プログラマーズJavaチートシート4. JavaストリームAPI

Integer sumOddOld = 0; for(Integer i: collection) { if(i % 2 != 0) { sumOddOld += i; } }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  Integer sumOdd = collection.stream().filter(o -> o % 2 != 0).reduce((s1, s2) -> s1 + s2).orElse(0);
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  Integer sumOdd = collection.parallelStream().filter(o -> o % 2 != 0).reduce((s1, s2) -> s1 + s2).orElse(0);
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  System.out.println("Test buildStream start"); //     Stream<String> streamFromValues = Stream.of("a1", "a2", "a3"); System.out.println("streamFromValues = " + streamFromValues.collect(Collectors.toList())); //  streamFromValues = [a1, a2, a3] //     String[] array = {"a1","a2","a3"}; Stream<String> streamFromArrays = Arrays.stream(array); System.out.println("streamFromArrays = " + streamFromArrays.collect(Collectors.toList())); //  streamFromArrays = [a1, a2, a3] Stream<String> streamFromArrays1 = Stream.of(array); System.out.println("streamFromArrays1 = " + streamFromArrays1.collect(Collectors.toList())); //  streamFromArrays = [a1, a2, a3] //     (        ) File file = new File("1.tmp"); file.deleteOnExit(); PrintWriter out = new PrintWriter(file); out.println("a1"); out.println("a2"); out.println("a3"); out.close(); Stream<String> streamFromFiles = Files.lines(Paths.get(file.getAbsolutePath())); System.out.println("streamFromFiles = " + streamFromFiles.collect(Collectors.toList())); //  streamFromFiles = [a1, a2, a3] //     Collection<String> collection = Arrays.asList("a1", "a2", "a3"); Stream<String> streamFromCollection = collection.stream(); System.out.println("streamFromCollection = " + streamFromCollection.collect(Collectors.toList())); //  streamFromCollection = [a1, a2, a3] //      IntStream streamFromString = "123".chars(); System.out.print("streamFromString = "); streamFromString.forEach((e)->System.out.print(e + " , ")); //  streamFromString = 49 , 50 , 51 , System.out.println(); //   Stream.builder Stream.Builder<String> builder = Stream.builder(); Stream<String> streamFromBuilder = builder.add("a1").add("a2").add("a3").build(); System.out.println("streamFromBuilder = " + streamFromBuilder.collect((Collectors.toList()))); //  streamFromFiles = [a1, a2, a3] //    //   Stream.iterate Stream<Integer> streamFromIterate = Stream.iterate(1, n -> n + 2); System.out.println("streamFromIterate = " + streamFromIterate.limit(3).collect(Collectors.toList())); //  streamFromIterate = [1, 3, 5] //   Stream.generate Stream<String> streamFromGenerate = Stream.generate(() -> "a1"); System.out.println("streamFromGenerate = " + streamFromGenerate.limit(3).collect(Collectors.toList())); //  streamFromGenerate = [a1, a1, a1] //    Stream<String> streamEmpty = Stream.empty(); System.out.println("streamEmpty = " + streamEmpty.collect(Collectors.toList())); //  streamEmpty = [] //      Stream<String> parallelStream = collection.parallelStream(); System.out.println("parallelStream = " + parallelStream.collect(Collectors.toList())); //  parallelStream = [a1, a2, a3]
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  // filter -  stream,     ,    // count -      // collect -  stream       // mapToInt -      (,  ) private static void testFilterAndCount() { System.out.println(); System.out.println("Test filter and count start"); Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); Collection<People> peoples = Arrays.asList( new People("", 16, Sex.MAN), new People("", 23, Sex.MAN), new People("", 42, Sex.WOMEN), new People(" ", 69, Sex.MAN) ); //     long count = collection.stream().filter("a1"::equals).count(); System.out.println("count = " + count); //  count = 2 //      List<String> select = collection.stream().filter((s) -> s.contains("1")).collect(Collectors.toList()); System.out.println("select = " + select); //  select = [a1, a1] //  - List<People> militaryService = peoples.stream().filter((p)-> p.getAge() >= 18 && p.getAge() < 27 && p.getSex() == Sex.MAN).collect(Collectors.toList()); System.out.println("militaryService = " + militaryService); //  militaryService = [{name='', age=23, sex=MAN}] //      double manAverageAge = peoples.stream().filter((p) -> p.getSex() == Sex.MAN). mapToInt(People::getAge).average().getAsDouble(); System.out.println("manAverageAge = " + manAverageAge); //  manAverageAge = 36.0 //  -      (..  