mediator (#743)

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* commit

* update

* update

* update

* update

Author: kanake10

kotlin-patterns/src/main/kotlin/com/baeldung/mediator/Mediator.kt

@@ -0,0 +1,46 @@
+package com.baeldung.mediator
+
+interface AirTrafficController {
+    fun registerAirplane(airplane: Airplane)
+    fun deregisterAirplane(airplane: Airplane)
+    fun requestTakeOff(airplane: Airplane)
+    fun requestLanding(airplane: Airplane)
+}
+class Airplane(private val registrationNumber: String, private val controller: AirTrafficController) {
+    fun takeOff() {
+        println("$registrationNumber is requesting takeoff.")
+        controller.requestTakeOff(this)
+    }
+    fun land() {
+        println("$registrationNumber is requesting landing.")
+        controller.requestLanding(this)
+    }
+    fun notifyTakeOff() {
+        println("$registrationNumber has taken off.")
+        controller.deregisterAirplane(this)
+    }
+    fun notifyLanding() {
+        println("$registrationNumber has landed.")
+        controller.registerAirplane(this)
+    }
+}
+
+class AirTrafficControlTower : AirTrafficController {
+    private val registeredAirplanes: MutableSet<Airplane> = mutableSetOf()
+    override fun registerAirplane(airplane: Airplane) {
+        registeredAirplanes.add(airplane)
+    }
+    override fun deregisterAirplane(airplane: Airplane) {
+        registeredAirplanes.remove(airplane)
+    }
+    override fun requestTakeOff(airplane: Airplane) {
+        if (registeredAirplanes.contains(airplane)) {
+            airplane.notifyTakeOff()
+        }
+    }
+    override fun requestLanding(airplane: Airplane) {
+        if (!registeredAirplanes.contains(airplane)) {
+            airplane.notifyLanding()
+        }
+    }
+}