The Collectors.joining() method is a handy utility in the java.util.stream.Collectors class that provides a Collector which concatenates input elements from a stream into a String.
Here are three versions of Collectors.joining():
joining(): Concatenates the input elements, separated by the empty string.joining(CharSequence delimiter): Concatenates the input elements, separated by the specified delimiter.joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix): Concatenates the input elements, separated by the delimiter, with the specified prefix and suffix.
Let’s see an example of each:
package org.kodejava.stream;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class CollectorsJoining {
public static void main(String[] args) {
// Using joining()
String joined1 = Stream.of("Hello", "world")
.collect(Collectors.joining());
System.out.println(joined1);
// Using joining(CharSequence delimiter)
String joined2 = Stream.of("Hello", "world")
.collect(Collectors.joining(" "));
System.out.println(joined2);
// Using joining(CharSequence delimiter, CharSequence prefix,
// CharSequence suffix)
String joined3 = Stream.of("Hello", "world")
.collect(Collectors.joining(", ", "[", "]"));
System.out.println(joined3);
}
}
Output:
Helloworld
Hello world
[Hello, world]
In these examples, we use Stream.of() to create a stream of strings, and Collectors.joining() to concatenate them together, with or without delimiters, prefix, and suffix as needed.
Now, let’s consider we have a Person class and a list of Person objects. We want to join the names of all persons. Here is how to do that:
package org.kodejava.stream;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
class Person {
private final String name;
public Person(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
public class CollectorsJoiningObjectProperty {
public static void main(String[] args) {
List<Person> people = Arrays.asList(
new Person("John"),
new Person("Rosa"),
new Person("Doe")
);
String names = people.stream()
.map(Person::getName)
.collect(Collectors.joining(", "));
System.out.println(names);
}
}
Output:
John, Rosa, Doe
In the above example, we start with a List of Person objects. We create a stream from the list, then use the map() function to transform each Person into a String (their name). The collect() method is then used with Collectors.joining(), which joins all the names together into one String, separated by commas.