BAEL-5784 Added Flux Vs Mono Unit Tests (#12815)

reactor-core/src/test/java/com/baeldung/reactor/core/FluxVsMonoUnitTest.java

@@ -0,0 +1,45 @@
+package com.baeldung.reactor.core;
+
+import org.junit.jupiter.api.Test;
+
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+
+public class FluxVsMonoUnitTest {
+
+    @Test
+    public void givenMonoPublisher_whenSubscribeThenReturnSingleValue() {
+        Mono<String> helloMono = Mono.just("Hello");
+        StepVerifier.create(helloMono)
+          .expectNext("Hello")
+          .expectComplete()
+          .verify();
+    }
+
+    @Test
+    public void givenFluxPublisher_whenSubscribeThenReturnMultipleValues() {
+        Flux<String> stringFlux = Flux.just("Hello", "Baeldung");
+        StepVerifier.create(stringFlux)
+          .expectNext("Hello")
+          .expectNext("Baeldung")
+          .expectComplete()
+          .verify();
+    }
+
+    @Test
+    public void givenFluxPublisher_whenSubscribeThenReturnMultipleValuesWithError() {
+        Flux<String> stringFlux = Flux.just("Hello", "Baeldung", "Error")
+          .map(str -> {
+              if (str.equals("Error"))
+                  throw new RuntimeException("Throwing Error");
+              return str;
+          });
+        StepVerifier.create(stringFlux)
+          .expectNext("Hello")
+          .expectNext("Baeldung")
+          .expectError()
+          .verify();
+    }
+
+}