Join strings with different last delimiter

If you are looking for old Java solution, using Guava libraries would be easy.

    List<String> values = Arrays.asList("a", "b", "c");
    String output = Joiner.on(",").join(values);
    output = output.substring(0, output.lastIndexOf(","))+" and "+values.get(values.size()-1);
    System.out.println(output);//a,b and c

Try joining the last 2 strings first with stream.collect(Collectors.joining(" and "))

Then join all remaining strings and this new string with the code you used in your question: stream.collect(Collectors.joining(", ")).

If they are already in a list, no stream is needed; simply join a sublist of all but the last element and concat the other delimiter and the final element:

int last = list.size() - 1;
String joined = String.join(" and ",
                    String.join(", ", list.subList(0, last)),
                    list.get(last));

Here's a version that does the above using Collectors.collectingAndThen:

stream.collect(Collectors.collectingAndThen(Collectors.toList(),
    joiningLastDelimiter(", ", " and ")));

public static Function<List<String>, String> joiningLastDelimiter(
        String delimiter, String lastDelimiter) {
    return list -> {
                int last = list.size() - 1;
                if (last < 1) return String.join(delimiter, list);
                return String.join(lastDelimiter,
                    String.join(delimiter, list.subList(0, last)),
                    list.get(last));
            };
}

This version can also handle the case where the stream is empty or only has one value. Thanks to Holger and Andreas for their suggestions which greatly improved this solution.

I had suggested in a comment that the Oxford comma could be accomplished with this using ", " and ", and" as the delimiters, but that yields incorrect results of "a, and b" for two elements, so just for fun here's one that does Oxford commas correctly:

stream.collect(Collectors.collectingAndThen(Collectors.toList(),
    joiningOxfordComma()));

public static Function<List<String>, String> joiningOxfordComma() {
    return list -> {
                int last = list.size() - 1;
                if (last < 1) return String.join("", list);
                if (last == 1) return String.join(" and ", list);
                return String.join(", and ",
                    String.join(", ", list.subList(0, last)),
                    list.get(last));
            };
}

If you're fine with "a, b, and c", then it's possible to use mapLast method of my StreamEx library which extends standard Stream API with additional operations:

String result = StreamEx.of("a", "b", "c")
                        .mapLast("and "::concat)
                        .joining(", "); // "a, b, and c"

The mapLast method applies given mapping to the last stream element keeping others unchanged. I even have similar unit-test.