Using Java 8 to convert a list of objects into a string obtained from the toString method

Question

There are a lot of useful new things in Java 8  E g   I can iterate with a stream over a list of objects and then sum the values from a specific field of the Object s instances  E g   public class AClass     private int value    public int getValue     return value       Integer sum   list stream   mapToInt AClass  getValue  sum      Thus  I m asking if there is any way to build a String that concatenates the output of the toString   method from the instances in a single line   List lt Integer gt  list        String concatenated   list stream         concatenate here with toString   method from java lang Integer class   Suppose that list contains integers 1  2 and 3  I expect that concatenated is  123  or  1 2 3

User · Answer

There is a method in the String API for those  joining list of string  usecases  you don t even need Stream   List lt String gt  myStringIterable   Arrays asList  baguette    bonjour     String myReducedString   String join      myStringIterable       And here you obtain  baguette bonjour  in your myReducedString variable

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String actual   list stream   reduce  t  u  - gt  t         u  get

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Also  you can do like this       List lt String gt  list   Arrays asList  One    Two    Three        String result   String join       list       System out println result

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The other answers are fine  However  you can also pass Collectors toList   as parameter to Stream collect   to return the elements as an ArrayList   System out println  list stream   map  e - gt  e toString     collect  toList

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I m going to use the streams api to convert a stream of integers into a single string  The problem with some of the provided answers is that they produce a O n 2  runtime because of String building  A better solution is to use a StringBuilder  and then join the strings together as the final step                   Create a stream of integers      String result   Arrays stream new int   1 2 3 4 5 6                                   collect into a single StringBuilder              collect StringBuilder  new     supplier function                        accumulator - converts cur integer into a string and appends it to the string builder                      builder  cur  - gt  builder append Integer toString cur                           combiner - combines two string builders if running in parallel                     StringBuilder  append                  convert StringBuilder into a single string              toString      You can take this process a step further by converting the collection of object to a single string      Start with a class definition public static class AClass       private int value      public int getValue     return value        public AClass int value    this value   value          Override     public String toString             return Integer toString value               Create a stream of AClass objects         String resultTwo   Arrays stream new AClass                    new AClass 1                   new AClass 2                   new AClass 3                   new AClass 4                                transform stream of objects into a single string                  collect StringBuilder  new                           builder  curObj  - gt  builder append curObj toString                             StringBuilder  append                                  finally transform string builder into a single string              toString

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With Java 8   String s   Arrays toString list stream   toArray AClass    new      Not the most efficient  but it is a solution with a small amount of code

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There is a collector joining in the API  It s a static method in Collectors   list stream   map Object  toString  collect Collectors joining         Not perfect because of the necessary call of toString  but works  Different delimiters are possible

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List lt String gt  list   Arrays asList  One    Two    Three        list stream                reduce     org apache commons lang3 StringUtils  join     Or   List lt String gt  list   Arrays asList  One    Two    Three            list stream                    reduce      s1 s2 - gt s1 s2     This approach allows you also build a string result from a list of objects Example   List lt Wrapper gt  list   Arrays asList w1  w2  w2           list stream                    map w- gt w getStringValue                   reduce     org apache commons lang3 StringUtils  join     Here the reduce function allows you to have some initial value to which you want to append new string Example    List lt String gt  errors   Arrays asList  er1    er2    er3                list stream                        reduce  Found next errors     s1 s2 - gt s1 s2

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Can we try this   public static void main String   args           List lt String gt  stringList   new ArrayList lt  gt             for int i 0 i lt  10 i                 stringList add    i                     String stringConcated   String join      stringList           System out println stringConcated

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Testing both approaches suggested in Shail016 and bpedroso answer  https   stackoverflow com a 24883180 2832140   the simple StringBuilder   append String  within a for loop  seems to execute much faster than list stream   map         Example  This code walks through a Map lt Long  List lt Long gt  gt  builds a json string  using list stream   map         if  mapSize  gt  0        StringBuilder sb   new StringBuilder            for  Map Entry lt Long  List lt Long gt  gt  entry   threadsMap entrySet               sb append         entry getKey   toString                      sb append entry getValue   stream   map Object  toString  collect Collectors joining                   sb delete sb length  -2  sb length         sb append           System out println sb toString         On my dev VM  junit usually takes between 0 35 and 1 2 seconds to execute the test  While  using this following code  it takes between 0 15 and 0 33 seconds   if  mapSize  gt  0        StringBuilder sb   new StringBuilder            for  Map Entry lt Long  List lt Long gt  gt  entry   threadsMap entrySet               sb append         entry getKey   toString                       for  Long tid   entry getValue                  sb append tid toString                              sb delete sb length  -2  sb length             sb append                    sb delete sb length  -2  sb length         sb append           System out println sb toString

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StringListName   ObjectListName stream   map  m -  m toString     collect  Collectors toList

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Just in case anyone is trying to do this without java 8  there is a pretty good trick   List toString   already returns a collection that looks like this    1 2 3   Depending on your specific requirements  this can be post-processed to whatever you want as long as your list items don t contain    or      For instance   list toString   replace         replace            or if your data might contain square brackets this   String s list toString    s   s substring 1 s length  -1     will get you a pretty reasonable output   One array item on each line can be created like this   list toString   replace         replace         replaceAll       r n     I used this technique to make html tooltips from a list in a small app  with something like   list toString   replace       lt html gt    replace       lt  html gt    replaceAll       lt br gt      If you have an array then start with Arrays asList list  toString   instead  I ll totally own the fact that this is not optimal  but it s not as inefficient as you might think and is pretty straightforward to read and understand   It is  however  quite inflexible--in particular don t try to separate the elements with replaceAll if your data might contain commas and use the substring version if you have square brackets in your data  but for an array of numbers it s pretty much perfect

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One simple way is to append your list items in a StringBuilder      List lt Integer gt  list   new ArrayList lt  gt        list add 1      list add 2      list add 3       StringBuilder b   new StringBuilder       list forEach b  append       System out println b     you can also try   String s   list stream   map e - gt  e toString    reduce     String  concat     Explanation  map converts Integer stream to String stream  then its reduced as concatenation of all the elements   Note  This is normal reduction which performs in O n2   for better performance use a StringBuilder or mutable reduction similar to F  B  ller s answer   String s   list stream   map Object  toString  collect Collectors joining          Ref  Stream Reduction

[java] Using Java 8 to convert a list of objects into a string obtained from the toString() method

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