Java的23種設計模式，詳細講解（三）

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1. 適配器（Adapter）

Intent

把一個類接口轉換成另一個用戶需要的接口。

Class Diagram

Implementation

鴨子（Duck）和火雞（Turkey）擁有不同的叫聲，Duck 的叫聲調用 quack() 方法，而 Turkey 調用 gobble() 方法。

要求將 Turkey 的 gobble() 方法適配成 Duck 的 quack() 方法，從而讓火雞冒充鴨子！

public?interface?Duck?{
????void?quack();}public?interface?Turkey?{
????void?gobble();}public?class?WildTurkey?implements?Turkey?{
????@Override
????public?void?gobble()?{
????????System.out.println("gobble!");
????}}public?class?TurkeyAdapter?implements?Duck?{
????Turkey?turkey;

????public?TurkeyAdapter(Turkey?turkey)?{
????????this.turkey?=?turkey;
????}

????@Override
????public?void?quack()?{
????????turkey.gobble();
????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????Turkey?turkey?=?new?WildTurkey();
????????Duck?duck?=?new?TurkeyAdapter(turkey);
????????duck.quack();
????}}

JDK

• java.util.Arrays#asList()

• java.util.Collections#list()

• java.util.Collections#enumeration()

• javax.xml.bind.annotation.adapters.XMLAdapter

2. 橋接（Bridge）

Intent

將抽象與實現分離開來，使它們可以獨立變化。

Class Diagram

• Abstraction：定義抽象類的接口

• Implementor：定義實現類接口

Implementation

RemoteControl 表示遙控器，指代 Abstraction。

TV 表示電視，指代 Implementor。

橋接模式將遙控器和電視分離開來，從而可以獨立改變遙控器或者電視的實現。

public?abstract?class?TV?{
????public?abstract?void?on();

????public?abstract?void?off();

????public?abstract?void?tuneChannel();}public?class?Sony?extends?TV?{
????@Override
????public?void?on()?{
????????System.out.println("Sony.on()");
????}

????@Override
????public?void?off()?{
????????System.out.println("Sony.off()");
????}

????@Override
????public?void?tuneChannel()?{
????????System.out.println("Sony.tuneChannel()");
????}}public?class?RCA?extends?TV?{
????@Override
????public?void?on()?{
????????System.out.println("RCA.on()");
????}

????@Override
????public?void?off()?{
????????System.out.println("RCA.off()");
????}

????@Override
????public?void?tuneChannel()?{
????????System.out.println("RCA.tuneChannel()");
????}}public?abstract?class?RemoteControl?{
????protected?TV?tv;

????public?RemoteControl(TV?tv)?{
????????this.tv?=?tv;
????}

????public?abstract?void?on();

????public?abstract?void?off();

????public?abstract?void?tuneChannel();}public?class?ConcreteRemoteControl1?extends?RemoteControl?{
????public?ConcreteRemoteControl1(TV?tv)?{
????????super(tv);
????}

????@Override
????public?void?on()?{
????????System.out.println("ConcreteRemoteControl1.on()");
????????tv.on();
????}

????@Override
????public?void?off()?{
????????System.out.println("ConcreteRemoteControl1.off()");
????????tv.off();
????}

????@Override
????public?void?tuneChannel()?{
????????System.out.println("ConcreteRemoteControl1.tuneChannel()");
????????tv.tuneChannel();
????}}public?class?ConcreteRemoteControl2?extends?RemoteControl?{
????public?ConcreteRemoteControl2(TV?tv)?{
????????super(tv);
????}

????@Override
????public?void?on()?{
????????System.out.println("ConcreteRemoteControl2.on()");
????????tv.on();
????}

????@Override
????public?void?off()?{
????????System.out.println("ConcreteRemoteControl2.off()");
????????tv.off();
????}

????@Override
????public?void?tuneChannel()?{
????????System.out.println("ConcreteRemoteControl2.tuneChannel()");
????????tv.tuneChannel();
????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????RemoteControl?remoteControl1?=?new?ConcreteRemoteControl1(new?RCA());
????????remoteControl1.on();
????????remoteControl1.off();
????????remoteControl1.tuneChannel();
????????RemoteControl?remoteControl2?=?new?ConcreteRemoteControl2(new?Sony());
?????????remoteControl2.on();
?????????remoteControl2.off();
?????????remoteControl2.tuneChannel();
????}}

JDK

• AWT (It provides an abstraction layer which maps onto the native OS the windowing support.)

