Streams - Beginning Java 8 Language Features - page 640

■ the toMap() method returns a non-concurrent map that has performance overhead when streams are
processed in parallel. It has a companion method called toConcurrentMap() that returns a concurrent collector that
should be used when streams are processed in parallel.

Tip

Joining Strings Using Collectors
The joining() method of the Collectors class returns a collector that concatenates the elements of a stream of
CharSequence and returns the result as a String . The concatenation occurs in the encounter order. The joining()
method is overloaded and it has three versions:

•

joining()

•

joining(CharSequence delimiter)

joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
The version with no arguments simply concatenates all elements. The second version uses a delimiter to be used
between two elements. The third version uses a delimiter, a prefix and a suffix. The prefix is added to the beginning of
the result and the suffix is added to end of the result. Listing 13-10 shows how to use the joining() method.

•

Listing 13-10. Joining a Stream of CharSequence Using a Collector

// CollectJoiningTest.java
package com.jdojo.streams;

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

public class CollectJoiningTest {
public static void main(String[] args) {
List<Person> persons = Person.persons();
String names = persons.stream()
.map(Person::getName)
.collect(Collectors.joining());

String delimitedNames = persons.stream()
.map(Person::getName)
.collect(Collectors.joining(", "));

String prefixedNames = persons.stream()
.map(Person::getName)
.collect(Collectors.joining(", ", "Hello ", ". Goodbye."));

System.out.println("Joined names: " + names);
System.out.println("Joined, delimited names: " + delimitedNames);
System.out.println(prefixedNames);
}
}