18        55 ,    60) long peopleHowCanWork = peoples.stream().filter((p) -> p.getAge() >= 18).filter( (p) -> (p.getSex() == Sex.WOMEN && p.getAge() < 55) || (p.getSex() == Sex.MAN && p.getAge() < 60)).count(); System.out.println("peopleHowCanWork = " + peopleHowCanWork); //  manAverageAge = 2 } // findFirst -   Optional    // skip -  N   ( N  ) // collect  stream       private static void testFindFirstSkipCount() { Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); System.out.println("Test findFirst and skip start"); //     String first = collection.stream().findFirst().orElse("1"); System.out.println("first = " + first); //  first = a1 //     String last = collection.stream().skip(collection.size() - 1).findAny().orElse("1"); System.out.println("last = " + last ); //  last = a1 //     String find = collection.stream().filter("a3"::equals).findFirst().get(); System.out.println("find = " + find); //  find = a3 //       String third = collection.stream().skip(2).findFirst().get(); System.out.println("third = " + third); //  third = a3 System.out.println(); System.out.println("Test collect start"); //      List<String> select = collection.stream().filter((s) -> s.contains("1")).collect(Collectors.toList()); System.out.println("select = " + select); //  select = [a1, a1] } //  Limit        private static void testLimit() { System.out.println(); System.out.println("Test limit start"); Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); //     List<String> limit = collection.stream().limit(2).collect(Collectors.toList()); System.out.println("limit = " + limit); //  limit = [a1, a2] //       List<String> fromTo = collection.stream().skip(1).limit(2).collect(Collectors.toList()); System.out.println("fromTo = " + fromTo); //  fromTo = [a2, a3] //     String last = collection.stream().skip(collection.size() - 1).findAny().orElse("1"); System.out.println("last = " + last ); //  last = a1 } private enum Sex { MAN, WOMEN } private static class People { private final String name; private final Integer age; private final Sex sex; public People(String name, Integer age, Sex sex) { this.name = name; this.age = age; this.sex = sex; } public String getName() { return name; } public Integer getAge() { return age; } public Sex getSex() { return sex; } @Override public String toString() { return "{" + "name='" + name + '\'' + ", age=" + age + ", sex=" + sex + '}'; } }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

 //  distinct  stream  ,      (,    list)   ,   -    //  collect  stream       private static void testDistinct() { System.out.println(); System.out.println("Test distinct start"); Collection<String> ordered = Arrays.asList("a1", "a2", "a2", "a3", "a1", "a2", "a2"); Collection<String> nonOrdered = new HashSet<>(ordered); //     List<String> distinct = nonOrdered.stream().distinct().collect(Collectors.toList()); System.out.println("distinct = " + distinct); //  distinct = [a1, a2, a3] -    List<String> distinctOrdered = ordered.stream().distinct().collect(Collectors.toList()); System.out.println("distinctOrdered = " + distinctOrdered); //  distinct = [a1, a2, a3] -   }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  //  anyMatch -  true,         //  noneMatch -  true,         //  allMatch -  true,       private static void testMatch() { System.out.println(); System.out.println("Test anyMatch, allMatch, noneMatch start"); Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); //           boolean isAnyOneTrue = collection.stream().anyMatch("a1"::equals); System.out.println("anyOneTrue " + isAnyOneTrue); //  true boolean isAnyOneFalse = collection.stream().anyMatch("a8"::equals); System.out.println("anyOneFlase " + isAnyOneFalse); //  false //          boolean isAll = collection.stream().allMatch((s) -> s.contains("1")); System.out.println("isAll " + isAll); //  false //    boolean isNotEquals = collection.stream().noneMatch("a7"::equals); System.out.println("isNotEquals " + isNotEquals); //  true }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  //  Map     ,  stream    private static void testMap() { System.out.println(); System.out.println("Test map start"); Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); //     List<String> transform = collection.stream().map((s) -> s + "_1").collect(Collectors.toList()); System.out.println("transform = " + transform); //  transform = [a1_1, a2_1, a3_1, a1_1] //       List<Integer> number = collection.stream().map((s) -> Integer.parseInt(s.substring(1))).collect(Collectors.toList()); System.out.println("number = " + number); //  transform = [1, 2, 3, 1] } //  MapToInt -     ,  stream     private static void testMapToInt() { System.out.println(); System.out.println("Test mapToInt start"); Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); //       int[] number = collection.stream().mapToInt((s) -> Integer.parseInt(s.substring(1))).toArray(); System.out.println("number = " + Arrays.toString(number)); //  number = [1, 2, 3, 1] } //  FlatMap -   Map -    ,    stream  private static void testFlatMap() { System.out.println(); System.out.println("Test flat map start"); Collection<String> collection = Arrays.asList("1,2,0", "4,5"); //    ,      collection String[] number = collection.stream().flatMap((p) -> Arrays.asList(p.split(",")).stream()).toArray(String[]::new); System.out.println("number = " + Arrays.toString(number)); //  number = [1, 2, 0, 4, 5] } //  FlatMapToInt -   MapToInt -    ,    stream  private static void testFlatMapToInt() { System.out.println(); System.out.println("Test flat map start"); Collection<String> collection = Arrays.asList("1,2,0", "4,5"); //     ,      collection int sum = collection.stream().flatMapToInt((p) -> Arrays.asList(p.split(",")).stream().mapToInt(Integer::parseInt)).sum(); System.out.println("sum = " + sum); //  sum = 12 }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  //  Sorted       ,   Comparator private static void testSorted() { System.out.println(); System.out.println("Test sorted start"); // ************    Collection<String> collection = Arrays.asList("a1", "a4", "a3", "a2", "a1", "a4"); //     List<String> sorted = collection.stream().sorted().collect(Collectors.toList()); System.out.println("sorted = " + sorted); //  sorted = [a1, a1, a2, a3, a4, a4] //        List<String> sortedDistinct = collection.stream().sorted().distinct().collect(Collectors.toList()); System.out.println("sortedDistinct = " + sortedDistinct); //  sortedDistinct = [a1, a2, a3, a4] //        List<String> sortedReverse = collection.stream().sorted((o1, o2) -> -o1.compareTo(o2)).collect(Collectors.toList()); System.out.println("sortedReverse = " + sortedReverse); //  sortedReverse = [a4, a4, a3, a2, a1, a1] //           List<String> distinctReverse = collection.stream().sorted((o1, o2) -> -o1.compareTo(o2)).distinct().collect(Collectors.toList()); System.out.println("distinctReverse = " + distinctReverse); //  sortedReverse = [a4, a3, a2, a1] // ************    //    Collection<People> peoples = Arrays.asList( new People("", 16, Sex.MAN), new People("", 23, Sex.MAN), new People("", 42, Sex.WOMEN), new People(" ", 69, Sex.MAN) ); //        Collection<People> byName = peoples.stream().sorted((o1,o2) -> -o1.getName().compareTo(o2.getName())).collect(Collectors.toList()); System.out.println("byName = " + byName); // byName = [{name='', age=23, sex=MAN}, {name=' ', age=69, sex=MAN}, {name='', age=42, sex=WOMEN}, {name='', age=16, sex=MAN}] //    ,     Collection<People> bySexAndAge = peoples.stream().sorted((o1, o2) -> o1.getSex() != o2.getSex() ? o1.getSex(). compareTo(o2.getSex()) : o1.getAge().compareTo(o2.getAge())).collect(Collectors.toList()); System.out.println("bySexAndAge = " + bySexAndAge); // bySexAndAge = [{name='', age=16, sex=MAN}, {name='', age=23, sex=MAN}, {name=' ', age=69, sex=MAN}, {name='', age=42, sex=WOMEN}] } private enum Sex { MAN, WOMEN } private static class People { private final String name; private final Integer age; private final Sex sex; public People(String name, Integer age, Sex sex) { this.name = name; this.age = age; this.sex = sex; } public String getName() { return name; } public Integer getAge() { return age; } public Sex getSex() { return sex; } @Override public String toString() { return "{" + "name='" + name + '\'' + ", age=" + age + ", sex=" + sex + '}'; } @Override public boolean equals(Object o) { if (this == o) return true; if (!(o instanceof People)) return false; People people = (People) o; return Objects.equals(name, people.name) && Objects.equals(age, people.age) && Objects.equals(sex, people.sex); } @Override public int hashCode() { return Objects.hash(name, age, sex); } }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  //  max   ,      //  min   ,      private static void testMinMax() { System.out.println(); System.out.println("Test min and max start"); // ************    Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); //    String max = collection.stream().max(String::compareTo).get(); System.out.println("max " + max); //  a3 //    String min = collection.stream().min(String::compareTo).get(); System.out.println("min " + min); //  a1 // ************     //    Collection<People> peoples = Arrays.asList( new People("", 16, Sex.MAN), new People("", 23, Sex.MAN), new People("", 42, Sex.WOMEN), new People(" ", 69, Sex.MAN) ); //      People older = peoples.stream().max((p1, p2) -> p1.getAge().compareTo(p2.getAge())).get(); System.out.println("older " + older); //  {name=' ', age=69, sex=MAN} //      People younger = peoples.stream().min((p1, p2) -> p1.getAge().compareTo(p2.getAge())).get(); System.out.println("younger " + younger); //  {name='', age=16, sex=MAN} } private enum Sex { MAN, WOMEN } private static class People { private final String name; private final Integer age; private final Sex sex; public People(String name, Integer age, Sex sex) { this.name = name; this.age = age; this.sex = sex; } public String getName() { return name; } public Integer getAge() { return age; } public Sex getSex() { return sex; } @Override public String toString() { return "{" + "name='" + name + '\'' + ", age=" + age + ", sex=" + sex + '}'; } @Override public boolean equals(Object o) { if (this == o) return true; if (!(o instanceof People)) return false; People people = (People) o; return Objects.equals(name, people.name) && Objects.equals(age, people.age) && Objects.equals(sex, people.sex); } @Override public int hashCode() { return Objects.hash(name, age, sex); } }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  //  ForEach             private static void testForEach() { System.out.println(); System.out.println("For each start"); Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); //        System.out.print("forEach = "); collection.stream().map(String::toUpperCase).forEach((e) -> System.out.print(e + ",")); //  forEach = A1,A2,A3,A1, System.out.println(); Collection<StringBuilder> list = Arrays.asList(new StringBuilder("a1"), new StringBuilder("a2"), new StringBuilder("a3")); list.stream().forEachOrdered((p) -> p.append("_new")); System.out.println("forEachOrdered = " + list); //  forEachOrdered = [a1_new, a2_new, a3_new] } //  Peek    ,           private static void testPeek() { System.out.println(); System.out.println("Test peek start"); Collection<String> collection = Arrays.asList("a1", "a2", "a3", "a1"); //        System.out.print("peak1 = "); List<String> peek = collection.stream().map(String::toUpperCase).peek((e) -> System.out.print(e + ",")). collect(Collectors.toList()); System.out.println(); //  peak1 = A1,A2,A3,A1, System.out.println("peek2 = " + peek); //  peek2 = [A1, A2, A3, A1] Collection<StringBuilder> list = Arrays.asList(new StringBuilder("a1"), new StringBuilder("a2"), new StringBuilder("a3")); List<StringBuilder> newList = list.stream().peek((p) -> p.append("_new")).collect(Collectors.toList()); System.out.println("newList = " + newList); //  newList = [a1_new, a2_new, a3_new] }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  //  reduce        (  ,       ..) //    Optional  // map -      (,     ) // mapToInt -      (,    int) private static void testReduce() { System.out.println(); System.out.println("Test reduce start"); // ************     Collection<Integer> collection = Arrays.asList(1, 2, 3, 4, 2); //   Integer sum = collection.stream().reduce((s1, s2) -> s1 + s2).orElse(0); //  stream Api Integer sumOld = 0; //    for(Integer i: collection) { sumOld += i; } System.out.println("sum = " + sum + " : " + sumOld); //  sum = 12 : 12 //   Integer max1 = collection.stream().reduce((s1, s2) -> s1 > s2 ? s1 : s2).orElse(0); //  stream Api Integer max2 = collection.stream().reduce(Integer::max).orElse(0); //  stream Api  Integer::max Integer maxOld = null; //    for(Integer i: collection) { maxOld = maxOld != null && maxOld > i? maxOld: i; } maxOld = maxOld == null? 0 : maxOld; System.out.println("max = " + max1 + " : " + max2 + " : " + maxOld); //  max1 = 4 : 4 : 4 //   Integer min = collection.stream().reduce((s1, s2) -> s1 < s2 ? s1 : s2).orElse(0); //  stream Api Integer minOld = null; //    for(Integer i: collection) { minOld = minOld != null && minOld < i? minOld: i; } minOld = minOld == null? 0 : minOld; System.out.println("min = " + min+ " : " + minOld); //  min = 1 : 1 //    Integer last = collection.stream().reduce((s1, s2) -> s2).orElse(0); //  stream Api Integer lastOld = null; //    for(Integer i: collection) { lastOld = i; } lastOld = lastOld == null? 0 : lastOld; System.out.println("last = " + last + " : " + lastOld); //  last = 2 : 2 //   ,   2 Integer sumMore2 = collection.stream().filter(o -> o > 2).reduce((s1, s2) -> s1 + s2).orElse(0); //  stream Api Integer sumMore2Old = 0; //    for(Integer i: collection) { if(i > 2) { sumMore2Old += i; } } System.out.println("sumMore2 = " + sumMore2 + " : " + sumMore2Old); //  sumMore2 = 7 : 7 //     Integer sumOdd = collection.stream().filter(o -> o % 2 != 0).reduce((s1, s2) -> s1 + s2).orElse(0); //  stream Api Integer sumOddOld = 0; //    for(Integer i: collection) { if(i % 2 != 0) { sumOddOld += i; } } System.out.println("sumOdd = " + sumOdd + " : " + sumOddOld); //  sumOdd = 4 : 4 // ************     //    Collection<People> peoples = Arrays.asList( new People("", 16, Sex.MAN), new People("", 23, Sex.MAN), new People("", 42, Sex.WOMEN), new People(" ", 69, Sex.MAN) ); //     int oldMan = peoples.stream().filter((p) -> p.getSex() == Sex.MAN).map(People::getAge).reduce((s1, s2) -> s1 > s2 ? s1 : s2).get(); System.out.println("oldMan = " + oldMan); //  69 //             int younger = peoples.stream().filter((p) -> p.getName().contains("")).mapToInt(People::getAge).reduce((s1, s2) -> s1 < s2 ? s1 : s2).orElse(0); System.out.println("younger = " + younger); //  23 } private enum Sex { MAN, WOMEN } private static class People { private final String name; private final Integer age; private final Sex sex; public People(String name, Integer age, Sex sex) { this.name = name; this.age = age; this.sex = sex; } public String getName() { return name; } public Integer getAge() { return age; } public Sex getSex() { return sex; } @Override public String toString() { return "{" + "name='" + name + '\'' + ", age=" + age + ", sex=" + sex + '}'; } @Override public boolean equals(Object o) { if (this == o) return true; if (!