added Composite design pattern

Author: DmitryTsyvtsyn

src/main/kotlin/design_patterns/Composite.kt

@@ -0,0 +1,53 @@
+package design_patterns
+
+/**
+ *
+ * Composite is a structural design pattern that organizes objects into a tree structure
+ *
+ * and allows clients to access individual objects and groups of objects in the same way.
+ *
+ */
+
+// all components have a common interface
+abstract class MenuComponent(val title: String) {
+    // menus and items can give information about themselves
+    abstract fun fetchMenuInformation() : String
+}
+
+class Menu(title: String) : MenuComponent(title) {
+
+    // menu can contain other menus and items
+    private val childMenuComponents = mutableListOf<MenuComponent>()
+
+    // addComponent/removeComponent operations are only available for the menu
+    fun addComponent(component: MenuComponent) {
+        childMenuComponents.add(component)
+    }
+
+    fun removeComponent(component: MenuComponent) {
+        childMenuComponents.remove(component)
+    }
+
+    override fun fetchMenuInformation(): String {
+        val builder = StringBuilder()
+        builder.append("Menu: $title")
+        childMenuComponents.forEach { component ->
+            builder.append("\n")
+            builder.append(component.fetchMenuInformation())
+        }
+        return builder.toString()
+    }
+
+}
+
+// the simple component that contains no others
+class MenuItem(title: String, private val price: Int) : MenuComponent(title) {
+
+    override fun fetchMenuInformation(): String =
+        """
+            title: $title
+            price: $price
+            -------------
+        """.trimIndent()
+
+}

src/test/kotlin/design_patterns/CompositeTest.kt

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+package design_patterns
+
+import org.junit.Test
+import org.junit.Assert.assertEquals
+
+class CompositeTest {
+
+    @Test
+    fun test() {
+        val menu = Menu("Delicious Restaurant")
+
+        val pizzasMenu = Menu("Pizzas")
+        pizzasMenu.addComponent(MenuItem("Cheese pizza", 10))
+        pizzasMenu.addComponent(MenuItem("Pepperoni pizza", 11))
+        menu.addComponent(pizzasMenu)
+
+        val cakesMenu = Menu("Cakes")
+        cakesMenu.addComponent(MenuItem("Chocolate cake", 13))
+        cakesMenu.addComponent(MenuItem("Cheese cake", 13))
+        menu.addComponent(cakesMenu)
+
+        assertEquals("""
+            Menu: Delicious Restaurant
+            Menu: Pizzas
+            title: Cheese pizza
+            price: 10
+            -------------
+            title: Pepperoni pizza
+            price: 11
+            -------------
+            Menu: Cakes
+            title: Chocolate cake
+            price: 13
+            -------------
+            title: Cheese cake
+            price: 13
+            -------------
+        """.trimIndent(), menu.fetchMenuInformation())
+    }
+
+}