Java 8 Streams | Collectors.joining() method with Examples
Last Updated : 17 Jan, 2023
The joining() method of Collectors Class, in Java, is used to join various elements of a character or string array into a single string object. This method uses the stream to do so. There are various overloads of joining methods present in the Collector class. The class hierarchy is as follows:
java.lang.Object ↳ java.util.stream.Collectors
joining()
java.util.stream.Collectors.joining() is the most simple joining method which does not take any parameter. It returns a Collector that joins or concatenates the input streams into String in the order of their appearance.
Syntax:
public static Collector<CharSequence, ?, String> joining()
Illustration: Usage of joining() method
Program 1: Using joining() with an array of characters
In the below program, a character array is created in 'ch'. Then this array is fed to be converted into Stream using Stream.of(). Then the resulted stream is mapped for a sequential series using map(). At last, the sequential stream containing the character array is joined into a String using Collectors.joining() method. It is stored in the 'chString' variable.
Example
Java // Java Program to demonstrate the working // of the Collectors.joining() method import java.util.stream.Collectors; import java.util.stream.Stream; // Class public class GFG { // Main driver method public static void main(String[] args) { // Creating a custom character array char[] ch = { 'G', 'e', 'e', 'k', 's', 'f', 'o', 'r', 'G', 'e', 'e', 'k', 's' }; // Converting character array into string // using joining() method of Collectors class String chString = Stream.of(ch) .map(arr -> new String(arr)) .collect(Collectors.joining()); // Printing concatenated string System.out.println(chString); } } Program 2: Using joining() with a list of characters
In the below program, a character list is created in 'ch'. Then this list is fed to be converted into Stream using ch.stream() method. Then the resulted stream is mapped for a sequential series using map(). At last, the sequential stream containing the character list is joined into a String using Collectors.joining() method. It is stored in 'chString' variable.
Example
Java // Java Program to demonstrate Working of joining() Method // of Collectors Class // Importing required classes import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; // Main class public class GFG { // Main driver method public static void main(String[] args) { // Creating a character list List<Character> ch = Arrays.asList( 'G', 'e', 'e', 'k', 's', 'f', 'o', 'r', 'G', 'e', 'e', 'k', 's'); // Converting character list into string // using joining() method of Collectors class String chString = ch.stream() .map(String::valueOf) .collect(Collectors.joining()); // Printing the concatenated string System.out.println(chString); } } Program 3: Using joining() with n list of string
In the below program, a String list is created in 'str'. Then this list is fed to be converted into Stream using str.stream() method. Then the resulted stream is mapped for a sequential series using map(). At last, the sequential stream containing the character list is joined into a String using Collectors.joining() method. It is stored in 'chString' variable.
Example
Java // Java Program to demonstrate the working // of the Collectors.joining() method import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class GFG { public static void main(String[] args) { // Create a string list List<String> str = Arrays.asList("Geeks", "for", "Geeks"); // Convert the string list into String // using Collectors.joining() method String chString = str.stream().collect(Collectors.joining()); // Print the concatenated String System.out.println(chString); } } Output:
GeeksforGeeks
joining(delimiter)
java.util.stream.Collectors.joining(CharSequence delimiter) is an overload of joining() method which takes delimiter as a parameter, of the type CharSequence. A delimiter is a symbol or a CharSequence that is used to separate words from each other. For example, in every sentence, space ' ' is used as the default delimiter for the words in it. It returns a Collector that joins or concatenates the input elements into String in the order of their appearance, separated by the delimiter.
Syntax:
public static Collector<CharSequence, ?, String> joining(CharSequence delimiter)
Below are the illustration for how to use joining(delimiter) method:
Program 1: Using joining(delimiter) with a list of characters: In the below program, a character list is created in 'ch'. Then this list is fed to be converted into Stream using ch.stream() method. Then the resulted stream is mapped for a sequential series using map(). At last, the sequential stream containing the character list is joined into a String using Collectors.joining() method with ", " passed as the delimiter. It is stored in 'chString' variable.
