joining

Learn string joining with Collectors.joining.

Joining strings: joining

Collectors.joining merges the strings in a stream into a single string. Its simplest form takes no delimiter and just concatenates the strings. The delimiter version inserts a separator between elements.

Example: names.stream().collect(Collectors.joining(", ")). This call returns a single string such as "Alex, Mia, Sam". The delimiter is placed only between elements, never at the start or end. The result is always a String.

joining with prefix and suffix

The three-argument joining overload takes a delimiter, a prefix and a suffix. This is handy for turning a list into a one-line text snippet.

Example: Collectors.joining(", ", "[", "]"). This call returns a string such as "[Alex, Mia, Sam]". The delimiter sits between elements, the prefix at the front and the suffix at the end. For an empty stream only the prefix and suffix are joined; the delimiter is never used.

Exercise Join names with a comma

Create a list of three names. Join the stream with Collectors.joining(", ") and print the result with System.out.println. The output must be a single line.

Show example solution

import java.util.List;
import java.util.stream.Collectors;

public class JoinNames {
    public static void main(String[] args) {
        List<String> names = List.of("Alex", "Mia", "Sam");
        String result = names.stream().collect(Collectors.joining(", "));
        System.out.println(result);
    }
}

Test yourself

What does names.stream().collect(Collectors.joining(", ")) return?
- String
- Map<String, String>
- List<String>
- Optional<String>
Show answer

String — joining always returns a String.

Complete the joining call

String result = names.stream()
    .collect(Collectors.joining(____));

Show answer

String result = names.stream()
    .collect(Collectors.joining(", "));

Match the joining overloads

joining()

Concatenates directly
joining(delim)

Separates with delimiter
joining(delim, pre, suf)

Adds prefix and suffix

Next lesson: reduce

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