Java 8 Streams | Collectors.joining() method with Examples

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 

Lazyroar

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

Lazyroar

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

Output:

Lazyroar

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. 

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. 

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. 

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. 

Output:

{Geeks, for, Geeks}