How to join strings with a separator

Short answer

String.join() glues elements together with a separator:

void main() {
    var names = List.of("Anna", "Oleg", "Boris");
    IO.println(String.join(", ", names));
}

Output:

Anna, Oleg, Boris

Non-string values

String.join() accepts only strings. Convert numbers and objects to strings in a stream first and collect with Collectors.joining():

void main() {
    var numbers = List.of(1, 2, 3);
    String text = numbers.stream()
            .map(String::valueOf)
            .collect(Collectors.joining(" + "));
 
    IO.println(text);
}

Output:

1 + 2 + 3

Prefix and suffix

joining() has a version with a start and an end:

void main() {
    var tags = List.of("java", "sql");
    IO.println(tags.stream().collect(Collectors.joining(", ", "[", "]")));
}

Output:

[java, sql]

Joining in a loop

If elements arrive one by one, use StringJoiner — it adds separators only between elements:

void main() {
    var joiner = new StringJoiner(" | ");
    for (int i = 1; i <= 3; i++) {
        joiner.add("item " + i);
    }
    IO.println(joiner);
}

Output:

item 1 | item 2 | item 3

Common mistake

Concatenating strings with += in a big loop. Each += creates a new string and the code gets slow. Use StringBuilder for building strings in a loop:

void main() {
    var builder = new StringBuilder();
    for (int i = 0; i < 5; i++) {
        builder.append(i);
    }
    IO.println(builder);
}

Output:

Thanks for reading!

Short answer

Non-string values

Prefix and suffix

Joining in a loop

Common mistake

Functions used in this recipe

Thanks for reading!