[JAVA-12051] Moved code to core-java-streams-simple for ebook (#14889)

core-java-modules/core-java-streams-simple/README.md

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+## Java Streams Ebook
+
+This module contains articles about Streams that are part of the Java Streams Ebook.
+
+### Relevant Articles
+
+- [Introduction to Java 8 Streams](https://www.baeldung.com/java-8-streams-introduction)
+- [Guide to Java 8’s Collectors](https://www.baeldung.com/java-8-collectors)
+- [Java Stream Filter with Lambda Expression](https://www.baeldung.com/java-stream-filter-lambda)
+- [Working With Maps Using Streams](https://www.baeldung.com/java-maps-streams)
+- [The Difference Between map() and flatMap()](https://www.baeldung.com/java-difference-map-and-flatmap)
+- [When to Use a Parallel Stream in Java](https://www.baeldung.com/java-when-to-use-parallel-stream)
+- [Guide to Java 8 groupingBy Collector](https://www.baeldung.com/java-groupingby-collector)
+- [Guide to Stream.reduce()](https://www.baeldung.com/java-stream-reduce)

core-java-modules/core-java-streams-simple/pom.xml

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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>
+    <artifactId>core-java-streams-simple</artifactId>
+    <name>core-java-streams-simple</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.pivovarit</groupId>
+            <artifactId>throwing-function</artifactId>
+            <version>${throwing-function.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${commons-lang3.version}</version>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-streams-simple</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+            </plugin>
+        </plugins>
+    </build>
+
+    <properties>
+        <throwing-function.version>1.5.1</throwing-function.version>
+        <maven.compiler.source.version>17</maven.compiler.source.version>
+        <maven.compiler.target.version>17</maven.compiler.target.version>
+        <maven.compiler.release>17</maven.compiler.release>
+    </properties>
+
+</project>

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/filter/Customer.java

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+package com.baeldung.streams.filter;
+
+import java.io.IOException;
+import java.net.HttpURLConnection;
+import java.net.URL;
+
+import javax.net.ssl.HttpsURLConnection;
+
+public class Customer {
+    private String name;
+    private int points;
+    private String profilePhotoUrl;
+
+    public Customer(String name, int points) {
+        this(name, points, "");
+    }
+
+    public Customer(String name, int points, String profilePhotoUrl) {
+        this.name = name;
+        this.points = points;
+        this.profilePhotoUrl = profilePhotoUrl;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public int getPoints() {
+        return points;
+    }
+
+    public boolean hasOver(int points) {
+        return this.points > points;
+    }
+
+    public boolean hasOverHundredPoints() {
+        return this.points > 100;
+    }
+
+    public boolean hasValidProfilePhoto() throws IOException {
+        URL url = new URL(this.profilePhotoUrl);
+        HttpsURLConnection connection = (HttpsURLConnection) url.openConnection();
+        return connection.getResponseCode() == HttpURLConnection.HTTP_OK;
+    }
+
+    public boolean hasValidProfilePhotoWithoutCheckedException() {
+        try {
+            URL url = new URL(this.profilePhotoUrl);
+            HttpsURLConnection connection = (HttpsURLConnection) url.openConnection();
+            return connection.getResponseCode() == HttpURLConnection.HTTP_OK;
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/groupingby/BlogPost.java

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+package com.baeldung.streams.groupingby;
+
+import java.util.IntSummaryStatistics;
+
+
+public class BlogPost {
+
+    private String title;
+    private String author;
+    private BlogPostType type;
+    private int likes;
+    record AuthPostTypesLikes(String author, BlogPostType type, int likes) {};
+    record PostCountTitlesLikesStats(long postCount, String titles, IntSummaryStatistics likesStats){};
+    record TitlesBoundedSumOfLikes(String titles, int boundedSumOfLikes) {};
+
+    public BlogPost(String title, String author, BlogPostType type, int likes) {
+        this.title = title;
+        this.author = author;
+        this.type = type;
+        this.likes = likes;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public String getAuthor() {
+        return author;
+    }
+
+    public BlogPostType getType() {
+        return type;
+    }
+
+    public int getLikes() {
+        return likes;
+    }
+
+    @Override
+    public String toString() {
+        return "BlogPost{" + "title='" + title + '\'' + ", type=" + type + ", likes=" + likes + '}';
+    }
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/groupingby/BlogPostType.java

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+package com.baeldung.streams.groupingby;
+
+public enum BlogPostType {
+    NEWS, REVIEW, GUIDE
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/groupingby/Tuple.java

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+package com.baeldung.streams.groupingby;
+
+import java.util.Objects;
+
+public class Tuple {
+    private final BlogPostType type;
+    private final String author;
+
+    public Tuple(BlogPostType type, String author) {
+        this.type = type;
+        this.author = author;
+    }
+
+    public BlogPostType getType() {
+        return type;
+    }
+
+    public String getAuthor() {
+        return author;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+        Tuple tuple = (Tuple) o;
+        return type == tuple.type && author.equals(tuple.author);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(type, author);
+    }
+
+    @Override
+    public String toString() {
+        return "Tuple{" + "type=" + type + ", author='" + author + '\'' + '}';
+    }
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/parallel/BenchmarkRunner.java

