How to print collection elements separated by a delimeter, without creating a large string?

Question

Very similar question was close due someone mark it as a duplicate of question: "A quick and easy way to join array elements with a separator (the opposite of split) in Java".

I expected that producing a huge String that represents collection is not a best approach. Just provide me a method implementation. Thanks.

Answer 1

If you just need to print the elements and that you worried about building a huge String just for printing it, you can still write a good old for loop that prints the elements one by one followed by the separator.

By that I mean that the new Java 8 features does not make these constructions deprecated.

For example, you could write a utility method if needed:

public static void printCollection(Collection<?> coll, String delimiter) {
    int size = coll.size();
    int i = 0;
    for(Object elem : coll) {
        System.out.print(elem);
        if(++i < size) {
            System.out.print(delimiter);
        }
    }
}

With Java 8, you could maybe make this sligthly more compact:

public static void printCollection(Collection<?> coll, String delimiter) {
    //you can still avoid the map call and use two print statements in the forEach
    coll.stream().limit(coll.size()-1).map(o -> o+delimiter).forEach(System.out::print);
    coll.stream().skip(coll.size()-1).forEach(System.out::print);
}

Answer 2

In general case the input collection could contains thousands or millions of elements, so better approach than producing a huge String object as a result would be to direct printing into a specific output stream.

• in worst case it leads to exceeding the heap memory, in other words method/app could be terminated with java.lang.OutOfMemoryError
• in other case list could not be transformed into a String because it could exceeded the maximum String length This approach let you to print ArrayList elements separated by coma directly to a specific PrintStream (e.g. System.out).

In my dump implementation I simply iterate through collection. Printing steps are separated to avoid creating a new String for each concat operation (element.toString() + delimiter). As @Pshemo notice Stream API approach "will end up printing delimiter even after last element".

    public static <T> void nicePrint(final Collection<T> collection,
                                     final PrintStream printStream,
                                     final Optional<String> delimiter) {
        final Iterator<T> iterator = collection.iterator();
        while (iterator.hasNext()) {
            printStream.print(iterator.next());
            if (iterator.hasNext()) {
                delimiter.ifPresent(printStream::print);
            }
        }
    }

Example of use:

final String DELIMITER = ", ";
nicePrint(Arrays.asList(1, 2, 3), printStream, Optional.of(DELIMITER));

Answer 3

From what I understand, you are looking for Scala's mkString like ability.

public static String mkString(Stream<String> s, String delimeter){
    return s.collect(Collectors.joining(delimeter));
}

System.out.println(print(Stream.of("1", "2", "3"), ", "));
//prints: 1, 2, 3