(o instanceof People)) return false; People people = (People) o; return Objects.equals(name, people.name) && Objects.equals(age, people.age) && Objects.equals(sex, people.sex); } @Override public int hashCode() { return Objects.hash(name, age, sex); } }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  //  collect  stream       //     Collectors: // toList, toCollection, toSet -     ,    // toConcurrentMap, toMap -     map,    // averagingInt, averagingDouble, averagingLong -    // summingInt, summingDouble, summingLong -   // summarizingInt, summarizingDouble, summarizingLong -  SummaryStatistics     // partitioningBy -             Map<Boolean, List> // groupingBy -       Map<N, List<T>>,  T -   , N -   // mapping -      Collector' private static void testCollect() { System.out.println(); System.out.println("Test distinct start"); // ********    Collection<String> strings = Arrays.asList("a1", "b2", "c3", "a1"); //        List<String> distinct = strings.stream().distinct().collect(Collectors.toList()); System.out.println("distinct = " + distinct); //  distinct = [a1, b2, c3] //        String[] array = strings.stream().distinct().map(String::toUpperCase).toArray(String[]::new); System.out.println("array = " + Arrays.asList(array)); //  array = [A1, B2, C3] //         :    <b> ... </b> String join = strings.stream().collect(Collectors.joining(" : ", "<b> ", " </b>")); System.out.println("join = " + join); //  <b> a1 : b2 : c3 : a1 </b> //   map,    ,    Map<String, String> map = strings.stream().distinct().collect(Collectors.toMap((p) -> p.substring(0, 1), (p) -> p.substring(1, 2))); System.out.println("map = " + map); //  map = {a=1, b=2, c=3} //   map,      Map<String, List<String>> groups = strings.stream().collect(Collectors.groupingBy((p) -> p.substring(0, 1))); System.out.println("groups = " + groups); //  groups = {a=[a1, a1], b=[b2], c=[c3]} //   map,               : Map<String, String> groupJoin = strings.stream().collect(Collectors.groupingBy((p) -> p.substring(0, 1), Collectors.mapping((p) -> p.substring(1, 2), Collectors.joining(":")))); System.out.println("groupJoin = " + groupJoin); //  groupJoin = groupJoin = {a=1/1, b=2, c=3} // ********    Collection<Integer> numbers = Arrays.asList(1, 2, 3, 4); //     long sumOdd = numbers.stream().collect(Collectors.summingInt(((p) -> p % 2 == 1 ? p : 0))); System.out.println("sumOdd = " + sumOdd); //  sumEven = 4 //     1    double average = numbers.stream().collect(Collectors.averagingInt((p) -> p - 1)); System.out.println("average = " + average); //  average = 1.5 //    3    IntSummaryStatistics statistics = numbers.stream().collect(Collectors.summarizingInt((p) -> p + 3)); System.out.println("statistics = " + statistics); //  statistics = IntSummaryStatistics{count=4, sum=22, min=4, average=5.500000, max=7} //      IntSummaryStatistics long sumEven = numbers.stream().collect(Collectors.summarizingInt((p) -> p % 2 == 0 ? p : 0)).getSum(); System.out.println("sumEven = " + sumEven); //  sumEven = 6 //       Map<Boolean, List<Integer>> parts = numbers.stream().collect(Collectors.partitioningBy((p) -> p % 2 == 0)); System.out.println("parts = " + parts); //  parts = {false=[1, 3], true=[2, 4]} }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

  //   Collector,        StringBuilder Collector<String,StringBuilder, String> stringBuilderCollector = Collector.of( StringBuilder::new, //    (b ,s) -> b.append(s).append(" , "), //     (b1, b2) -> b1.append(b2).append(" , "), //        StringBuilder::toString // ,     ); String joinBuilder = strings.stream().collect(stringBuilderCollector); System.out.println("joinBuilder = " + joinBuilder); //  joinBuilder = a1 , b2 , c3 , a1 , //  Collector'   JDK7   StringBuilder b = new StringBuilder(); //    for(String s: strings) { b.append(s).append(" , "); //     } String joinBuilderOld = b.toString(); // ,     System.out.println("joinBuilderOld = " + joinBuilderOld); //  joinBuilderOld = a1 , b2 , c3 , a1 , //    toList         Collector<String, List<String>, List<String>> toList = Collector.of( ArrayList::new, //    List::add, //     (l1, l2) -> { l1.addAll(l2); return l1; } //        ); List<String> distinct1 = strings.stream().distinct().collect(toList); System.out.println("distinct1 = " + distinct1); //  distinct1 = [a1, b2, c3]