Java // Java Program to demonstrate the working // of the Collectors.joining() method import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class GFG { public static void main(String[] args) { // Create a character list List<Character> ch = Arrays.asList('G', 'e', 'e', 'k', 's', 'f', 'o', 'r', 'G', 'e', 'e', 'k', 's'); // Convert the character list into String // using Collectors.joining() method // with, as the delimiter String chString = ch.stream() .map(String::valueOf) .collect(Collectors.joining(", ")); // Print the concatenated String System.out.println(chString); } } Output:
G, e, e, k, s, f, o, r, G, e, e, k, s
Program 2: Using joining(delimiter) with a list of string:
In the below program, a String list is created in 'str'. Then this list is fed to be converted into Stream using str.stream() method. Then the resulted stream is mapped for a sequential series using map(). At last, the sequential stream containing the character list is joined into a String using Collectors.joining() method with ", " passed as the delimiter. It is stored in 'chString' variable.
Java // Java Program to demonstrate the working // of the Collectors.joining() method import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class GFG { public static void main(String[] args) { // Create a string list List<String> str = Arrays.asList("Geeks", "for", "Geeks"); // Convert the string list into String // using Collectors.joining() method String chString = str.stream().collect( Collectors.joining(", ")); // Print the concatenated String System.out.println(chString); } } Output:
Geeks, for, Geeks
joining(delimiter, prefix, suffix)
java.util.stream.Collectors.joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix) is an overload of joining() method which takes delimiter, prefix and suffix as parameter, of the type CharSequence. A delimiter is a symbol or a CharSequence that is used to separate words from each other. A prefix is a symbol or a CharSequence that is joined at the starting of the 1st element of the String. Then suffix is also a CharSequence parameter but this is joined after the last element of the string. i.e. at the end. For example, in every {Geeks, for, Geeks}, space ' ' is used as the by default delimiter for the words in it. The '{' is the prefix and '}' is the suffix. It returns a Collector that joins or concatenates the input elements into String in the order of their appearance, separated by the delimiter.
Syntax:
public static Collector<CharSequence, ?, String> joining(CharSequence delimiter. CharSequence prefix, CharSequence suffix))
Below are the illustration for how to use joining(delimiter, prefix, suffix) method:
Program 1: Using joining() with a list of characters: In the below program, a character list is created in 'ch'. Then this list is fed to be converted into Stream using ch.stream() method. Then the resulted stream is mapped for a sequential series using map(). At last, the sequential stream containing the character list is joined into a String using Collectors.joining() method with ", " passed as the delimiter, "[" as the prefix and "]" as the suffix. It is stored in 'chString' variable.
Java // Java Program to demonstrate the working // of the Collectors.joining() method import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class GFG { public static void main(String[] args) { // Create a character list List<Character> ch = Arrays.asList('G', 'e', 'e', 'k', 's', 'f', 'o', 'r', 'G', 'e', 'e', 'k', 's'); // Convert the character list into String // using Collectors.joining() method // with, as the delimiter String chString = ch.stream() .map(String::valueOf) .collect(Collectors.joining(", ", "[", "]")); // Print the concatenated String System.out.println(chString); } } Output:
[G, e, e, k, s, f, o, r, G, e, e, k, s]
Program 2: Using joining() with a list of string: In the below program, a String list is created in 'str'. Then this list is fed to be converted into Stream using str.stream() method. Then the resulted stream is mapped for a sequential series using map(). At last, the sequential stream containing the character list is joined into a String using Collectors.joining() method with ", " passed as the delimiter, "{" as the prefix and "}" as the suffix. It is stored in 'chString' variable.
Java // Java Program to demonstrate the working // of the Collectors.joining() method import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class GFG { public static void main(String[] args) { // Create a string list List<String> str = Arrays.asList("Geeks", "for", "Geeks"); // Convert the string list into String // using Collectors.joining() method String chString = str.stream().collect(Collectors.joining(", ", " {", "} ")); // Print the concatenated String System.out.println(chString); } } Output:
{Geeks, for, Geeks} Similar Reads
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