added Mediator design pattern

Author: DmitryTsyvtsyn

src/main/kotlin/design_patterns/Mediator.kt

@@ -0,0 +1,88 @@
+package design_patterns
+
+/**
+ *
+ * Mediator is a behavioral design pattern that reduces the coupling of many classes among themselves
+ *
+ * by moving these couplings into one mediator class
+ *
+ */
+
+// SoftwareDevelopmentManager is a mediator for the customer, designer, programmer and tester
+class SoftwareDevelopmentManager {
+
+    private lateinit var customer: SoftwareDevelopmentMember
+    private lateinit var designer: SoftwareDevelopmentMember
+    private lateinit var programmer: SoftwareDevelopmentMember
+    private lateinit var tester: SoftwareDevelopmentMember
+
+    fun changeCustomer(member: SoftwareDevelopmentMember) { customer = member }
+    fun changeDesigner(member: SoftwareDevelopmentMember) { designer = member }
+    fun changeProgrammer(member: SoftwareDevelopmentMember) { programmer = member }
+    fun changeTester(member: SoftwareDevelopmentMember) { tester = member }
+
+    private val stages = mutableListOf<String>()
+    val stagesAsString: String
+        get() = stages.joinToString("\n")
+
+    // members do not interact directly with each other because the mediator does it
+    // which reduces the coupling between them
+    fun nextStage(work: String, member: SoftwareDevelopmentMember) {
+        val finishedStage = when (member) {
+
+            is CustomerSoftwareDevelopmentMember -> designer.receiveWork(work)
+
+            is DesignerSoftwareDevelopmentMember -> programmer.receiveWork(work)
+
+            is ProgrammerSoftwareDevelopmentMember -> tester.receiveWork(work)
+
+            is TesterSoftwareDevelopmentMember -> customer.receiveWork(work)
+
+            else -> ""
+        }
+
+        stages.add(finishedStage)
+    }
+
+}
+
+// SoftwareDevelopmentMember works with other members through the mediator
+abstract class SoftwareDevelopmentMember(protected val mediator: SoftwareDevelopmentManager) {
+
+    abstract fun receiveWork(work: String): String
+
+    abstract fun finishWork()
+
+}
+
+class CustomerSoftwareDevelopmentMember(mediator: SoftwareDevelopmentManager) : SoftwareDevelopmentMember(mediator) {
+    override fun receiveWork(work: String) = "Customer accepted the work: $work"
+
+    override fun finishWork() {
+        mediator.nextStage("design development", this)
+    }
+}
+
+class DesignerSoftwareDevelopmentMember(mediator: SoftwareDevelopmentManager) : SoftwareDevelopmentMember(mediator) {
+    override fun receiveWork(work: String) = "Designer accepted the work: $work"
+
+    override fun finishWork() {
+        mediator.nextStage("writing code", this)
+    }
+}
+
+class ProgrammerSoftwareDevelopmentMember(mediator: SoftwareDevelopmentManager) : SoftwareDevelopmentMember(mediator) {
+    override fun receiveWork(work: String) = "Programmer accepted the work: $work"
+
+    override fun finishWork() {
+        mediator.nextStage("application testing", this)
+    }
+}
+
+class TesterSoftwareDevelopmentMember(mediator: SoftwareDevelopmentManager) : SoftwareDevelopmentMember(mediator) {
+    override fun receiveWork(work: String) = "Tester accepted the work: $work"
+
+    override fun finishWork() {
+        mediator.nextStage("business valuation", this)
+    }
+}

src/test/kotlin/design_patterns/MediatorTest.kt

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+package design_patterns
+
+import org.junit.Test
+import org.junit.Assert.assertEquals
+
+class MediatorTest {
+
+    @Test
+    fun test() {
+        val manager = SoftwareDevelopmentManager()
+
+        val customer = CustomerSoftwareDevelopmentMember(manager)
+        manager.changeCustomer(customer)
+        val designer = DesignerSoftwareDevelopmentMember(manager)
+        manager.changeDesigner(designer)
+        val programmer = ProgrammerSoftwareDevelopmentMember(manager)
+        manager.changeProgrammer(programmer)
+        val tester = TesterSoftwareDevelopmentMember(manager)
+        manager.changeTester(tester)
+
+        customer.finishWork()
+        designer.finishWork()
+        programmer.finishWork()
+        tester.finishWork()
+
+        assertEquals("""
+            Designer accepted the work: design development
+            Programmer accepted the work: writing code
+            Tester accepted the work: application testing
+            Customer accepted the work: business valuation
+        """.trimIndent(), manager.stagesAsString)
+    }
+
+}