Add collector examples

Author: winterbe

src/com/winterbe/java8/Streams10.java

@@ -1,8 +1,11 @@
 package com.winterbe.java8;
 
 import java.util.Arrays;
+import java.util.IntSummaryStatistics;
 import java.util.List;
 import java.util.Map;
+import java.util.StringJoiner;
+import java.util.stream.Collector;
 import java.util.stream.Collectors;
 
 /**
@@ -26,23 +29,154 @@ public String toString() {
  }
 
  public static void main(String[] args) {
- test1();
- }
-
- private static void test1() {
  List<Person> persons =
  Arrays.asList(
  new Person("Max", 18),
  new Person("Peter", 23),
- new Person("Brenda", 23),
+ new Person("Pamela", 23),
  new Person("David", 12));
 
+// test1(persons);
+// test2(persons);
+// test3(persons);
+// test4(persons);
+// test5(persons);
+// test6(persons);
+// test7(persons);
+// test8(persons);
+ test9(persons);
+ }
+
+ private static void test1(List<Person> persons) {
+ List<Person> filtered =
+ persons
+ .stream()
+ .filter(p -> p.name.startsWith("P"))
+ .collect(Collectors.toList());
+
+ System.out.println(filtered); // [Peter, Pamela]
+ }
+
+ private static void test2(List<Person> persons) {
  Map<Integer, List<Person>> personsByAge = persons
  .stream()
  .collect(Collectors.groupingBy(p -> p.age));
 
  personsByAge
- .forEach((age, p) -> System.out.format("age: %s; persons: %s\n", age, p));
+ .forEach((age, p) -> System.out.format("age %s: %s\n", age, p));
+
+ // age 18: [Max]
+ // age 23:[Peter, Pamela]
+ // age 12:[David]
+ }
+
+ private static void test3(List<Person> persons) {
+ Double averageAge = persons
+ .stream()
+ .collect(Collectors.averagingInt(p -> p.age));
+
+ System.out.println(averageAge); // 19.0
+ }
+
+ private static void test4(List<Person> persons) {
+ IntSummaryStatistics ageSummary =
+ persons
+ .stream()
+ .collect(Collectors.summarizingInt(p -> p.age));
+
+ System.out.println(ageSummary);
+ // IntSummaryStatistics{count=4, sum=76, min=12, average=19,000000, max=23}
+ }
+
+ private static void test5(List<Person> persons) {
+ String names = persons
+ .stream()
+ .filter(p -> p.age >= 18)
+ .map(p -> p.name)
+ .collect(Collectors.joining(" and ", "In Germany ", " are of legal age."));
+
+ System.out.println(names);
+ // In Germany Max and Peter and Pamela are of legal age.
+ }
+
+ private static void test6(List<Person> persons) {
+ Map<Integer, String> map = persons
+ .stream()
+ .collect(Collectors.toMap(
+ p -> p.age,
+ p -> p.name,
+ (name1, name2) -> name1 + ";" + name2));
+
+ System.out.println(map);
+ // {18=Max, 23=Peter;Pamela, 12=David}
+ }
+
+ private static void test7(List<Person> persons) {
+ Collector<Person, StringJoiner, String> personNameCollector =
+ Collector.of(
+ () -> new StringJoiner(" | "), // supplier
+ (j, p) -> j.add(p.name.toUpperCase()), // accumulator
+ (j1, j2) -> j1.merge(j2), // combiner
+ StringJoiner::toString); // finisher
+
+ String names = persons
+ .stream()
+ .collect(personNameCollector);
+
+ System.out.println(names); // MAX | PETER | PAMELA | DAVID
  }
 
+ private static void test8(List<Person> persons) {
+ 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); // MAX | PETER | PAMELA | DAVID
+ }
+
+ private static void test9(List<Person> persons) {
+ 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
+ .parallelStream()
+ .collect(personNameCollector);
+
+ System.out.println(names); // MAX | PETER | PAMELA | DAVID
+ }
 }