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+package com.baeldung.streams.parallel;
+
+public class BenchmarkRunner {
+
+    public static void main(String[] args) throws Exception {
+        org.openjdk.jmh.Main.main(args);
+    }
+
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/parallel/DifferentSourceSplitting.java

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+package com.baeldung.streams.parallel;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+public class DifferentSourceSplitting {
+
+    private static final List<Integer> arrayListOfNumbers = new ArrayList<>();
+    private static final List<Integer> linkedListOfNumbers = new LinkedList<>();
+
+    static {
+        IntStream.rangeClosed(1, 1_000_000).forEach(i -> {
+            arrayListOfNumbers.add(i);
+            linkedListOfNumbers.add(i);
+        });
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void differentSourceArrayListSequential() {
+        arrayListOfNumbers.stream().reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void differentSourceArrayListParallel() {
+        arrayListOfNumbers.parallelStream().reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void differentSourceLinkedListSequential() {
+        linkedListOfNumbers.stream().reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void differentSourceLinkedListParallel() {
+        linkedListOfNumbers.parallelStream().reduce(0, Integer::sum);
+    }
+
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/parallel/FileSearchCost.java

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+package com.baeldung.streams.parallel;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.Collectors;
+
+import org.apache.commons.io.FileUtils;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Level;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.TearDown;
+
+@State(Scope.Benchmark)
+public class FileSearchCost {
+
+	private final static String FILE_NAME = "src/main/resources/Test";
+
+	@Setup(Level.Trial)
+	public void setup() throws IOException {
+		for (int i = 0; i < 1500; i++) {
+			File targetFile = new File(FILE_NAME + i);
+			FileUtils.writeStringToFile(targetFile, "Test", "UTF8");
+		}
+	}
+
+	@TearDown(Level.Trial)
+	public void tearDown() {
+		for (int i = 0; i < 1500; i++) {
+			File fileToDelete = new File(FILE_NAME + i);
+			fileToDelete.delete();
+		}
+	}
+
+	@Benchmark
+	@BenchmarkMode(Mode.AverageTime)
+	@OutputTimeUnit(TimeUnit.NANOSECONDS)
+	public static void textFileSearchSequential() throws IOException {
+		Files.walk(Paths.get("src/main/resources/")).map(Path::normalize).filter(Files::isRegularFile)
+				.filter(path -> path.getFileName().toString().endsWith(".txt")).collect(Collectors.toList());
+	}
+
+	@Benchmark
+	@BenchmarkMode(Mode.AverageTime)
+	@OutputTimeUnit(TimeUnit.NANOSECONDS)
+	public static void textFileSearchParallel() throws IOException {
+		Files.walk(Paths.get("src/main/resources/")).parallel().map(Path::normalize).filter(Files::isRegularFile)
+				.filter(path -> path.getFileName().toString().endsWith(".txt")).collect(Collectors.toList());
+	}
+
+}
+

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/parallel/MemoryLocalityCosts.java

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+package com.baeldung.streams.parallel;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+
+import java.util.Arrays;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+public class MemoryLocalityCosts {
+
+    private static final int[] intArray = new int[1_000_000];
+    private static final Integer[] integerArray = new Integer[1_000_000];
+
+    static {
+        IntStream.rangeClosed(1, 1_000_000).forEach(i -> {
+            intArray[i-1] = i;
+            integerArray[i-1] = i;
+        });
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void localityIntArraySequential() {
+        Arrays.stream(intArray).reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void localityIntArrayParallel() {
+        Arrays.stream(intArray).parallel().reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void localityIntegerArraySequential() {
+        Arrays.stream(integerArray).reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void localityIntegerArrayParallel() {
+        Arrays.stream(integerArray).parallel().reduce(0, Integer::sum);
+    }
+
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/parallel/MergingCosts.java

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+package com.baeldung.streams.parallel;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+public class MergingCosts {
+
+    private static final List<Integer> arrayListOfNumbers = new ArrayList<>();
+
+    static {
+        IntStream.rangeClosed(1, 1_000_000).forEach(i -> {
+            arrayListOfNumbers.add(i);
+        });
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void mergingCostsSumSequential() {
+        arrayListOfNumbers.stream().reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void mergingCostsSumParallel() {
+        arrayListOfNumbers.stream().parallel().reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void mergingCostsGroupingSequential() {
+        arrayListOfNumbers.stream().collect(Collectors.toSet());
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void mergingCostsGroupingParallel() {
+        arrayListOfNumbers.stream().parallel().collect(Collectors.toSet());
+    }
+
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/parallel/ParallelStream.java

@@ -0,0 +1,15 @@
+package com.baeldung.streams.parallel;
+
+import java.util.Arrays;
+import java.util.List;
+
+public class ParallelStream {
+
+    public static void main(String[] args) {
+        List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4);
+        listOfNumbers.parallelStream().forEach(number ->
+          System.out.println(number + " " + Thread.currentThread().getName())
+        );
+    }
+
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/parallel/SequentialStream.java

