Java8-Stream-No.10
2024-09-04 16:07:55
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;
public class Streams10 {
static class Person {
String name;
int age;
Person(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public String toString() {
return name;
}
}
public static void main(String[] args) {
List<Person> persons =
Arrays.asList(
new Person("Max", 18),
new Person("Peter", 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: %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
}
}
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