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winterbewinterbe
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Add collector examples
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‎src/com/winterbe/java8/Streams10.java

Lines changed: 140 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -1,8 +1,11 @@
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package com.winterbe.java8;
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import java.util.Arrays;
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import java.util.IntSummaryStatistics;
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import java.util.List;
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import java.util.Map;
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import java.util.StringJoiner;
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import java.util.stream.Collector;
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import java.util.stream.Collectors;
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/**
@@ -26,23 +29,154 @@ public String toString() {
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}
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public static void main(String[] args) {
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test1();
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}
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private static void test1() {
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List<Person> persons =
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Arrays.asList(
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new Person("Max", 18),
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new Person("Peter", 23),
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new Person("Brenda", 23),
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new Person("Pamela", 23),
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new Person("David", 12));
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// test1(persons);
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// test2(persons);
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// test3(persons);
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// test4(persons);
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// test5(persons);
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// test6(persons);
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// test7(persons);
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// test8(persons);
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test9(persons);
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}
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private static void test1(List<Person> persons) {
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List<Person> filtered =
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persons
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.stream()
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.filter(p -> p.name.startsWith("P"))
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.collect(Collectors.toList());
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System.out.println(filtered); // [Peter, Pamela]
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}
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private static void test2(List<Person> persons) {
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Map<Integer, List<Person>> personsByAge = persons
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.stream()
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.collect(Collectors.groupingBy(p -> p.age));
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personsByAge
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.forEach((age, p) -> System.out.format("age: %s; persons: %s\n", age, p));
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.forEach((age, p) -> System.out.format("age %s: %s\n", age, p));
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// age 18: [Max]
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// age 23:[Peter, Pamela]
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// age 12:[David]
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}
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private static void test3(List<Person> persons) {
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Double averageAge = persons
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.stream()
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.collect(Collectors.averagingInt(p -> p.age));
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System.out.println(averageAge); // 19.0
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}
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private static void test4(List<Person> persons) {
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IntSummaryStatistics ageSummary =
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persons
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.stream()
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.collect(Collectors.summarizingInt(p -> p.age));
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System.out.println(ageSummary);
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// IntSummaryStatistics{count=4, sum=76, min=12, average=19,000000, max=23}
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}
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private static void test5(List<Person> persons) {
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String names = persons
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.stream()
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.filter(p -> p.age >= 18)
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.map(p -> p.name)
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.collect(Collectors.joining(" and ", "In Germany ", " are of legal age."));
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System.out.println(names);
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// In Germany Max and Peter and Pamela are of legal age.
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}
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private static void test6(List<Person> persons) {
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Map<Integer, String> map = persons
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.stream()
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.collect(Collectors.toMap(
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p -> p.age,
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p -> p.name,
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(name1, name2) -> name1 + ";" + name2));
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System.out.println(map);
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// {18=Max, 23=Peter;Pamela, 12=David}
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}
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private static void test7(List<Person> persons) {
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Collector<Person, StringJoiner, String> personNameCollector =
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Collector.of(
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() -> new StringJoiner(" | "), // supplier
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(j, p) -> j.add(p.name.toUpperCase()), // accumulator
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(j1, j2) -> j1.merge(j2), // combiner
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StringJoiner::toString); // finisher
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String names = persons
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.stream()
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.collect(personNameCollector);
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System.out.println(names); // MAX | PETER | PAMELA | DAVID
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}
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private static void test8(List<Person> persons) {
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Collector<Person, StringJoiner, String> personNameCollector =
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Collector.of(
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() -> {
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System.out.println("supplier");
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return new StringJoiner(" | ");
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},
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(j, p) -> {
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System.out.format("accumulator: p=%s; j=%s\n", p, j);
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j.add(p.name.toUpperCase());
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},
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(j1, j2) -> {
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System.out.println("merge");
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return j1.merge(j2);
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},
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j -> {
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System.out.println("finisher");
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return j.toString();
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});
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String names = persons
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.stream()
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.collect(personNameCollector);
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System.out.println(names); // MAX | PETER | PAMELA | DAVID
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}
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private static void test9(List<Person> persons) {
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Collector<Person, StringJoiner, String> personNameCollector =
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Collector.of(
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() -> {
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System.out.println("supplier");
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return new StringJoiner(" | ");
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},
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(j, p) -> {
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System.out.format("accumulator: p=%s; j=%s\n", p, j);
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j.add(p.name.toUpperCase());
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},
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(j1, j2) -> {
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System.out.println("merge");
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return j1.merge(j2);
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},
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j -> {
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System.out.println("finisher");
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return j.toString();
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});
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String names = persons
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.parallelStream()
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.collect(personNameCollector);
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System.out.println(names); // MAX | PETER | PAMELA | DAVID
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}
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}

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