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  1. import java.util.*;
  2. import java.util.function.*;
  3. import java.util.stream.*;
  4.  
  5. class Ideone {
  6.     public static void main(String args[]){
  7.         List<String> strings = List.of("a", "bb", "cc", "ddd");
  8.  
  9.         Map<Integer, List<Character>> result1 = strings.stream()
  10.                 .map(toStringList())
  11.                 .collect(Collectors.groupingBy(List::size
  12.                         , Collectors.reducing(List.of(),concat())
  13.                 ));
  14.         System.out.println(result1);
  15.  
  16.         Map<Integer,ObjStr2> result2 = strings.stream()
  17.                 .map(ObjStr1::new)
  18.                 .collect(Collectors.groupingBy(ObjStr1::getLen
  19.                         , Collectors.reducing(new ObjStr2(), ObjStr1::to2, ObjStr2::doReduce)));
  20.  
  21.         System.out.println(result2);
  22.     }
  23.  
  24.     private static Function<String, List<Character>> toStringList(){
  25.         return s-> s.chars()
  26.                 .mapToObj(c->(char) c)
  27.                 .collect(Collectors.toList());
  28.     }
  29.  
  30.     private static BinaryOperator<List<Character>> concat(){
  31.         return (l1, l2) -> {
  32.             return Stream.concat(l1.stream(), l2.stream()).collect(Collectors.toList());
  33.         };
  34.     }
  35.  
  36.  
  37. }
  38.  
  39. class ObjStr1{
  40.     String str = "";
  41.  
  42.     ObjStr1(String str) {
  43.         this.str = str;
  44.     }
  45.  
  46.     static ObjStr2 to2(ObjStr1 o){
  47.         return new ObjStr2(o.str);
  48.     }
  49.  
  50.     Integer getLen(){return str.length(); };
  51. }
  52.  
  53. class ObjStr2{
  54.     String str = "";
  55.  
  56.     ObjStr2(){}
  57.  
  58.     ObjStr2(String str) {
  59.         this.str = str;
  60.     }
  61.  
  62.     static ObjStr2 doReduce(ObjStr2 a, ObjStr2 b){
  63.         return new ObjStr2(a.str + b.str);
  64.     }
  65.  
  66.     @Override
  67.     public String toString(){
  68.         return str;
  69.     }
  70. }
Success #stdin #stdout 0.17s 37504KB
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Standard input is empty
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{1=[a], 2=[b, b, c, c], 3=[d, d, d]}
{1=a, 2=bbcc, 3=ddd}
https://ideone.com/j57GWw
language:
Java (12.0.1)
created:
4 years ago
visibility:
public

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