BAEL-4884 Add unit tests for map vs. flatMap (#10836)

reactor-core/src/test/java/com/baeldung/reactor/mapping/MappingUnitTest.java

@@ -0,0 +1,37 @@
+package com.baeldung.reactor.mapping;
+
+import org.junit.Test;
+import org.reactivestreams.Publisher;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Function;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class MappingUnitTest {
+    @Test
+    public void givenInputStream_whenCallingTheMapOperator_thenItemsAreTransformed() {
+        Function<String, String> mapper = String::toUpperCase;
+        Flux<String> inFlux = Flux.just("baeldung", ".", "com");
+        Flux<String> outFlux = inFlux.map(mapper);
+
+        StepVerifier.create(outFlux)
+                .expectNext("BAELDUNG", ".", "COM")
+                .expectComplete()
+                .verify();
+    }
+
+    @Test
+    public void givenInputStream_whenCallingTheFlatMapOperator_thenItemsAreFlatten() {
+        Function<String, Publisher<String>> mapper = s -> Flux.just(s.toUpperCase().split(""));
+        Flux<String> inFlux = Flux.just("baeldung", ".", "com");
+        Flux<String> outFlux = inFlux.flatMap(mapper);
+
+        List<String> output = new ArrayList<>();
+        outFlux.subscribe(output::add);
+        assertThat(output).containsExactlyInAnyOrder("B", "A", "E", "L", "D", "U", "N", "G", ".", "C", "O", "M");
+    }
+}