@@ -0,0 +1,15 @@
+package com.baeldung.streams.parallel;
+
+import java.util.Arrays;
+import java.util.List;
+
+public class SequentialStream {
+
+    public static void main(String[] args) {
+        List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4);
+        listOfNumbers.stream().forEach(number ->
+          System.out.println(number + " " + Thread.currentThread().getName())
+        );
+    }
+
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/parallel/SplittingCosts.java

@@ -0,0 +1,27 @@
+package com.baeldung.streams.parallel;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+public class SplittingCosts {
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void sourceSplittingIntStreamSequential() {
+        IntStream.rangeClosed(1, 100).reduce(0, Integer::sum);
+    }
+
+    @Benchmark
+    @BenchmarkMode(Mode.AverageTime)
+    @OutputTimeUnit(TimeUnit.NANOSECONDS)
+    public static void sourceSplittingIntStreamParallel() {
+        IntStream.rangeClosed(1, 100).parallel().reduce(0, Integer::sum);
+    }
+
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/reduce/application/Application.java

@@ -0,0 +1,38 @@
+package com.baeldung.streams.reduce.application;
+
+import com.baeldung.streams.reduce.entities.User;
+
+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);
+    }    
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/reduce/benchmarks/JMHStreamReduceBenchMark.java

@@ -0,0 +1,52 @@
+package com.baeldung.streams.reduce.benchmarks;
+
+import com.baeldung.streams.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);
+    }
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/reduce/entities/Rating.java

@@ -0,0 +1,39 @@
+package com.baeldung.streams.reduce.entities;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Rating {
+
+    double points;
+    List<Review> reviews = new ArrayList<>();
+
+    public Rating() {}
+
+    public void add(Review review) {
+        reviews.add(review);
+        computeRating();
+    }
+
+    private double computeRating() {
+        double totalPoints = reviews.stream().map(Review::getPoints).reduce(0, Integer::sum);
+        this.points = totalPoints / reviews.size();
+        return this.points;
+    }
+
+    public static Rating average(Rating r1, Rating r2) {
+        Rating combined = new Rating();
+        combined.reviews = new ArrayList<>(r1.reviews);
+        combined.reviews.addAll(r2.reviews);
+        combined.computeRating();
+        return combined;
+    }
+
+    public double getPoints() {
+        return points;
+    }
+
+    public List<Review> getReviews() {
+        return reviews;
+    }
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/reduce/entities/Review.java

@@ -0,0 +1,28 @@
+package com.baeldung.streams.reduce.entities;
+
+public class Review {
+
+    int points;
+    String review;
+
+    public Review(int points, String review) {
+        this.points = points;
+        this.review = review;
+    }
+
+    public int getPoints() {
+        return points;
+    }
+
+    public void setPoints(int points) {
+        this.points = points;
+    }
+
+    public String getReview() {
+        return review;
+    }
+
+    public void setReview(String review) {
+        this.review = review;
+    }
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/reduce/entities/User.java

@@ -0,0 +1,25 @@
+package com.baeldung.streams.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 + '}';
+    }
+}

core-java-modules/core-java-streams-simple/src/main/java/com/baeldung/streams/reduce/utilities/NumberUtils.java

@@ -0,0 +1,52 @@
+package com.baeldung.streams.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;
+    }
+}

core-java-modules/core-java-streams-simple/src/main/resources/logback.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>

core-java-modules/core-java-streams-simple/src/test/java/com/baeldung/streams/Detail.java

@@ -0,0 +1,13 @@
+package com.baeldung.streams;
+
+import java.util.Arrays;
+import java.util.List;
+
+public class Detail {
+
+    private static final List<String> PARTS = Arrays.asList("turbine", "pump");
+
+    public List<String> getParts() {
+        return PARTS;
+    }
+}

core-java-modules/core-java-streams-simple/src/test/java/com/baeldung/streams/Java8StreamsUnitTest.java

