feat: Java 8 flatMap example.

src/main/java/br/com/somejava8examples/commons/entities/Student.java

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+package br.com.somejava8examples.commons.entities;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class Student {
+
+    private String name;
+    private Set<String> book;
+
+    public Student() {
+    }
+
+    public Student(String name, Set<String> book) {
+        this.name = name;
+        this.book = book;
+    }
+
+    public void addBook(String book) {
+        if (this.book == null) {
+            this.book = new HashSet<>();
+        }
+        this.book.add(book);
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public Set<String> getBook() {
+        return book;
+    }
+
+    public void setBook(Set<String> book) {
+        this.book = book;
+    }
+
+    @Override
+    public String toString() {
+        return "Student{" +
+                "name='" + name + '\'' +
+                ", book=" + book +
+                '}';
+    }
+}

src/main/java/br/com/somejava8examples/map/FlatMapExampleService.java

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+package br.com.somejava8examples.map;
+
+import br.com.somejava8examples.commons.entities.Student;
+import org.springframework.stereotype.Service;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+@Service
+public class FlatMapExampleService {
+
+    public Stream<String> streamStringFlatMap() {
+        String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};
+
+        //Stream<String[]>
+        Stream<String[]> temp = Arrays.stream(data);
+
+        //filter a stream of string[], and return a string[]?
+        Stream<String[]> stream = temp.filter(x -> "a".equals(x.toString()));
+
+        stream.forEach(System.out::println);
+
+        //Stream<String[]>
+        Stream<String[]> temp2 = Arrays.stream(data);
+
+        //Stream<String>, GOOD!
+        Stream<String> stringStream = temp.flatMap(x -> Arrays.stream(x));
+
+        Stream<String> stream2 = stringStream.filter(x -> "a".equals(x.toString()));
+
+        stream2.forEach(System.out::println);
+
+        return stream2;
+    }
+
+    public List<String> streamSetFlatMap() {
+        Student obj1 = new Student();
+        obj1.setName("mkyong");
+        obj1.addBook("Java 8 in Action");
+        obj1.addBook("Spring Boot in Action");
+        obj1.addBook("Effective Java (2nd Edition)");
+
+        Student obj2 = new Student();
+        obj2.setName("zilap");
+        obj2.addBook("Learning Python, 5th Edition");
+        obj2.addBook("Effective Java (2nd Edition)");
+
+        List<Student> list = new ArrayList<>();
+        list.add(obj1);
+        list.add(obj2);
+
+        List<String> collect =
+                list.stream()
+                        .map(x -> x.getBook())      //Stream<Set<String>>
+                        .flatMap(x -> x.stream())   //Stream<String>
+                        .distinct()
+                        .collect(Collectors.toList());
+
+        collect.forEach(x -> System.out.println(x));
+
+        return collect;
+    }
+
+    public IntStream streamPrimitiveFlatMapToInt() {
+        int[] intArray = {1, 2, 3, 4, 5, 6};
+
+        //1. Stream<int[]>
+        Stream<int[]> streamArray = Stream.of(intArray);
+
+        //2. Stream<int[]> -> flatMap -> IntStream
+        IntStream intStream = streamArray.flatMapToInt(x -> Arrays.stream(x));
+
+        intStream.forEach(x -> System.out.println(x));
+
+        return intStream;
+    }
+
+}

src/test/java/br/com/somejava8examples/map/FlatMapExampleTest.java

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+package br.com.somejava8examples.map;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.context.ActiveProfiles;
+
+@SpringBootTest
+@ActiveProfiles("test")
+public class FlatMapExampleTest {
+
+    @Autowired
+    private FlatMapExampleService service;
+
+    @Test
+    public void sortByKeysTest() {
+        Assertions.assertNotNull(service.streamStringFlatMap());
+    }
+
+    @Test
+    public void streamSetFlatMapTest() {
+        Assertions.assertNotNull(service.streamSetFlatMap());
+    }
+
+    @Test
+    public void streamPrimitiveFlatMapToIntTest() {
+        Assertions.assertNotNull(service.streamPrimitiveFlatMapToInt());
+    }
+
+}