StringJoiner Class in Java

Last Updated: 13 September, 2023

In Java, StringJoiner is a final class introduced in Java 8 as part of the java.util package.

StringJoiner class is used to construct a sequence of characters (strings) separated by a delimiter like a comma (,), hyphen (-), etc. and optionally starting with a supplied prefix and ending with a given suffix.

The StringJoiner class provides a convenient way to build a sequence of strings separated by a delimiter.

StringJoiner Class Declaration:

  public final class StringJoiner extends Object

Constructors of StringJoiner Class

1. public StringJoiner (CharSequence delimiter)

It constructs a StringJoiner with no characters in it, with no prefix or suffix, and a copy of the supplied delimiter.

Parameter:

delimiter - the sequence of characters to be used between each element added to the StringJoiner value

Throws:

It throws a NullPointerException if the delimiter is null.

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

It constructs a StringJoiner with no characters in it using copies of the supplied prefix, delimiter and suffix.

Parameters:

delimiter - the sequence of characters to be used between each element added to the StringJoiner.
prefix - the sequence of characters to be used at the beginning.
suffix - the sequence of characters to be used at the end.

Throws:

It throws a NullPointerException if the prefix, delimiter, or suffix is null.

Methods of StringJoiner Class

Methods	Description
public StringJoiner add (CharSequence newElement)	Adds a copy of the given CharSequence value as the next element of the StringJoiner value. If newElement is null, then “null” is added.
public int length()	Returns the length of the String representation of this StringJoiner.
public StringJoiner merge (StringJoiner other)	Adds the contents of the given StringJoiner without prefix and suffix as the next element if it is non-empty.
public String toString()	Returns the String object of this StringJoiner.
public StringJoiner setEmptyValue (CharSequence emptyValue)	Sets the string to be used when determining the string representation of this StringJoiner, and no elements have been added yet; that is when it is empty.

Example 1: Java StringJoiner class example

  
  import java.util.StringJoiner;
  public class StringJoinerExample1 {
    public static void main(String[] args) {
      StringJoiner frutisJoiner = new StringJoiner(","); // passing comma(,) as delimiter
      // Adding values to StringJoiner
      frutisJoiner.add("Apple");
      frutisJoiner.add("Mango");
      frutisJoiner.add("Orange");
      frutisJoiner.add("Grapes");
      frutisJoiner.add("Pear");
      System.out.println(frutisJoiner);
      
      StringJoiner vegJoiner = new StringJoiner("|"); // passing comma(,) as delimiter
      
      // Adding values to StringJoiner
      vegJoiner.add("Potato");
      vegJoiner.add("Onion");
      vegJoiner.add("Tomato");
      vegJoiner.add("Pea");
      vegJoiner.add("Brinjal");
      System.out.println(vegJoiner);
    }
  }

Output

Apple,Mango,Orange,Grapes,Pear
Potato|Onion|Tomato|Pea|Brinjal

Example 2: Java StringJoiner class example

  
  import java.util.StringJoiner;
  public class StringJoinerExample2 {
    public static void main(String[] args) {
      /*
      * Passing comma(,) as delimiter and opening bracket "(" as prefix and closing
      * bracket ")" as suffix
      */
      StringJoiner frutisJoiner = new StringJoiner(",", "(", ")");
      
      // Joining multiple strings by using add() method
      frutisJoiner.add("Apple");
      frutisJoiner.add("Mango");
      frutisJoiner.add("Orange");
      frutisJoiner.add("Grapes");
      frutisJoiner.add("Pear");
      
      // Printing StringJoiner
      System.out.println(frutisJoiner);
    }
  }

Output

(Apple,Mango,Orange,Grapes,Pear)

Example 3: Java StringJoiner class example

  
  import java.util.StringJoiner;
  // Merging two StringJoiner objects
  public class StringJoinerExample3 {
    public static void main(String[] args) {
      /*
      * Passing comma(,) as delimiter and 
      * opening bracket "(" as prefix and closing bracket ")" as suffix
      */
      StringJoiner frutisJoiner = new StringJoiner(",", "(", ")");
      
      frutisJoiner.add("Apple");
      frutisJoiner.add("Mango");
      frutisJoiner.add("Orange");
      frutisJoiner.add("Grapes");
      frutisJoiner.add("Pear");
      
      System.out.println("Fruits: " + frutisJoiner);
      
      /*
      * Passing hyphen(-) as delimiter and 
      * string "pre" as prefix and string "suff" as suffix
      */
      StringJoiner vegJoiner = new StringJoiner("-", "pre", "suff");
      
      vegJoiner.add("Potato");
      vegJoiner.add("Onion");
      vegJoiner.add("Tomato");
      vegJoiner.add("Pea");
      vegJoiner.add("Brinjal");
      
      System.out.println("Vegetables: " + vegJoiner);
      
      /*
      * Merging both the strings The important point to note here is that the output
      * string will be having the delimiter prefix and suffix of the first string
      * (the string which is calling the merge method of StringJoiner)
      */
      StringJoiner mergedString = frutisJoiner.merge(vegJoiner);
      System.out.println("Fruits + Vegetables: "+ mergedString);
    }
  }

Output

Fruits: (Apple,Mango,Orange,Grapes,Pear)
Vegetables: prePotato-Onion-Tomato-Pea-Brinjalsuff
Fruits + Vegetables: (Apple,Mango,Orange,Grapes,Pear,Potato-Onion-Tomato-Pea-Brinjal)

Example 4: Java StringJoiner class example

  
  import java.util.StringJoiner;
  public class StringJoinerExample4 {
    public static void main(String[] args) {
      StringJoiner frutisJoiner = new StringJoiner(","); // passing comma(,) as delimiter
      
      // Prints nothing because it is empty
      System.out.println("StringJoiner: " + frutisJoiner); // Prints nothing because StringJoiner is empty. Print blank line as an output
      
      // We can set default empty value.
      frutisJoiner.setEmptyValue("It is an empty.");
      System.out.println(frutisJoiner);
      
      // Adding values to StringJoiner
      frutisJoiner.add("Apple");
      frutisJoiner.add("Mango");
      frutisJoiner.add("Orange");
      frutisJoiner.add("Grapes");
      frutisJoiner.add("Pear");
      
      // Printing StringJoiner
      System.out.println(frutisJoiner);
      
      // Printing length of StringJoiner using length() method
      System.out.println("Length: " + frutisJoiner.length());
      
      // Converting StringJoiner to String
      String str = frutisJoiner.toString();
      System.out.println(str);
      
      // Using String class methods over the converted object
      char ch = str.charAt(3);
      System.out.println("Character at index 3: " + ch);
      
      // Adding one more element
      frutisJoiner.add("Coconut");
      System.out.println(frutisJoiner);
      
      // After adding element, printing updated length of StringJoiner
      int newLength = frutisJoiner.length();
      System.out.println("New Length: " + newLength);
    }
  }

Output

StringJoiner:
It is an empty.
Apple,Mango,Orange,Grapes,Pear
Length: 30
Apple,Mango,Orange,Grapes,Pear
Character at index 3: l
Apple,Mango,Orange,Grapes,Pear,Coconut
New Length: 38

Reference: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/StringJoiner.html