@@ -0,0 +1,104 @@
+package com.baeldung.streams;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.*;
+
+public class Java8StreamsUnitTest {
+
+    private List<String> list;
+
+    @Before
+    public void init() {
+        list = new ArrayList<>();
+        list.add("One");
+        list.add("OneAndOnly");
+        list.add("Derek");
+        list.add("Change");
+        list.add("factory");
+        list.add("justBefore");
+        list.add("Italy");
+        list.add("Italy");
+        list.add("Thursday");
+        list.add("");
+        list.add("");
+    }
+
+    @Test
+    public void checkStreamCount_whenCreating_givenDifferentSources() {
+        String[] arr = new String[] { "a", "b", "c" };
+        Stream<String> streamArr = Arrays.stream(arr);
+        assertEquals(streamArr.count(), 3);
+
+        Stream<String> streamOf = Stream.of("a", "b", "c");
+        assertEquals(streamOf.count(), 3);
+
+        long count = list.stream().distinct().count();
+        assertEquals(count, 9);
+    }
+
+    @Test
+    public void checkStreamCount_whenOperationFilter_thanCorrect() {
+        Stream<String> streamFilter = list.stream().filter(element -> element.isEmpty());
+        assertEquals(streamFilter.count(), 2);
+    }
+
+    @Test
+    public void checkStreamCount_whenOperationMap_thanCorrect() {
+        List<String> uris = new ArrayList<>();
+        uris.add("C:\\My.txt");
+        Stream<Path> streamMap = uris.stream().map(uri -> Paths.get(uri));
+        assertEquals(streamMap.count(), 1);
+
+        List<Detail> details = new ArrayList<>();
+        details.add(new Detail());
+        details.add(new Detail());
+        Stream<String> streamFlatMap = details.stream().flatMap(detail -> detail.getParts().stream());
+        assertEquals(streamFlatMap.count(), 4);
+    }
+
+    @Test
+    public void checkStreamCount_whenOperationMatch_thenCorrect() {
+        boolean isValid = list.stream().anyMatch(element -> element.contains("h"));
+        boolean isValidOne = list.stream().allMatch(element -> element.contains("h"));
+        boolean isValidTwo = list.stream().noneMatch(element -> element.contains("h"));
+        assertTrue(isValid);
+        assertFalse(isValidOne);
+        assertFalse(isValidTwo);
+    }
+
+    @Test
+    public void checkStreamReducedValue_whenOperationReduce_thenCorrect() {
+        List<Integer> integers = new ArrayList<>();
+        integers.add(1);
+        integers.add(1);
+        integers.add(1);
+        Integer reduced = integers.stream().reduce(23, (a, b) -> a + b);
+        assertTrue(reduced == 26);
+    }
+
+    @Test
+    public void checkStreamContains_whenOperationCollect_thenCorrect() {
+        List<String> resultList = list.stream().map(element -> element.toUpperCase()).collect(Collectors.toList());
+        assertEquals(resultList.size(), list.size());
+        assertTrue(resultList.contains(""));
+    }
+
+    @Test
+    public void checkParallelStream_whenDoWork() {
+        list.parallelStream().forEach(element -> doWork(element));
+    }
+
+    private void doWork(String string) {
+        assertTrue(true); // just imitate an amount of work
+    }
+}

core-java-modules/core-java-streams-simple/src/test/java/com/baeldung/streams/collectors/Java8CollectorsUnitTest.java

