Java StringJoiner

last modified July 5, 2024

In this article we cover Java StringJoiner.

StringJoiner is used to construct a sequence of characters separated by a delimiter and optionally starting with a supplied prefix and ending with a supplied suffix.

StringJoiner is also used internally by the join method of the String class.

Using StringJoiner

The following example joins numbers with the StringJoiner class.

import java.util.StringJoiner;

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.

import java.util.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.

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.StringJoiner;

void main() throws FileNotFoundException {

    var fileName = "src/main/resources/numbers.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.

import java.io.File;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.StringJoiner;

void main() throws IOException {

    var fileName = "src/main/resources/numbers2.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.

import java.util.stream.Collectors;
import java.util.stream.Stream;

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 many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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