get a better understanding of flat map

src/test/java/com/finance/programming/UsageOfFlatMap.java

@@ -0,0 +1,56 @@
+package com.finance.programming;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@DisplayName("Usage of flatMap in Java 8 Streams")
+public class UsageOfFlatMap {
+
+    @Test
+    @DisplayName("Flattening a 2D array using flatMap")
+    void testFlattening2DArray() {
+        int[][] twoDArray = {{1, 2}, {3, 4}, {5, 6}};
+        int[] expectedArray = {1, 2, 3, 4, 5, 6};
+
+        int[] oneDArray = Arrays.stream(twoDArray)
+                        .flatMapToInt(Arrays::stream)
+                        .toArray();
+
+        assertArrayEquals(expectedArray, oneDArray);
+
+    }
+
+    @Test
+    @DisplayName("Flattening a 2D array using flatMap")
+    void testFlatteningAListOfInteger() {
+        List<List<Integer>> twoDList = List.of(
+                List.of(1, 2),
+                List.of(3, 4),
+                List.of(5, 6)
+        );
+
+        List<Integer> expectedList = List.of(1, 2, 3, 4, 5, 6);
+
+        Function<List<Integer>, Stream<Integer>> returnJustOneList = Collection::stream;
+
+        List<Integer> actualList = twoDList.stream()
+                .flatMap(returnJustOneList::apply)
+                .toList();
+
+        assertIterableEquals(expectedList, actualList, () -> {
+            System.out.println("Expected List:  " + expectedList);
+            System.out.println("Actual List:  " + actualList);
+            return "The content of the list doesn't match";
+        });
+
+    }
+
+}

tests.md

@@ -28,7 +28,7 @@ Description: Take a list of strings and use map to convert all strings to upperc
 ✔️ **Exercise 8: Reducing a list**
 Description: Use reduce to sum all the elements of a list of integers.
 
-❌ **Exercise 9: Using flatMap**
+✔️ **Exercise 9: Using flatMap**
 Description: Take a list of lists of integers and use flatMap to convert it into a flat list of integers.
 
 ❌ **Exercise 10: Sorting a list**