@@ -0,0 +1,237 @@
+package com.baeldung.streams.collectors;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Sets;
+import org.junit.Test;
+
+import java.util.*;
+import java.util.function.BiConsumer;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Collector;
+
+import static com.google.common.collect.Sets.newHashSet;
+import static java.util.stream.Collectors.*;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class Java8CollectorsUnitTest {
+
+    private final List<String> givenList = Arrays.asList("a", "bb", "ccc", "dd");
+    private final List<String> listWithDuplicates = Arrays.asList("a", "bb", "c", "d", "bb");
+
+    @Test
+    public void whenCollectingToList_shouldCollectToList() throws Exception {
+        final List<String> result = givenList.stream().collect(toList());
+
+        assertThat(result).containsAll(givenList);
+    }
+
+    @Test
+    public void whenCollectingToUnmodifiableList_shouldCollectToUnmodifiableList() {
+        final List<String> result = givenList.stream().collect(toUnmodifiableList());
+
+        assertThatThrownBy(() -> result.add("foo"))
+          .isInstanceOf(UnsupportedOperationException.class);
+    }
+
+    @Test
+    public void whenCollectingToSet_shouldCollectToSet() throws Exception {
+        final Set<String> result = givenList.stream().collect(toSet());
+
+        assertThat(result).containsAll(givenList);
+    }
+
+    @Test
+    public void whenCollectingToUnmodifiableSet_shouldCollectToUnmodifiableSet() {
+        final Set<String> result = givenList.stream().collect(toUnmodifiableSet());
+
+        assertThatThrownBy(() -> result.add("foo"))
+          .isInstanceOf(UnsupportedOperationException.class);
+    }
+
+    @Test
+    public void givenContainsDuplicateElements_whenCollectingToSet_shouldAddDuplicateElementsOnlyOnce() throws Exception {
+        final Set<String> result = listWithDuplicates.stream().collect(toSet());
+
+        assertThat(result).hasSize(4);
+    }
+
+    @Test
+    public void whenCollectingToCollection_shouldCollectToCollection() throws Exception {
+        final List<String> result = givenList.stream().collect(toCollection(LinkedList::new));
+
+        assertThat(result).containsAll(givenList).isInstanceOf(LinkedList.class);
+    }
+
+    @Test
+    public void whenCollectingToImmutableCollection_shouldThrowException() throws Exception {
+        assertThatThrownBy(() -> {
+            givenList.stream().collect(toCollection(ImmutableList::of));
+        }).isInstanceOf(UnsupportedOperationException.class);
+
+    }
+
+    @Test
+    public void whenCollectingToMap_shouldCollectToMap() throws Exception {
+        final Map<String, Integer> result = givenList.stream().collect(toMap(Function.identity(), String::length));
+
+        assertThat(result).containsEntry("a", 1).containsEntry("bb", 2).containsEntry("ccc", 3).containsEntry("dd", 2);
+    }
+
+    @Test
+    public void whenCollectingToUnmodifiableMap_shouldCollectToUnmodifiableMap() {
+        final Map<String, Integer> result = givenList.stream()
+          .collect(toUnmodifiableMap(Function.identity(), String::length));
+
+        assertThatThrownBy(() -> result.put("foo", 3))
+                .isInstanceOf(UnsupportedOperationException.class);
+    }
+
+    @Test
+    public void whenCollectingToMapwWithDuplicates_shouldCollectToMapMergingTheIdenticalItems() throws Exception {
+        final Map<String, Integer> result = listWithDuplicates.stream().collect(
+          toMap(
+            Function.identity(),
+            String::length,
+            (item, identicalItem) -> item
+          )
+        );
+
+        assertThat(result).containsEntry("a", 1).containsEntry("bb", 2).containsEntry("c", 1).containsEntry("d", 1);
+    }
+
+    @Test
+    public void givenContainsDuplicateElements_whenCollectingToMap_shouldThrowException() throws Exception {
+        assertThatThrownBy(() -> {
+            listWithDuplicates.stream().collect(toMap(Function.identity(), String::length));
+        }).isInstanceOf(IllegalStateException.class);
+    }
+
+    @Test
+    public void whenCollectingAndThen_shouldCollect() throws Exception {
+        final List<String> result = givenList.stream().collect(collectingAndThen(toList(), ImmutableList::copyOf));
+
+        assertThat(result).containsAll(givenList).isInstanceOf(ImmutableList.class);
+    }
+
+    @Test
+    public void whenJoining_shouldJoin() throws Exception {
+        final String result = givenList.stream().collect(joining());
+
+        assertThat(result).isEqualTo("abbcccdd");
+    }
+
+    @Test
+    public void whenJoiningWithSeparator_shouldJoinWithSeparator() throws Exception {
+        final String result = givenList.stream().collect(joining(" "));
+
+        assertThat(result).isEqualTo("a bb ccc dd");
+    }
+
+    @Test
+    public void whenJoiningWithSeparatorAndPrefixAndPostfix_shouldJoinWithSeparatorPrePost() throws Exception {
+        final String result = givenList.stream().collect(joining(" ", "PRE-", "-POST"));
+
+        assertThat(result).isEqualTo("PRE-a bb ccc dd-POST");
+    }
+
+    @Test
+    public void whenPartitioningBy_shouldPartition() throws Exception {
+        final Map<Boolean, List<String>> result = givenList.stream().collect(partitioningBy(s -> s.length() > 2));
+
+        assertThat(result).containsKeys(true, false).satisfies(booleanListMap -> {
+            assertThat(booleanListMap.get(true)).contains("ccc");
+
+            assertThat(booleanListMap.get(false)).contains("a", "bb", "dd");
+        });
+    }
+
+    @Test
+    public void whenCounting_shouldCount() throws Exception {
+        final Long result = givenList.stream().collect(counting());
+
+        assertThat(result).isEqualTo(4);
+    }
+
+    @Test
+    public void whenSummarizing_shouldSummarize() throws Exception {
+        final DoubleSummaryStatistics result = givenList.stream().collect(summarizingDouble(String::length));
+
+        assertThat(result.getAverage()).isEqualTo(2);
+        assertThat(result.getCount()).isEqualTo(4);
+        assertThat(result.getMax()).isEqualTo(3);
+        assertThat(result.getMin()).isEqualTo(1);
+        assertThat(result.getSum()).isEqualTo(8);
+    }
+
+    @Test
+    public void whenAveraging_shouldAverage() throws Exception {
+        final Double result = givenList.stream().collect(averagingDouble(String::length));
+
+        assertThat(result).isEqualTo(2);
+    }
+
+    @Test
+    public void whenSumming_shouldSum() throws Exception {
+        final Double result = givenList.stream().filter(i -> true).collect(summingDouble(String::length));
+
+        assertThat(result).isEqualTo(8);
+    }
+
+    @Test
+    public void whenMaxingBy_shouldMaxBy() throws Exception {
+        final Optional<String> result = givenList.stream().collect(maxBy(Comparator.naturalOrder()));
+
+        assertThat(result).isPresent().hasValue("dd");
+    }
+
+    @Test
+    public void whenGroupingBy_shouldGroupBy() throws Exception {
+        final Map<Integer, Set<String>> result = givenList.stream().collect(groupingBy(String::length, toSet()));
+
+        assertThat(result).containsEntry(1, newHashSet("a")).containsEntry(2, newHashSet("bb", "dd")).containsEntry(3, newHashSet("ccc"));
+    }
+
+    @Test
+    public void whenCreatingCustomCollector_shouldCollect() throws Exception {
+        final ImmutableSet<String> result = givenList.stream().collect(toImmutableSet());
+
+        assertThat(result).isInstanceOf(ImmutableSet.class).contains("a", "bb", "ccc", "dd");
+
+    }
+
+    private static <T> ImmutableSetCollector<T> toImmutableSet() {
+        return new ImmutableSetCollector<>();
+    }
+
+    private static class ImmutableSetCollector<T> implements Collector<T, ImmutableSet.Builder<T>, ImmutableSet<T>> {
+
+        @Override
+        public Supplier<ImmutableSet.Builder<T>> supplier() {
+            return ImmutableSet::builder;
+        }
+
+        @Override
+        public BiConsumer<ImmutableSet.Builder<T>, T> accumulator() {
+            return ImmutableSet.Builder::add;
+        }
+
+        @Override
+        public BinaryOperator<ImmutableSet.Builder<T>> combiner() {
+            return (left, right) -> left.addAll(right.build());
+        }
+
+        @Override
+        public Function<ImmutableSet.Builder<T>, ImmutableSet<T>> finisher() {
+            return ImmutableSet.Builder::build;
+        }
+
+        @Override
+        public Set<Characteristics> characteristics() {
+            return Sets.immutableEnumSet(Characteristics.UNORDERED);
+        }
+    }
+}

