public static void main(String[] args) {

List<User> persons = List.of(

new User("Robert", 28),

new User("Peter", 37),

new User("Lucy", 23),

new User("David", 28));

Collector<User, StringJoiner, String> personNameCollector =

Collector.of(

() -> new StringJoiner(" | "), // supplier

(j, p) -> j.add(p.getName()), // accumulator

(j1, j2) -> j1.merge(j2), // combiner

StringJoiner::toString); // finisher

String names = persons

.stream()

.collect(personNameCollector);

System.out.println(names);

}

private String name;

private int age;

User(String name, int age) {

this.name = name;

this.age = age;

}

public String getName() {

return this.name;

}

@Override

public String toString() {

return String.format("%s is %d years old", name, age);

}