Java String join() method

The join() method of String class is introduced in Java 8. This method is used to concatenate strings with specified delimiter. It is also used to combine multiple elements of a collection with the specified separator. In this guide, we will see several programs to discuss Java String join() method.

Note: Java 8 also introduced a new StringJoiner class for the same purpose.

Syntax

There are two main overloaded versions of the String.join() method:

Joining CharSequence elements

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

Joining Iterable elements

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

Parameters

delimiter: The delimiter that separates each element, for example comma(,), space(” “) etc. are delimiters.
elements: The sequence of elements to be joined.

Examples of Java String join() method

1. Joining Strings with a Delimiter

In the following example, we are concatenating the strings with the specified delimiter.

public class StringJoinExample {
    public static void main(String[] args) {
        // Here , is a delimiter and elements are strings
        String result = String.join(", ", "Cricket", "Hockey", "Tennis");
        System.out.println(result); // Output: "Cricket, Hockey, Tennis"
    }
}

2. Joining Elements of a List (Collection)

import java.util.Arrays;
import java.util.List;

public class StringJoinExample {
    public static void main(String[] args) {
        List<String> sports = Arrays.asList("Cricket", "Hockey", "Tennis");
        // Here, we are joining elements of an ArrayList with 
        // comma as delimiter
        String result = String.join(", ", sports);
        System.out.println(result); // Output: "Cricket", "Hockey", "Tennis"
    }
}

3. Joining Varargs of Strings

In the following example, we are using String.join() method with a variable number of string arguments. Refer: Java varargs Tutorial

public class StringJoinExample {
    public static void main(String[] args) {
        String result = String.join(" | ", "Java", "Python", "C++", "JavaScript");
        System.out.println(result); // Output: "Java | Python | C++ | JavaScript"
    }
}

4. Joining Elements of a Collection

Here, we are using String.join() method to concatenate the elements of a HashSet.

import java.util.HashSet;

public class StringJoinHashSetExample {
    public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();
        fruits.add("apple");
        fruits.add("banana");
        fruits.add("cherry");
        fruits.add("date");

        // Joining the elements of HashSet with comma as delimiter
        String result = String.join(", ", fruits);
        
        // Output: "banana, cherry, apple, date" 
        // (order can be different)
        System.out.println(result); 
    }
}

Practical Uses of String join() method

CSV Generation: This method can be used to join elements with a comma to generate CSV records.
Logging: Concatenating multiple values with a separator for logging purposes.

❮ Java String Class