Join strings with different last delimiter

问题:

Using stream.collect(Collectors.joining(", ")) I can easily join all the strings of my stream delimited by a comma. A possible result would be "a, b, c". But what if I want the last delimiter to be different. For example to be " and " such that I get "a, b and c" as result. Is there an easy solution?

回答1:

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));
            };
}

回答2:

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.

回答3:

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(", ")).

回答4:

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

回答5:

    List<String> names = Arrays.asList("Thomas", "Pierre", "Yussef", "Rick");
    int length = names.size();
    String result = IntStream.range(0, length - 1).mapToObj(i -> {
        if (i == length - 2) {
            return names.get(i) + " and " + names.get(length - 1);
        } else {
            return names.get(i);
        }
    }).collect(Collectors.joining(", "));

回答6:

This is not a Streams-API solution but is pretty fast. Enjoy!

public static final <E> String join(
    Iterable<E> objects, String separator, String lastSeparator) 
{
    Objects.requireNonNull(objects);

    final String sep = separator == null ? "" : separator;
    final String lastSep = lastSeparator == null ? sep : lastSeparator;

    final StringBuilder builder = new StringBuilder();

    final Iterator<E> iterator = objects.iterator();
    while (iterator.hasNext()) {
        final E next = iterator.next();
        if (builder.length() > 0) {
            if (iterator.hasNext()) {
                builder.append(sep);
            }
            else {
                builder.append(lastSep);
            }
        }
        builder.append(next);
    }

    return builder.toString();
}

回答7:

String str = "a , b , c , d";
String what_you_want = str.substring(0, str.lastIndexOf(","))
        + str.substring(str.lastIndexOf(",")).replace(",", "and");

// what_you_want is : a , b , c and d