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Guide to Stream.reduce() (#6480) 

* Initial Commit

* Update Application.java

* Update Application.java
This commit is contained in:
Alejandro Gervasio 2019-03-08 11:20:52 -03:00 committed by Eugen
parent c81651ba42
commit ae6c29dd19
6 changed files with 286 additions and 0 deletions
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39
java-streams-2/pom.xml Normal file
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<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.baeldung.javastreams2</groupId>
<artifactId>javastreams2</artifactId>
<version>1.0</version>
<packaging>jar</packaging>
<dependencies>
<dependency>
<groupId>org.openjdk.jmh</groupId>
<artifactId>jmh-core</artifactId>
<version>1.21</version>
</dependency>
<dependency>
<groupId>org.openjdk.jmh</groupId>
<artifactId>jmh-generator-annprocess</artifactId>
<version>1.21</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
<type>jar</type>
</dependency>
<dependency>
<groupId>org.assertj</groupId>
<artifactId>assertj-core</artifactId>
<version>3.11.1</version>
<scope>test</scope>
</dependency>
</dependencies>
<name>Stream Reduce</name>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
</properties>
</project>

39
java-streams-2/src/main/java/com/baeldung/reduce/application/Application.java Normal file
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package com.baeldung.reduce.application;
import com.baeldung.reduce.entities.User;
import com.baeldung.reduce.utilities.NumberUtils;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Application {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int result1 = numbers.stream().reduce(0, (subtotal, element) -> subtotal + element);
System.out.println(result1);
int result2 = numbers.stream().reduce(0, Integer::sum);
System.out.println(result2);
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result3 = letters.stream().reduce("", (partialString, element) -> partialString + element);
System.out.println(result3);
String result4 = letters.stream().reduce("", String::concat);
System.out.println(result4);
String result5 = letters.stream().reduce("", (partialString, element) -> partialString.toUpperCase() + element.toUpperCase());
System.out.println(result5);
List<User> users = Arrays.asList(new User("John", 30), new User("Julie", 35));
int result6 = users.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
System.out.println(result6);
String result7 = letters.parallelStream().reduce("", String::concat);
System.out.println(result7);
int result8 = users.parallelStream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
System.out.println(result8);
}
}

52
java-streams-2/src/main/java/com/baeldung/reduce/benchmarks/JMHStreamReduceBenchMark.java Normal file
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package com.baeldung.reduce.benchmarks;
import com.baeldung.reduce.entities.User;
import java.util.ArrayList;
import java.util.List;
import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.Scope;
import org.openjdk.jmh.annotations.State;
import org.openjdk.jmh.runner.Runner;
import org.openjdk.jmh.runner.RunnerException;
import org.openjdk.jmh.runner.options.Options;
import org.openjdk.jmh.runner.options.OptionsBuilder;
@State(Scope.Thread)
@BenchmarkMode(Mode.AverageTime)
public class JMHStreamReduceBenchMark {
private final List<User> userList = createUsers();
public static void main(String[] args) throws RunnerException {
Options options = new OptionsBuilder()
.include(JMHStreamReduceBenchMark.class.getSimpleName()).threads(1)
.forks(1).shouldFailOnError(true).shouldDoGC(true)
.jvmArgs("-server").build();
new Runner(options).run();
}
private List<User> createUsers() {
List<User> users = new ArrayList<>();
for (int i = 0; i <= 1000000; i++) {
users.add(new User("John" + i, i));
}
return users;
}
@Benchmark
public Integer executeReduceOnParallelizedStream() {
return this.userList
.parallelStream()
.reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
}
@Benchmark
public Integer executeReduceOnSequentialStream() {
return this.userList
.stream()
.reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
}
}

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java-streams-2/src/main/java/com/baeldung/reduce/entities/User.java Normal file
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package com.baeldung.reduce.entities;
public class User {
private final String name;
private final int age;
public User(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
@Override
public String toString() {
return "User{" + "name=" + name + ", age=" + age + '}';
}
}

