Added Mono.single()

Author: sinwe

src/main/scala/reactor/core/scala/publisher/Mono.scala

@@ -1371,6 +1371,19 @@ class Mono[T] private(private val jMono: JMono[T]) extends Publisher[T] with Map
     */
   final def retryWhen(whenFactory: Flux[Throwable] => Publisher[_]): Mono[T] = Mono[T](jMono.retryWhen(whenFactory))
 
+  /**
+    * Expect exactly one item from this [[Mono]] source or signal
+    * [[java.util.NoSuchElementException]] for an empty source.
+    * <p>
+    * <img class="marble" src="https://raw.githubusercontent.com/reactor/reactor-core/v3.1.1.RELEASE/src/docs/marble/single.png" alt="">
+    * <p>
+    * Note Mono doesn't need [[Flux.single(AnyRef)]], since it is equivalent to
+    * [[Mono.defaultIfEmpty(AnyRef)]] in a [[Mono]].
+    *
+    * @return a [[Mono]] with the single item or an error signal
+    */
+  final def single() = Mono(jMono.single())
+
   /**
     * Subscribe to this [[Mono]] and request unbounded demand.
     * <p>

src/test/scala/reactor/core/scala/publisher/MonoTest.scala

@@ -1027,6 +1027,22 @@ class MonoTest extends FreeSpec with Matchers with TableDrivenPropertyChecks wit
         .verifyComplete()
     }
 
+    ".single" - {
+      "should enforce the existence of element" in {
+        val mono = Mono.just(randomValue).single()
+        StepVerifier.create(mono)
+          .expectNext(randomValue)
+          .verifyComplete()
+      }
+      "should throw exception if it is empty" in {
+        val mono = Mono.empty.single()
+        StepVerifier.create(mono)
+          .expectError(classOf[NoSuchElementException])
+          .verify()
+      }
+
+    }
+
     ".subscribe" - {
       "without parameter should return Disposable" in {
         val x = Mono.just(randomValue).subscribe()