core-java-modules/core-java-streams-simple/src/test/java/com/baeldung/streams/filter/StreamFilterUnitTest.java

@@ -0,0 +1,159 @@
+package com.baeldung.streams.filter;
+
+import org.junit.jupiter.api.Test;
+import com.pivovarit.function.ThrowingPredicate;
+import com.pivovarit.function.exception.WrappedException;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.AssertionsForClassTypes.assertThatThrownBy;
+
+class StreamFilterUnitTest {
+
+    @Test
+    void givenListOfCustomers_whenFilterByPoints_thenGetTwo() {
+        Customer john = new Customer("John P.", 15);
+        Customer sarah = new Customer("Sarah M.", 200);
+        Customer charles = new Customer("Charles B.", 150);
+        Customer mary = new Customer("Mary T.", 1);
+        List<Customer> customers = Arrays.asList(john, sarah, charles, mary);
+
+        List<Customer> customersWithMoreThan100Points = customers
+          .stream()
+          .filter(c -> c.getPoints() > 100)
+          .collect(Collectors.toList());
+
+        assertThat(customersWithMoreThan100Points).hasSize(2);
+        assertThat(customersWithMoreThan100Points).contains(sarah, charles);
+    }
+
+    @Test
+    public void givenListOfCustomers_whenFilterByPointsAndName_thenGetOne() {
+        Customer john = new Customer("John P.", 15);
+        Customer sarah = new Customer("Sarah M.", 200);
+        Customer charles = new Customer("Charles B.", 150);
+        Customer mary = new Customer("Mary T.", 1);
+        List<Customer> customers = Arrays.asList(john, sarah, charles, mary);
+
+        List<Customer> charlesWithMoreThan100Points = customers
+          .stream()
+          .filter(c -> c.getPoints() > 100 && c
+            .getName()
+            .startsWith("Charles"))
+          .collect(Collectors.toList());
+
+        assertThat(charlesWithMoreThan100Points).hasSize(1);
+        assertThat(charlesWithMoreThan100Points).contains(charles);
+    }
+
+    @Test
+    void givenListOfCustomers_whenFilterByMethodReference_thenGetTwo() {
+        Customer john = new Customer("John P.", 15);
+        Customer sarah = new Customer("Sarah M.", 200);
+        Customer charles = new Customer("Charles B.", 150);
+        Customer mary = new Customer("Mary T.", 1);
+        List<Customer> customers = Arrays.asList(john, sarah, charles, mary);
+
+        List<Customer> customersWithMoreThan100Points = customers
+          .stream()
+          .filter(Customer::hasOverHundredPoints)
+          .collect(Collectors.toList());
+
+        assertThat(customersWithMoreThan100Points).hasSize(2);
+        assertThat(customersWithMoreThan100Points).contains(sarah, charles);
+    }
+
+    @Test
+    void givenListOfCustomersWithOptional_whenFilterBy100Points_thenGetTwo() {
+        Optional<Customer> john = Optional.of(new Customer("John P.", 15));
+        Optional<Customer> sarah = Optional.of(new Customer("Sarah M.", 200));
+        Optional<Customer> mary = Optional.of(new Customer("Mary T.", 300));
+        List<Optional<Customer>> customers = Arrays.asList(john, sarah, Optional.empty(), mary, Optional.empty());
+
+        List<Customer> customersWithMoreThan100Points = customers
+          .stream()
+          .flatMap(c -> c
+            .map(Stream::of)
+            .orElseGet(Stream::empty))
+          .filter(Customer::hasOverHundredPoints)
+          .collect(Collectors.toList());
+
+        assertThat(customersWithMoreThan100Points).hasSize(2);
+        assertThat(customersWithMoreThan100Points).contains(sarah.get(), mary.get());
+    }
+
+    @Test
+    void givenListOfCustomers_whenFilterWithCustomHandling_thenThrowException() {
+        Customer john = new Customer("John P.", 15, "https://images.unsplash.com/photo-1543320485-d0d5a49c2b2e");
+        Customer sarah = new Customer("Sarah M.", 200);
+        Customer charles = new Customer("Charles B.", 150);
+        Customer mary = new Customer("Mary T.", 1, "https://images.unsplash.com/photo-1543297057-25167dfc180e");
+        List<Customer> customers = Arrays.asList(john, sarah, charles, mary);
+
+        assertThatThrownBy(() -> customers
+          .stream()
+          .filter(Customer::hasValidProfilePhotoWithoutCheckedException)
+          .count()).isInstanceOf(RuntimeException.class);
+    }
+
+    @Test
+    void givenListOfCustomers_whenFilterWithThrowingFunction_thenThrowException() {
+        Customer john = new Customer("John P.", 15, "https://images.unsplash.com/photo-1543320485-d0d5a49c2b2e");
+        Customer sarah = new Customer("Sarah M.", 200);
+        Customer charles = new Customer("Charles B.", 150);
+        Customer mary = new Customer("Mary T.", 1, "https://images.unsplash.com/photo-1543297057-25167dfc180e");
+        List<Customer> customers = Arrays.asList(john, sarah, charles, mary);
+
+        assertThatThrownBy(() -> customers
+          .stream()
+          .filter((ThrowingPredicate.unchecked(Customer::hasValidProfilePhoto)))
+          .collect(Collectors.toList())).isInstanceOf(WrappedException.class);
+    }
+
+    @Test
+    public void givenListOfCustomers_whenFilterWithTryCatch_thenGetTwo() {
+        Customer john = new Customer("John P.", 15, "https://images.unsplash.com/photo-1543320485-d0d5a49c2b2e");
+        Customer sarah = new Customer("Sarah M.", 200);
+        Customer charles = new Customer("Charles B.", 150);
+        Customer mary = new Customer("Mary T.", 1, "https://images.unsplash.com/photo-1543297057-25167dfc180e");
+        List<Customer> customers = Arrays.asList(john, sarah, charles, mary);
+
+        List<Customer> customersWithValidProfilePhoto = customers
+          .stream()
+          .filter(c -> {
+              try {
+                  return c.hasValidProfilePhoto();
+              } catch (IOException e) {
+                  //handle exception
+              }
+              return false;
+          })
+          .collect(Collectors.toList());
+
+        assertThat(customersWithValidProfilePhoto).hasSize(2);
+        assertThat(customersWithValidProfilePhoto).contains(john, mary);
+    }
+
+    @Test
+    public void givenListOfCustomers_whenFilterWithTryCatchAndRuntime_thenThrowException() {
+        List<Customer> customers = Arrays.asList(new Customer("John P.", 15, "https://images.unsplash.com/photo-1543320485-d0d5a49c2b2e"), new Customer("Sarah M.", 200), new Customer("Charles B.", 150),
+          new Customer("Mary T.", 1, "https://images.unsplash.com/photo-1543297057-25167dfc180e"));
+
+        assertThatThrownBy(() -> customers
+          .stream()
+          .filter(c -> {
+              try {
+                  return c.hasValidProfilePhoto();
+              } catch (IOException e) {
+                  throw new RuntimeException(e);
+              }
+          })
+          .collect(Collectors.toList())).isInstanceOf(RuntimeException.class);
+    }
+}

