0 1 78 12  String srt  list toString   replaceAll

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May not be the best way  but elegant way   Arrays deepToString Arrays asList  Test    Test2    import java util Arrays       public class Test           public static void main String   args                System out println Arrays deepToString Arrays asList  Test    Test2   toArray                        Output   Test  Test2

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In Java 8 or later   String listString   String join       list     In case the list is not of type String  a joining collector can be used   String listString   list stream   map Object  toString                           collect Collectors joining

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If you happen to be doing this on Android  there is a nice utility for this called TextUtils which has a  join String delimiter  Iterable  method   List lt String gt  list   new ArrayList lt String gt     list add  Item 1    list add  Item 2    String joined   TextUtils join       list     Obviously not much use outside of Android  but figured I d add it to this thread

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How about this function   public static String toString final Collection lt   gt  collection        final StringBuilder sb   new StringBuilder           boolean isFirst   true      for  final Object object   collection            if   isFirst              sb append               else             isFirst   false          sb append object             sb append           return sb toString        it works for any type of collection

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Most Java projects often have apache-commons lang available   StringUtils join   methods is very nice and has several flavors to meet almost every need   public static java lang String join java util Collection collection                                      char separator    public static String join Iterator iterator  String separator           handle null  zero and one elements before building a buffer      Object first   iterator next        if   iterator hasNext              return ObjectUtils toString first                two or more elements      StringBuffer buf            new StringBuffer 256      Java default is 16  probably too small      if  first    null            buf append first             while  iterator hasNext              if  separator    null                buf append separator                     Object obj   iterator next            if  obj    null                buf append obj                       return buf toString           Parameters       collection - the Collection of values to join together  may be null      separator - the separator character to use      Returns  the joined String  null if   null iterator input      Since    2 3

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In case you happen to be on Android and you are not using Jack yet  e g  because it s still lacking support for Instant Run   and if you want more control over formatting of the resulting string  e g  you would like to use the newline character as the divider of elements   and happen to use want to use the StreamSupport library  for using streams on Java 7 or earlier versions of the compiler   you could use something like this  I put this method in my ListUtils class    public static  lt T gt  String asString List lt T gt  list        return StreamSupport stream list               map Object  toString               collect Collectors joining   n         And of course  make sure to implement toString   on your list objects  class

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For seperating using tabs instead of using println you can use print       ArrayList lt String gt  mylist   new ArrayList lt String gt      mylist add  C Programming    mylist add  Java    mylist add  C      mylist add  Perl    mylist add  Python     for  String each   mylist               System out print each       System out print   t

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List lt String gt  stringList   getMyListOfStrings    StringJoiner sj   new StringJoiner       stringList stream   forEach e - gt  sj add e    String spaceSeparated   sj toString     You pass to the new StringJoiner the char sequence you want to be used as separator  If you want to do a CSV  new StringJoiner

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This is a pretty old question  but I figure I might as well add a more modern answer - use the Joiner class from Guava   String joined   Joiner on   t   join list

[java] Best way to convert an ArrayList to a string

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