Reduce a list of string add prefix only to last element using Java 8 Streams

Question

How can i reduce or collect a list of string delimitted by comma and prefixed "and" only to the last element using Java 8 Streams?

eg.

List<String> ls = Arrays.asList("tom","terry","john","kevin","steve");

String result = ls.stream().map(String::toString)
                .collect(Collectors.joining(", "));

System.out.println(result);

This statement prints => tom, terry, john, kevin, steve. But i wanted to print the list as tom, terry, john, kevin and steve.

Answer 1

Two solutions.

Clumsy, taking sublist actually:

    String result = ls.stream()
            .limit(ls.size() - 1)
            .collect(Collectors.joining(", ")) + " and " + ls.get(ls.size() - 1);

Using a class to shift one place:

    class SbS {
        StringBuilder sb = new StringBuilder();
        String s = "";

        @Override
        public String toString() {
            return sb + (s.empty() ? "" : " and " + s);
        }
    }
    result = ls.stream()
            .collect(SbS::new,
                (a, s) -> {
                    a.sb.append(a.sb.length() == 0 ? "" : ", ").append(a.s); a.s = s;
                },
                (a1, a2) -> {}).toString();

Mind: the class definition should be placed inside the method, just as above. Otherwise static would be needed. The lambda for the (a, s) could be a method in SbS. The support for parallelism is left as excercise to the reader (a1, a1).

Answer 2

Streams do not make this easier. You might just as well use String::replaceAll:

String input = "tom, terry, john, kevin, steve";
String result = input.replaceAll(", (\\w+)$", " and $1");

System.out.println(result);

Output:

tom, terry, john, kevin and steve

Answer 3

Stream API does not provide a standard way to achieve this. My StreamEx library which extends standard Stream API has a mapLast method which is helpful here. It allows to map only last stream element leaving everything else as is:

System.out.println(StreamEx.of(ls).mapLast("and "::concat).joining(", "));

The result is the following:

tom, terry, john, kevin, and steve

Note that it adds a comma before "and" which is ok (see Oxford Comma). If you don't like it (or actually using another language which forbids such punctuation), then it can be achieved in slightly longer manner:

StreamEx.of(ls)
        .mapLastOrElse(", "::concat, " and "::concat)
        .joining().substring(", ".length());