Java Stream How to - Output the steps for supplier, accumulator, combiner and finisher

Question

We would like to know how to output the steps for supplier, accumulator, combiner and finisher.

Answer

//  w  w w  . ja  va2s  .  c o m
import java.util.Arrays;
import java.util.List;
import java.util.StringJoiner;
import java.util.stream.Collector;

public class Main {

  public static void main(String[] args) throws Exception {
    List<Person> persons =
        Arrays.asList(
            new Person("Max", 18),
            new Person("Peter", 23),
            new Person("Pamela", 23),
            new Person("David", 12));   


    Collector<Person, StringJoiner, String> personNameCollector =
        Collector.of(
            () -> {
                System.out.println("supplier");
                return new StringJoiner(" | ");
            },
            (j, p) -> {
                System.out.format("accumulator: p=%s; j=%s\n", p, j);
                j.add(p.name.toUpperCase());
            },
            (j1, j2) -> {
                System.out.println("merge");
                return j1.merge(j2);
            },
            j -> {
                System.out.println("finisher");
                return j.toString();
            });

    String names = persons
        .stream()
        .collect(personNameCollector);

    System.out.println(names); 
  }

}
class Person {
  String name;
  int age;

  Person(String name, int age) {
      this.name = name;
      this.age = age;
  }

  @Override
  public String toString() {
      return name;
  }
}

The code above generates the following result.