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app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/command/Button.kt

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+package com.example.algorithmstudy.pattern.behavoiral.command
+
+class Button(private val command: Command) {
+    fun click() {
+        command.execute()
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/command/Command.kt

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+package com.example.algorithmstudy.pattern.behavoiral.command
+
+/**
+ * @reference : https://chercher.tech/kotlin/command-design-pattern-kotlin
+ */
+interface Command {
+    fun execute()
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/command/ConsumerWeapon.kt

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+package com.example.algorithmstudy.pattern.behavoiral.command
+
+interface ConsumerWeapon {
+    fun fire()
+    fun reload()
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/command/CruiseMissile.kt

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+package com.example.algorithmstudy.pattern.behavoiral.command
+
+class CruiseMissile : ConsumerWeapon {
+    override fun fire() {
+        println("Firing Cruise missile")
+    }
+
+    override fun reload() {
+        println("Reloading Crise missile")
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/command/FireCommand.kt

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+package com.example.algorithmstudy.pattern.behavoiral.command
+
+class FireCommand(private val consumerWeapon: ConsumerWeapon): Command {
+    override fun execute() {
+        consumerWeapon.fire()
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/command/GrenadeLauncher.kt

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+package com.example.algorithmstudy.pattern.behavoiral.command
+
+class GrenadeLauncher : ConsumerWeapon {
+    override fun fire() {
+        println("Firing Grenade Launcher")
+    }
+
+    override fun reload() {
+        println("Reloading Grenade Launcher")
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/command/ReloadAllCommand.kt

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+package com.example.algorithmstudy.pattern.behavoiral.command
+
+class ReloadAllCommand(private val consumerWeaponList: List<ConsumerWeapon>) : Command {
+    override fun execute() {
+        consumerWeaponList.forEach {
+            it.reload()
+        }
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/command/UniversalFiringSystem.kt

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+package com.example.algorithmstudy.pattern.behavoiral.command
+
+class UniversalFiringSystem {
+    fun getActiveWeapon(): ConsumerWeapon {
+        return CruiseMissile()
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/state/Complete.kt

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+package com.example.algorithmstudy.pattern.behavoiral.state
+
+object Complete : CookingStatus {
+    override val name = "Complete"
+
+    override fun foward(cooking: Cooking) = "다음 상태 없음"
+
+    override fun backward(cooking: Cooking): String {
+        cooking.status = OnCook
+        return cooking.status.name
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/state/Cooking.kt

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+package com.example.algorithmstudy.pattern.behavoiral.state
+
+class Cooking {
+    var status: CookingStatus = Wait
+
+    fun fowardStatus() = status.foward(this)
+    fun backwardStatus() = status.backward(this)
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/state/CookingStatus.kt

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+package com.example.algorithmstudy.pattern.behavoiral.state
+
+interface CookingStatus {
+    val name: String
+    fun foward(cooking: Cooking): String
+    fun backward(cooking: Cooking): String
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/state/OnCook.kt

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+package com.example.algorithmstudy.pattern.behavoiral.state
+
+object OnCook : CookingStatus {
+    override val name = "OnCook"
+
+    override fun foward(cooking: Cooking): String {
+        cooking.status = Complete
+        return cooking.status.name
+    }
+
+    override fun backward(cooking: Cooking): String {
+        cooking.status = Preparing
+        return cooking.status.name
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/state/Preparing.kt

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+package com.example.algorithmstudy.pattern.behavoiral.state
+
+object Preparing : CookingStatus {
+    override val name = "Preparing"
+
+    override fun foward(cooking: Cooking): String {
+        cooking.status = OnCook
+        return cooking.status.name
+    }
+
+    override fun backward(cooking: Cooking): String {
+        cooking.status = Wait
+        return cooking.status.name
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/state/Wait.kt

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+package com.example.algorithmstudy.pattern.behavoiral.state
+
+object Wait : CookingStatus {
+    override val name = "Waiting"
+
+    override fun foward(cooking: Cooking): String {
+        cooking.status = Preparing
+        return cooking.status.name
+    }
+
+    override fun backward(cooking: Cooking): String {
+        return "이전 상태 없음"
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/visitor/Euljiro.kt

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+package com.example.algorithmstudy.pattern.behavoiral.visitor
+
+class Euljiro : Region {
+    override fun accept(visitor: RegionVisitor) = visitor.visit(region = this)
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/visitor/Gangnam.kt

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+package com.example.algorithmstudy.pattern.behavoiral.visitor
+
+class Gangnam : Region {
+    override fun accept(visitor: RegionVisitor) = visitor.visit(region = this)
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/visitor/Hongdae.kt

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+package com.example.algorithmstudy.pattern.behavoiral.visitor
+
+class Hongdae : Region {
+    override fun accept(visitor: RegionVisitor) = visitor.visit(region = this)
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/visitor/Region.kt

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+package com.example.algorithmstudy.pattern.behavoiral.visitor
+
+interface Region {
+    fun accept(visitor: RegionVisitor)
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/visitor/RegionVisitor.kt