core-java-modules/core-java-streams-simple/src/test/java/com/baeldung/streams/flatmap/map/Java8MapAndFlatMapUnitTest.java

@@ -0,0 +1,51 @@
+package com.baeldung.streams.flatmap.map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static java.util.Arrays.asList;
+
+
+class Java8MapAndFlatMapUnitTest {
+
+    @Test
+    void givenStream_whenCalledMap_thenProduceList() {
+        List<String> myList = Stream.of("a", "b")
+            .map(String::toUpperCase)
+            .collect(Collectors.toList());
+        assertEquals(asList("A", "B"), myList);
+    }
+
+    @Test
+    void givenStream_whenCalledFlatMap_thenProduceFlattenedList() {
+        List<List<String>> list = Arrays.asList(Arrays.asList("a"), Arrays.asList("b"));
+        System.out.println(list);
+
+        System.out.println(list.stream()
+            .flatMap(Collection::stream)
+            .collect(Collectors.toList()));
+    }
+
+    @Test
+    void givenOptional_whenCalledMap_thenProduceOptional() {
+        Optional<String> s = Optional.of("test");
+        assertEquals(Optional.of("TEST"), s.map(String::toUpperCase));
+    }
+
+    @Test
+    void givenOptional_whenCalledFlatMap_thenProduceFlattenedOptional() {
+        assertEquals(Optional.of(Optional.of("STRING")), Optional.of("string")
+            .map(s -> Optional.of("STRING")));
+
+        assertEquals(Optional.of("STRING"), Optional.of("string")
+            .flatMap(s -> Optional.of("STRING")));
+    }
+
+}

core-java-modules/core-java-streams-simple/src/test/java/com/baeldung/streams/map/StreamMapUnitTest.java

