[KTLN-280] Add samples (#1179)

* [KTLN-280] Add samples

* [KTLN-280] Add samples

Author: LeoColman

core-kotlin-modules/core-kotlin-advanced-4/pom.xml

@@ -26,6 +26,21 @@
             <version>${kotlinx-coroutines.version}</version>
             <scope>test</scope>
         </dependency>
+        
+        <dependency>
+            <groupId>io.projectreactor</groupId>
+            <artifactId>reactor-core</artifactId>
+            <version>${reactor-core.version}</version>
+            <scope>test</scope>
+        </dependency>
+
+        <dependency>
+            <groupId>io.projectreactor</groupId>
+            <artifactId>reactor-test</artifactId>
+            <scope>test</scope>
+            <version>${reactor-core.version}</version>
+        </dependency>
+
         <dependency>
             <groupId>org.jetbrains.kotlin</groupId>
             <artifactId>kotlin-scripting-jsr223</artifactId>
@@ -56,6 +71,7 @@
     <properties>
         <kotlin-scripting.version>1.8.0</kotlin-scripting.version>
         <kotlin.version>2.0.21</kotlin.version>
+        <reactor-core.version>3.6.10</reactor-core.version>
     </properties>
 
 </project>

core-kotlin-modules/core-kotlin-advanced-4/src/test/kotlin/com/baeldung/mono/MonoUnitTest.kt

@@ -0,0 +1,73 @@
+package com.baeldung.mono
+
+import org.junit.jupiter.api.Assertions
+import org.junit.jupiter.api.Test
+import reactor.core.publisher.Mono
+import reactor.test.StepVerifier
+import java.util.concurrent.atomic.AtomicInteger
+
+class MonoUnitTest {
+
+  @Test
+  fun `mapping function is skipped for empty Mono`() {
+    val callCount = AtomicInteger(0)
+    val emptyMono = Mono.empty<String>()
+    val resultMono = emptyMono.map {
+      callCount.incrementAndGet()
+      it.uppercase()
+    }
+
+    StepVerifier.create(resultMono).verifyComplete() // No value emitted, Mono completes directly 
+    Assertions.assertEquals(0, callCount.get())
+  }
+
+  @Test
+  fun `should provide default value for empty Mono`() { 
+    val emptyMono = Mono.empty<String>() 
+    val resultMono = emptyMono.defaultIfEmpty("Default Value") 
+    StepVerifier.create(resultMono) 
+      .expectNext("Default Value")
+      .verifyComplete() 
+  }
+
+  @Test 
+  fun `should not override value for non-empty Mono`() { 
+    val nonEmptyMono = Mono.just("Actual Value")
+    val resultMono = nonEmptyMono.defaultIfEmpty("Default Value") 
+    StepVerifier.create(resultMono) 
+      .expectNext("Actual Value")
+      .verifyComplete() 
+  }
+
+  @Test fun `should detect empty Mono using hasElement`() { 
+    val emptyMono = Mono.empty<String>() 
+    StepVerifier.create(emptyMono.hasElement()) 
+      .expectNext(false) 
+      .verifyComplete() 
+  }
+
+  @Test fun `should detect non-empty Mono using hasElement`() { 
+    val nonEmptyMono = Mono.just("Hello, World!")
+    StepVerifier.create(nonEmptyMono.hasElement())
+      .expectNext(true)
+      .verifyComplete() 
+  }
+
+  @Test 
+  fun `should provide fallback value for empty Mono`() { 
+    val emptyMono = Mono.empty<String>() 
+    val fallbackMono = emptyMono.switchIfEmpty(Mono.just("Fallback Value"))
+    StepVerifier.create(fallbackMono) 
+      .expectNext("Fallback Value")
+      .verifyComplete() 
+  }
+
+  @Test 
+  fun `should not invoke fallback for non-empty Mono`() { 
+    val nonEmptyMono = Mono.just("Hello, World!") 
+    val resultMono = nonEmptyMono.switchIfEmpty(Mono.just("Fallback Value")) 
+    StepVerifier.create(resultMono) 
+      .expectNext("Hello, World!") 
+      .verifyComplete() 
+  }
+}