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+package com.example.algorithmstudy.pattern.behavoiral.visitor
+
+interface RegionVisitor {
+    fun visit(region: Region)
+}

app/src/main/java/com/example/algorithmstudy/pattern/behavoiral/visitor/Traveler.kt

@@ -0,0 +1,7 @@
+package com.example.algorithmstudy.pattern.behavoiral.visitor
+
+class Traveler : RegionVisitor {
+    override fun visit(region: Region) {
+        println("Visiting $region. It\'s so good!")
+    }
+}

app/src/main/java/com/example/algorithmstudy/pattern/creation/singleton/CustomSingleton.kt

@@ -1,7 +1,7 @@
 package com.example.algorithmstudy.pattern.creation.singleton
 
 /**
- *  ref : https://stackoverflow.com/questions/56825097/synchronized-singleton-in-kotlin
+ *  @ref : https://stackoverflow.com/questions/56825097/synchronized-singleton-in-kotlin
  */
 class CustomSingleton private constructor() {
     companion object {

app/src/main/java/com/example/algorithmstudy/pattern/structure/decorator/ChristmasTree.kt

@@ -1,7 +1,7 @@
 package com.example.algorithmstudy.pattern.structure.decorator
 
 /**
- *  reference : https://www.baeldung.com/kotlin/decorator-pattern
+ *  @reference : https://www.baeldung.com/kotlin/decorator-pattern
  */
 interface ChristmasTree {
     fun decorate(): String

app/src/test/java/com/example/algorithmstudy/design_pattern/CommandTest.kt

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+package com.example.algorithmstudy.design_pattern
+
+import com.example.algorithmstudy.pattern.behavoiral.command.*
+import org.junit.Test
+
+/**
+ *  Command Pattern
+ *
+ *  - 커맨터 패턴은 Command interface, invoker class, receiver class 로 구성된다.
+ *  - Command Interface는 execute function을 갖는다.
+ *  - Receiver에는 실제 수행해야 할 기능을 정의하며, invoker class가 Receiver의 function을 invoke 하는 역할을 맡게 된다.
+ *
+ */
+class CommandTest {
+
+    @Test
+    fun testCommand() {
+        // ConsumerWeapon의 구현체 : Receiver
+        // button: Invoker
+
+        val cw = UniversalFiringSystem().getActiveWeapon()
+        val fireCommand = FireCommand(cw)
+        val button = Button(fireCommand)
+        button.click()
+
+        val weaponList = listOf(CruiseMissile(), GrenadeLauncher())
+        val reloadAllCommand = ReloadAllCommand(weaponList)
+        val reloadButton = Button(reloadAllCommand)
+        reloadButton.click()
+    }
+}

app/src/test/java/com/example/algorithmstudy/design_pattern/StateTest.kt

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+package com.example.algorithmstudy.design_pattern
+
+import com.example.algorithmstudy.pattern.behavoiral.state.Cooking
+import org.junit.Test
+
+/**
+ *  State Pattern
+ *
+ * @reference : https://kimchanjung.github.io/design-pattern/2020/05/26/state-pattern/
+ *
+ *  - 각 상태를 단계별로 체이닝해 정의함으로써 특정 개념의 전후 상태를 명확하게 정의하고 관리할 수 있음.
+ *  - Client는 새로운 상태의 추가에 구애받지 않고 기존의 로직을 유지 가능
+ */
+class StateTest {
+
+    @Test
+    fun stateTest() {
+        val cooking = Cooking()
+
+        println(cooking.status)
+        println(cooking.fowardStatus())
+        println(cooking.fowardStatus())
+        println(cooking.fowardStatus())
+        println(cooking.fowardStatus())
+        println(cooking.backwardStatus())
+        println(cooking.backwardStatus())
+        println(cooking.backwardStatus())
+        println(cooking.backwardStatus())
+    }
+}

app/src/test/java/com/example/algorithmstudy/design_pattern/VisitorTest.kt

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+package com.example.algorithmstudy.design_pattern
+
+import com.example.algorithmstudy.pattern.behavoiral.visitor.Euljiro
+import com.example.algorithmstudy.pattern.behavoiral.visitor.Gangnam
+import com.example.algorithmstudy.pattern.behavoiral.visitor.Hongdae
+import com.example.algorithmstudy.pattern.behavoiral.visitor.Traveler
+import org.junit.Test
+
+/**
+ *  Visitor Pattern
+ *
+ *  @reference : https://chercher.tech/kotlin/visitor-design-pattern-kotlin
+ *
+ *  - Visitor pattern을 이용하면, 어느 클래스에서 수행하는 로직을 다른 클래스로 이전해 사용할 수 있게 된다.
+ *  - 즉, 아래의 테스트에서는 각 Region에서 수행하고자 하는 로직을 Traveler 클래스 안에 정의하고 실행하도록 하는 것이다.
+ */
+class VisitorTest {
+
+    @Test
+    fun visitorTest() {
+        val region = listOf(Gangnam(), Hongdae(), Euljiro())
+        val traveler = Traveler()
+        region.forEach {
+            it.accept(traveler)
+        }
+    }
+}