Stream equivalent for appending dash(-) after every 3rd char of string

Question

I have created simple for loop based solution to append dash(-) on every third digit.

 private String appendedDashOnEachThirdDigit(String digitString) {
    char digits[] = digitString.toCharArray();
    StringBuilder builder = new StringBuilder();

    for (int i = 0; i < digits.length; i++) {
      char ch = digits[i];
      builder.append(ch);
      if ((i + 1) % 3 == 0 && i - 1 != digits.length) {
        builder.append('-');
      }
    }
    return builder.toString();
  }

Now I want to create equivalent java 8 stream based solution. This is very basic case and I want to know how this can be done with stream api?

I have created this basic stream function:-

  private String appendedDashOnEachThirdDigitByStream(String digitString) {
    //char[] chars = digitString.toCharArray();
    //return IntStream.range(0, chars.length)
    //    .filter(i -> (i + 1) % 3 == 0 && i - 1 != chars.length)
    //    .collect(Collectors.collectingAndThen(Collectors.toList(),
    //        new Function<List<Object>, Object>() {
    //        }))
   return digitString.chars()
        .mapToObj(value -> String.valueOf((char) value))
        .collect(Collectors.joining("-"));
  }

https://stackoverflow.com/questions/537174/putting-char-into-a-java-string-for-each-n-characters — xingbin, Dec 31 '18 at 09:30
That's not even a particularly good way to do it without streams. — Andy Turner, Dec 31 '18 at 09:41

score 0 · Accepted Answer · answered Dec 31 '18 at 10:07

While it is possible it is not best solution.

    class T {
        final int i;
        final int c;

        public T(int i, int c) {

            this.i = i;
            this.c = c;
        }
    }
    int[] i = {0};


    String res = "123456789".chars()
            .mapToObj(c -> new T(i[0]++, c))
            .collect(Collectors.groupingBy(x -> x.i / 3))
            .entrySet()
            .stream()
            .sorted(Comparator.comparing(Map.Entry::getKey))
            .map(x ->
                    x.getValue().stream().mapToInt(y -> y.c)
                            .mapToObj(y -> String.valueOf((char)y))
                            .collect(Collectors.joining())

            ).collect(Collectors.joining("-"));


    System.out.println("res = " + res);

Main problem here is that stream is good when your elements is independent. In your scenario processing depend on position so it required wired grouping.

This works but definitely it is hard for me to read compared to for loop version. Its hard to interpret! But thanks for your help I have accepted your answer! :) — Kailash Dabhi, Dec 31 '18 at 10:46

Stream equivalent for appending dash(-) after every 3rd char of string

1 Answers1