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Optimised codes for various java files in the stream folder. #85

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196 changes: 73 additions & 123 deletions src/com/winterbe/java8/samples/stream/Streams10.java
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Original file line number Diff line number Diff line change
Expand Up @@ -4,18 +4,18 @@
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;

/**
* @author Benjamin Winterberg
* Optimized version of Streams10.
*/
public class Streams10 {

// ✅ Make fields final (immutability = safer + cleaner)
static class Person {
String name;
int age;
final String name;
final int age;

Person(String name, int age) {
this.name = name;
Expand All @@ -29,154 +29,104 @@ public String toString() {
}

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);
List<Person> persons = List.of(
new Person("Max", 18),
new Person("Peter", 23),
new Person("Pamela", 23),
new Person("David", 12)
);

filterByName(persons);
groupByAge(persons);
averageAge(persons);
ageStatistics(persons);
joiningNames(persons);
mapByAge(persons);
customCollector(persons);
parallelCollector(persons);
}

private static void test1(List<Person> persons) {
List<Person> filtered =
persons
.stream()
.filter(p -> p.name.startsWith("P"))
.collect(Collectors.toList());
// ✅ 1. Filter
private static void filterByName(List<Person> persons) {
List<Person> result = persons.stream()
.filter(p -> p.name.startsWith("P"))
.toList(); // Java 16+

System.out.println(filtered); // [Peter, Pamela]
System.out.println(result);
}

private static void test2(List<Person> persons) {
Map<Integer, List<Person>> personsByAge = persons
.stream()
.collect(Collectors.groupingBy(p -> p.age));
// ✅ 2. Grouping
private static void groupByAge(List<Person> persons) {
Map<Integer, List<Person>> grouped =
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]
grouped.forEach((age, p) ->
System.out.printf("age %d: %s%n", age, p));
}

private static void test3(List<Person> persons) {
Double averageAge = persons
.stream()
// ✅ 3. Average
private static void averageAge(List<Person> persons) {
double avg = persons.stream()
.collect(Collectors.averagingInt(p -> p.age));

System.out.println(averageAge); // 19.0
System.out.println(avg);
}

private static void test4(List<Person> persons) {
IntSummaryStatistics ageSummary =
persons
.stream()
.collect(Collectors.summarizingInt(p -> p.age));
// ✅ 4. Statistics
private static void ageStatistics(List<Person> persons) {
IntSummaryStatistics stats = persons.stream()
.collect(Collectors.summarizingInt(p -> p.age));

System.out.println(ageSummary);
// IntSummaryStatistics{count=4, sum=76, min=12, average=19,000000, max=23}
System.out.println(stats);
}

private static void test5(List<Person> persons) {
String names = persons
.stream()
// ✅ 5. Joining (cleaner mapping)
private static void joiningNames(List<Person> persons) {
String result = persons.stream()
.filter(p -> p.age >= 18)
.map(p -> p.name)
.collect(Collectors.joining(" and ", "In Germany ", " are of legal age."));
.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.
System.out.println(result);
}

private static void test6(List<Person> persons) {
Map<Integer, String> map = persons
.stream()
// ✅ 6. toMap with merge function
private static void mapByAge(List<Person> persons) {
Map<Integer, String> map = persons.stream()
.collect(Collectors.toMap(
p -> p.age,
p -> p.name,
(name1, name2) -> name1 + ";" + name2));
(n1, n2) -> n1 + ";" + n2
));

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);
// ✅ 7. Custom collector (simplified)
private static void customCollector(List<Person> persons) {
String result = persons.stream()
.map(p -> p.name.toUpperCase())
.collect(Collectors.joining(" | "));

System.out.println(names); // MAX | PETER | PAMELA | DAVID
System.out.println(result);
}

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
}
// ✅ 8. Parallel-safe collector
private static void parallelCollector(List<Person> persons) {
Collector<Person, ?, String> collector =
Collectors.mapping(
p -> p.name.toUpperCase(),
Collectors.joining(" | ")
);

String result = persons.parallelStream()
.collect(collector);

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
System.out.println(result);
}
}
}
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