BAEL-5853: added code snippets for first element of flux article (#12905)

spring-5-webflux-2/src/main/java/model/Payment.java

@@ -1,4 +1,4 @@
-package onsuccess;
+package model;
 
 
 public class Payment {

spring-5-webflux-2/src/main/java/onsuccess/PaymentService.java

@@ -1,5 +1,7 @@
 package onsuccess;
 
+import model.Payment;
+
 public class PaymentService {
 
     public void processPayment(Payment payment) {

spring-5-webflux-2/src/test/java/firstelementofflux/FirstElementOfFluxUnitTest.java

@@ -0,0 +1,112 @@
+package firstelementofflux;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Optional;
+
+import org.junit.jupiter.api.Test;
+
+import model.Payment;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+
+public class FirstElementOfFluxUnitTest {
+
+    private Payment paymentOf100 = new Payment(100);
+
+    private Flux<Payment> fluxOfThreePayments() {
+        return Flux.just(paymentOf100, new Payment(200), new Payment(300));
+    }
+
+    @Test
+    void givenAPaymentFlux_whenUsingNext_thenGetTheFirstPaymentAsMono() {
+        Mono<Payment> firstPayment = fluxOfThreePayments().next();
+
+        StepVerifier.create(firstPayment)
+            .expectNext(paymentOf100)
+            .verifyComplete();
+    }
+
+    @Test
+    void givenAnEmptyFlux_whenUsingNext_thenGetAEnEmptyMono() {
+        Flux<Payment> emptyFlux = Flux.empty();
+
+        Mono<Payment> firstPayment = emptyFlux.next();
+
+        StepVerifier.create(firstPayment)
+            .verifyComplete();
+    }
+
+    @Test
+    void givenAPaymentFlux_whenUsingTake_thenGetTheFirstPaymentAsFlux() {
+        Flux<Payment> firstPaymentFlux = fluxOfThreePayments().take(1);
+
+        StepVerifier.create(firstPaymentFlux)
+            .expectNext(paymentOf100)
+            .verifyComplete();
+    }
+
+    @Test
+    void givenAEmptyFlux_whenUsingNext_thenGetAnEmptyFlux() {
+        Flux<Payment> emptyFlux = Flux.empty();
+
+        Flux<Payment> firstPaymentFlux = emptyFlux.take(1);
+
+        StepVerifier.create(firstPaymentFlux)
+            .verifyComplete();
+    }
+
+    @Test
+    void givenAPaymentFlux_whenUsingElementAt_thenGetTheFirstPaymentAsMono() {
+        Mono<Payment> firstPayment = fluxOfThreePayments().elementAt(0);
+
+        StepVerifier.create(firstPayment)
+            .expectNext(paymentOf100)
+            .verifyComplete();
+    }
+
+    @Test
+    void givenAEmptyFlux_whenUsingElementAt_thenGetAnEmptyMono() {
+        Flux<Payment> emptyFlux = Flux.empty();
+
+        Mono<Payment> firstPayment = emptyFlux.elementAt(0);
+
+        StepVerifier.create(firstPayment)
+            .expectError(IndexOutOfBoundsException.class);
+    }
+
+    @Test
+    void givenAPaymentFlux_whenUsingBlockFirst_thenGetTheFirstPayment() {
+        Payment firstPayment = fluxOfThreePayments().blockFirst();
+
+        assertThat(firstPayment).isEqualTo(paymentOf100);
+    }
+
+    @Test
+    void givenAEmptyFlux_whenUsingElementAt_thenGetNull() {
+        Flux<Payment> emptyFlux = Flux.empty();
+
+        Payment firstPayment = emptyFlux.blockFirst();
+
+        assertThat(firstPayment).isNull();
+    }
+
+    @Test
+    void givenAPaymentFlux_whenUsingToStream_thenGetTheFirstPaymentAsOptional() {
+        Optional<Payment> firstPayment = fluxOfThreePayments().toStream()
+            .findFirst();
+
+        assertThat(firstPayment).contains(paymentOf100);
+    }
+
+    @Test
+    void givenAnEmptyPaymentFlux_whenUsingToStream_thenGetAnEmptyOptional() {
+        Flux<Payment> emptyFlux = Flux.empty();
+
+        Optional<Payment> firstPayment = emptyFlux.toStream()
+            .findFirst();
+
+        assertThat(firstPayment).isEmpty();
+    }
+}

spring-5-webflux-2/src/test/java/onsuccess/MonoOnSuccessVsOnNexUnitTest.java

@@ -10,6 +10,7 @@ import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.mockito.Mock;
 
+import model.Payment;
 import reactor.core.publisher.Mono;
 
 class MonoOnSuccessVsOnNexUnitTest {