52
java-streams-2/src/main/java/com/baeldung/reduce/utilities/NumberUtils.java Normal file
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package com.baeldung.reduce.utilities;
import java.util.List;
import java.util.function.BiFunction;
import java.util.logging.Level;
import java.util.logging.Logger;
public abstract class NumberUtils {
private static final Logger LOGGER = Logger.getLogger(NumberUtils.class.getName());
public static int divideListElements(List<Integer> values, Integer divider) {
return values.stream()
.reduce(0, (a, b) -> {
try {
return a / divider + b / divider;
} catch (ArithmeticException e) {
LOGGER.log(Level.INFO, "Arithmetic Exception: Division by Zero");
}
return 0;
});
}
public static int divideListElementsWithExtractedTryCatchBlock(List<Integer> values, int divider) {
return values.stream().reduce(0, (a, b) -> divide(a, divider) + divide(b, divider));
}
public static int divideListElementsWithApplyFunctionMethod(List<Integer> values, int divider) {
BiFunction<Integer, Integer, Integer> division = (a, b) -> a / b;
return values.stream().reduce(0, (a, b) -> applyFunction(division, a, divider) + applyFunction(division, b, divider));
}
private static int divide(int value, int factor) {
int result = 0;
try {
result = value / factor;
} catch (ArithmeticException e) {
LOGGER.log(Level.INFO, "Arithmetic Exception: Division by Zero");
}
return result;
}
private static int applyFunction(BiFunction<Integer, Integer, Integer> function, int a, int b) {
try {
return function.apply(a, b);
}
catch(Exception e) {
LOGGER.log(Level.INFO, "Exception thrown!");
}
return 0;
}
}

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java-streams-2/src/test/java/com/baeldung/reduce/tests/StreamReduceUnitTest.java Normal file
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package com.baeldung.reduce.tests;
import com.baeldung.reduce.entities.User;
import com.baeldung.reduce.utilities.NumberUtils;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import static org.assertj.core.api.Assertions.assertThat;
import org.junit.Test;
public class StreamReduceUnitTest {
@Test
public void givenIntegerList_whenReduceWithSumAccumulatorLambda_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int result = numbers.stream().reduce(0, (subtotal, element) -> subtotal + element);
assertThat(result).isEqualTo(21);
}
@Test
public void givenIntegerList_whenReduceWithSumAccumulatorMethodReference_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int result = numbers.stream().reduce(0, Integer::sum);
assertThat(result).isEqualTo(21);
}
@Test
public void givenStringList_whenReduceWithConcatenatorAccumulatorLambda_thenCorrect() {
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters.stream().reduce("", (partialString, element) -> partialString + element);
assertThat(result).isEqualTo("abcde");
}
@Test
public void givenStringList_whenReduceWithConcatenatorAccumulatorMethodReference_thenCorrect() {
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters.stream().reduce("", String::concat);
assertThat(result).isEqualTo("abcde");
}
@Test
public void givenStringList_whenReduceWithUppercaseConcatenatorAccumulator_thenCorrect() {
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters.stream().reduce("", (partialString, element) -> partialString.toUpperCase() + element.toUpperCase());
assertThat(result).isEqualTo("ABCDE");
}
@Test
public void givenUserList_whenReduceWithAgeAccumulatorAndSumCombiner_thenCorrect() {
List<User> users = Arrays.asList(new User("John", 30), new User("Julie", 35));
int result = users.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
assertThat(result).isEqualTo(65);
}
@Test
public void givenStringList_whenReduceWithParallelStream_thenCorrect() {
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters.parallelStream().reduce("", String::concat);
assertThat(result).isEqualTo("abcde");
}
@Test
public void givenNumberUtilsClass_whenCalledDivideListElements_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
assertThat(NumberUtils.divideListElements(numbers, 1)).isEqualTo(21);
}
@Test
public void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlock_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 1)).isEqualTo(21);
}
@Test
public void givenStream_whneCalleddivideListElementsWithApplyFunctionMethod_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
assertThat(NumberUtils.divideListElementsWithApplyFunctionMethod(numbers, 1)).isEqualTo(21);
}
}