Java StringJoiner

last modified May 31, 2025

This article explores StringJoiner in Java, a useful class for efficiently constructing delimited strings.

StringJoiner helps format sequences of characters by inserting a delimiter between elements. Additionally, it allows customization with an optional prefix and suffix, making it ideal for structured text formatting.

One important detail about StringJoiner is that it only inserts the delimiter between elements, but not at the end of the sequence. This means the last element will not have an extra trailing delimiter, ensuring clean and correct formatting without unnecessary characters.

Internally, StringJoiner is leveraged by the String.join method, providing a simple and efficient way to concatenate strings.

Using StringJoiner

The following example joins numbers with the StringJoiner class.

void main() {

    var joined = new StringJoiner(",");

    joined.add("1");
    joined.add("2");
    joined.add("3");
    joined.add("4");
    joined.add("5");

    System.out.println(joined);
}

The example concatenates five numbers and prints the final string to the console.

var joined = new StringJoiner(",");

A new instance of the StringJoiner class is created. The comma character is used as a delimiter.

joined.add("1");
joined.add("2");
joined.add("3");
joined.add("4");
joined.add("5");

Five values are added with the add method.

System.out.println(join);

The StringJoiner is converted to a string and printed to the console.

$ java Main.java
1,2,3,4,5

The String.join method

In the second example, we join strings with the String.join method.

void main() {

    var joined = String.join("/", "2024", "7", "1");
    System.out.println(joined);
}

The String.join method internally uses the StringJoiner.

var joined = String.join("/", "2024", "7", "1");

A date is concatenated with the String.join method.

$ java Main.java
2016/8/5

Joining list

The third example concatenates elements of a list.

void main() {

    var words = List.of("Today", "is", "a", "beautiful", "day");
    var joined = String.join(" ", words);

    System.out.println(joined);
}

A list can be passed as an argument to the String.join method.

var joined = String.join(" ", words);

The elements of the list are joined with a single space character.

$ java Main.java
Today is a beautiful day

Reading CSV file

The following example reads numbers from a CSV file and later joins them with a StringJoiner.

13,24,35,16,50

This is the numbers.csv file.

void main() throws FileNotFoundException {

    var fileName = "data.csv";

    StringJoiner joined;

    try (var scanner = new Scanner(new File(fileName))) {
        scanner.useDelimiter(",");

        joined = new StringJoiner("|");

        while (scanner.hasNext()) {

            joined.add(scanner.next());
        }
    }

    System.out.println(joined);
}

The example reads CSV file, containing numbers, and joins them with a StringJoiner using a different delimiter.

try (var scanner = new Scanner(new File(fileName))) {
    scanner.useDelimiter(",");

The values are read with the Scanner class. The numbers are separated by a comma character so we set the comma delimiter with the useDelimiter method.

var joined = new StringJoiner("|");

A StringJoiner class is instantiated with a "|" delimiter.

while (scanner.hasNext()) {

    join.add(scanner.next());
}

We retrieve the values with the scanner and concatenate them with the joiner.

Writing CSV file

The next example writes numbers to a CSV file.

void main() throws IOException {

    var fileName = "data.csv";
    var joined = new StringJoiner(",");

    joined.add("21");
    joined.add("43");
    joined.add("54");
    joined.add("76");
    joined.add("98");

    var newFile = new File(fileName);
    boolean created = newFile.createNewFile();

    if (created) {
        try (var pw = new PrintWriter(newFile)) {

            pw.write(joined.toString());
        }
    }
}

The example joins five numbers with a StringJoiner and writes the concatendated string to a CSV file.

var joined = new StringJoiner(",");

joined.add("21");
joined.add("43");
joined.add("54");
joined.add("76");
joined.add("98");

Five numbers are concatenated with the StringJoiner. The numbers are separated with a comma character.

var newFile = new File(fileName);
boolean created = newFile.createNewFile();

A new file object is created in the current working directory.

if (created) {
    try (var pw = new PrintWriter(newFile)) {

        pw.write(joined.toString());
    }
}

The joined values are written to the file.

The Collectors.joining method

Tthe Collectors.joining method returns a Collector that concatenates the input elements, separated by the specified delimiter, in encounter order.

void main() {

    Stream<String> stream = Stream.of("Jan", "Peter", "Robert");

    String names = stream.collect(Collectors.joining(" "));
    System.out.println(names);
}

The example uses the stream API to concatenate three names.

$ java Main.java
Jan Peter Robert

Source

Java StringJoiner - language reference

In this article we have covered StringJoiner and Collectors.joining.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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