Java StringJoiner Class

Last Updated : 10 Jan, 2025

StringJoiner class in Java provides an efficient way to concatenate multiple strings with a defined delimiter(character), optional prefix, and suffix. This class is especially useful when constructing formatted strings dynamically.

Example 1:

Java

// Demonstration of StringJoiner Class 
import java.util.StringJoiner;

public class Geeks{
    public static void main(String[] args) {
      
        // Create a StringJoiner with a delimiter
        StringJoiner sj = new StringJoiner(", ");

        // Add strings which you need to join
        sj.add("geeks").add("for").add("geeks");

        System.out.println("Joined String: " + sj.toString());
    }
}

Output

Joined String: geeks, for, geeks

Explanation: In the above example, we have created a StringJoiner object with a comma and space as the delimiter. The toString() method is used to make the addition of string with the specified delimiter.

Example 2: Here, we will use the StringJoiner to join elements of an array of Strings.

Java

// Using StringJoiner with Array of Strings
import java.util.StringJoiner;

public class Geeks {
    public static void main(String[] args) {
        String[] s1 = {"Geeks", "for", "Geeks"};

        // Create StringJoiner with a delimiter
        StringJoiner sj = new StringJoiner(", ");

        // Adding array elements to StringJoiner
        for (String s2 : s1) {
            sj.add(s2);
        }

        System.out.println(sj.toString());
    }
}

Output

Geeks, for, Geeks

Key Features:

Delimiters: Define a separator between the strings.
Prefix and Suffix: Add optional characters before and after the final joined string.
Chaining: Easily chain calls to add elements for a clean and concise implementation.

Constructors of StringJoiner Class

1. StringJoiner(CharSequence delimiter): It creates a StringJoiner with a specified delimiter.

Syntax:

public StringJoiner(CharSequence delimiter)

2. StringJoiner(CharSequence delimiter, CharSequence prefix, CharSequence suffix): It creates a StringJoiner with a delimiter, prefix, and suffix.

Syntax:

public StringJoiner(CharSequence delimiter, CharSequence prefix, CharSequence suffix)

Methods of StringJoiner Class

Method	Action Performed
add()	Adds a string to the StringJoiner.
length()	Returns the length of the joined string.
merge()	Merges the contents of another StringJoiner into the current one.
toString()	Returns the final joined string.
setEmptyValue()	Specifies a default value if no elements are added.

Example:

Java

// Demostrating different methods 
// of StringJoiner Class
import java.util.StringJoiner;

public class Geeks {
    public static void main(String[] args) {
        
        // add() method
        StringJoiner sj1 = new StringJoiner(", ");
        sj1.add("geeks").add("for").add("geeks");
        System.out.println("StringJoiner with elements: " + sj1.toString());
        
        // setEmptyValue() method 
        StringJoiner sj2 = new StringJoiner(", ");
        sj2.setEmptyValue("No elements to join");
        System.out.println("Empty StringJoiner: " + sj2.toString());  
        
        // toString() method
        StringJoiner sj3 = new StringJoiner(" | ", "[", "]");
        sj3.add("geeks").add("for").add("geeks");
        System.out.println("StringJoiner with prefix and suffix: " + sj3.toString());
        
        // Checking the length of the joined string
        StringJoiner sj4 = new StringJoiner(", ");
        sj4.add("geeks").add("for").add("geeks");
        System.out.println("Length of joined string: " + sj4.length());
        
    }
}

Output

StringJoiner with elements: geeks, for, geeks
Empty StringJoiner: No elements to join
StringJoiner with prefix and suffix: [geeks | for | geeks]
Length of joined string: 17