@@ -0,0 +1,80 @@
+package com.baeldung.streams.map;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.stream.Collectors;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class StreamMapUnitTest {
+
+    private Map<String, String> books;
+
+    @Before
+    public void setup() {
+        books = new HashMap<>();
+        books.put("978-0201633610", "Design patterns : elements of reusable object-oriented software");
+        books.put("978-1617291999", "Java 8 in Action: Lambdas, Streams, and functional-style programming");
+        books.put("978-0134685991", "Effective Java");
+    }
+
+
+    @Test
+    public void whenOptionalVersionCalledForExistingTitle_thenReturnOptionalWithISBN() {
+        Optional<String> optionalIsbn = books.entrySet().stream()
+                .filter(e -> "Effective Java".equals(e.getValue()))
+                .map(Map.Entry::getKey).findFirst();
+
+        assertEquals("978-0134685991", optionalIsbn.get());
+    }
+
+    @Test
+    public void whenOptionalVersionCalledForNonExistingTitle_thenReturnEmptyOptionalForISBN() {
+        Optional<String> optionalIsbn = books.entrySet().stream()
+                .filter(e -> "Non Existent Title".equals(e.getValue()))
+                .map(Map.Entry::getKey).findFirst();
+
+        assertEquals(false, optionalIsbn.isPresent());
+    }
+
+    @Test
+    public void whenMultipleResultsVersionCalledForExistingTitle_aCollectionWithMultipleValuesIsReturned() {
+        books.put("978-0321356680", "Effective Java: Second Edition");
+
+        List<String> isbnCodes = books.entrySet().stream()
+                .filter(e -> e.getValue().startsWith("Effective Java"))
+                .map(Map.Entry::getKey)
+                .collect(Collectors.toList());
+
+        assertTrue(isbnCodes.contains("978-0321356680"));
+        assertTrue(isbnCodes.contains("978-0134685991"));
+    }
+
+    @Test
+    public void whenMultipleResultsVersionCalledForNonExistingTitle_aCollectionWithNoValuesIsReturned() {
+        List<String> isbnCodes = books.entrySet().stream()
+                .filter(e -> e.getValue().startsWith("Spring"))
+                .map(Map.Entry::getKey)
+                .collect(Collectors.toList());
+
+        assertTrue(isbnCodes.isEmpty());
+    }
+
+    @Test
+    public void whenKeysFollowingPatternReturnsAllValuesForThoseKeys() {
+        List<String> titlesForKeyPattern = books.entrySet().stream()
+                .filter(e -> e.getKey().startsWith("978-0"))
+                .map(Map.Entry::getValue)
+                .collect(Collectors.toList());
+        assertEquals(2, titlesForKeyPattern.size());
+        assertTrue(titlesForKeyPattern.contains("Design patterns : elements of reusable object-oriented software"));
+        assertTrue(titlesForKeyPattern.contains("Effective Java"));
+    }
+    
+}

core-java-modules/core-java-streams-simple/src/test/java/com/baeldung/streams/parallel/ForkJoinUnitTest.java

@@ -0,0 +1,46 @@
+package com.baeldung.streams.parallel;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ForkJoinPool;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class ForkJoinUnitTest {
+
+    @Test
+    void givenSequentialStreamOfNumbers_whenReducingSumWithIdentityFive_thenResultIsCorrect() {
+        List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4);
+        int sum = listOfNumbers.stream().reduce(5, Integer::sum);
+        assertThat(sum).isEqualTo(15);
+    }
+
+    @Test
+    void givenParallelStreamOfNumbers_whenReducingSumWithIdentityFive_thenResultIsNotCorrect() {
+        List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4);
+        int sum = listOfNumbers.parallelStream().reduce(5, Integer::sum);
+        assertThat(sum).isNotEqualTo(15);
+    }
+
+    @Test
+    void givenParallelStreamOfNumbers_whenReducingSumWithIdentityZero_thenResultIsCorrect() {
+        List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4);
+        int sum = listOfNumbers.parallelStream().reduce(0, Integer::sum) + 5;
+        assertThat(sum).isEqualTo(15);
+    }
+
+    @Test
+    public void givenParallelStreamOfNumbers_whenUsingCustomThreadPool_thenResultIsCorrect()
+      throws InterruptedException, ExecutionException {
+        List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4);
+        ForkJoinPool customThreadPool = new ForkJoinPool(4);
+        int sum = customThreadPool.submit(
+          () -> listOfNumbers.parallelStream().reduce(0, Integer::sum)).get();
+        customThreadPool.shutdown();
+        assertThat(sum).isEqualTo(10);
+    }
+
+}

core-java-modules/core-java-streams-simple/src/test/java/com/baeldung/streams/reduce/StreamReduceUnitTest.java

@@ -0,0 +1,81 @@
+package com.baeldung.streams.reduce;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.baeldung.streams.reduce.entities.User;
+import com.baeldung.streams.reduce.utilities.NumberUtils;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.List;
+
+class StreamReduceUnitTest {
+
+    @Test
+    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
+    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
+    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
+    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
+    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
+    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
+    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
+    void givenNumberUtilsClass_whenCalledDivideListElements_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        assertThat(NumberUtils.divideListElements(numbers, 1)).isEqualTo(21);
+    }
+
+    @Test
+    void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlock_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 1)).isEqualTo(21);
+    }
+
+    @Test
+    void givenStream_whneCalleddivideListElementsWithApplyFunctionMethod_thenCorrect() {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        assertThat(NumberUtils.divideListElementsWithApplyFunctionMethod(numbers, 1)).isEqualTo(21);
+    }
+}