Java String join() Method Example Tutorial

By Ankit Lathiya Last updated Jan 10, 2020

Java String join() is an inbuilt method that is used to join character sequences with a specific delimiter. The delimiter is added between each different character sequence to combine all the character sequences into a single String. If you want to convert array to string, then Java String join() method is useful.

Java String join()

Java string join() method returns a string joined with the given delimiter. In the string join method, the delimiter is copied for each element.

In the case of a null element, “null” is added.

See the following figure.

See the following syntax.

public static String join(CharSequence delimiter, CharSequence... elements)

or 

public static String join(CharSequence delimiter, Iterable<? Extends CharSequence> elements)

Parameters:

delimiter: This is what the character sequences are joined with each other.

elements: The character sequences in question. This can also be entered utilizing the Collections framework extending Iterable.

Returns:

The concatenated String.

Throws:

NullPointerException if the delimiter is null. However, if the character sequence is null, the String “null” is added in place of it.

See the following code example.

public class Example1 {
  public static void main(String[] args) {
    String s1 = String.join("+", "One", "One");

    System.out.println(s1);
  }
}

Output

Let’s see an example two.

public class Example2 {
  public static void main(String[] args) {
    String s1 = String.join(null, "One", "One");

    System.out.println(s1);
  }
}

Output

In the above example, we got the output NullPointerException.

Let’s see the third example.

public class Example3 {
  public static void main(String[] args) {
    String s1 = String.join("+", null, "One");

    System.out.println(s1);
  }
}

Output

See the next example.

public class Example4 {
  public static void main(String[] args) {
    String s1 = null;

    String s2 = String.join("+", s1, "One");
    System.out.println(s2);
  }
}

Output

The following example demonstrates syntax of the form public static String join(CharSequence delimiter, Iterable<? Extends CharSequence> elements) with the use of an ArrayList.

See the following code.

import java.util.*;

public class Example5 {
  public static void main(String[] args) {

    ArrayList<String> arrlst = new ArrayList<>();
    arrlst.add("One");
    arrlst.add("Two");
    arrlst.add("Three");
    arrlst.add("Four");
    arrlst.add("Five");

    String s1 = String.join("+", arrlst);
    System.out.println(s1);

  }
}