• JDBC

3. 組合（Composite）

Intent

將對象組合成樹形結構來表示“整體/部分”層次關系，允許用戶以相同的方式處理單獨對象和組合對象。

Class Diagram

組件（Component）類是組合類（Composite）和葉子類（Leaf）的父類，可以把組合類看成是樹的中間節點。

組合對象擁有一個或者多個組件對象，因此組合對象的操作可以委托給組件對象去處理，而組件對象可以是另一個組合對象或者葉子對象。

Implementation

public?abstract?class?Component?{
????protected?String?name;

????public?Component(String?name)?{
????????this.name?=?name;
????}

????public?void?print()?{
????????print(0);
????}

????abstract?void?print(int?level);

????abstract?public?void?add(Component?component);

????abstract?public?void?remove(Component?component);}public?class?Composite?extends?Component?{

????private?List<Component>?child;

????public?Composite(String?name)?{
????????super(name);
????????child?=?new?ArrayList<>();
????}

????@Override
????void?print(int?level)?{
????????for?(int?i?=?0;?i?<?level;?i++)?{
????????????System.out.print("--");
????????}
????????System.out.println("Composite:"?+?name);
????????for?(Component?component?:?child)?{
????????????component.print(level?+?1);
????????}
????}

????@Override
????public?void?add(Component?component)?{
????????child.add(component);
????}

????@Override
????public?void?remove(Component?component)?{
????????child.remove(component);
????}}public?class?Leaf?extends?Component?{
????public?Leaf(String?name)?{
????????super(name);
????}

????@Override
????void?print(int?level)?{
????????for?(int?i?=?0;?i?<?level;?i++)?{
????????????System.out.print("--");
????????}
????????System.out.println("left:"?+?name);
????}

????@Override
????public?void?add(Component?component)?{
????????throw?new?UnsupportedOperationException();?//?犧牲透明性換取單一職責原則，這樣就不用考慮是葉子節點還是組合節點????}

????@Override
????public?void?remove(Component?component)?{
????????throw?new?UnsupportedOperationException();
????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????Composite?root?=?new?Composite("root");
????????Component?node1?=?new?Leaf("1");
????????Component?node2?=?new?Composite("2");
????????Component?node3?=?new?Leaf("3");
????????root.add(node1);
????????root.add(node2);
????????root.add(node3);
????????Component?node21?=?new?Leaf("21");
????????Component?node22?=?new?Composite("22");
????????node2.add(node21);
????????node2.add(node22);
????????Component?node221?=?new?Leaf("221");
????????node22.add(node221);
????????root.print();
????}}Composite:root--left:1--Composite:2----left:21----Composite:22------left:221--left:3

JDK

• javax.swing.JComponent#add(Component)

• java.awt.Container#add(Component)

• java.util.Map#putAll(Map)

• java.util.List#addAll(Collection)

• java.util.Set#addAll(Collection)

4. 裝飾（Decorator）

Intent

為對象動態添加功能。

Class Diagram

裝飾者（Decorator）和具體組件（ConcreteComponent）都繼承自組件（Component），具體組件的方法實現不需要依賴于其它對象，而裝飾者組合了一個組件，這樣它可以裝飾其它裝飾者或者具體組件。所謂裝飾，就是把這個裝飾者套在被裝飾者之上，從而動態擴展被裝飾者的功能。裝飾者的方法有一部分是自己的，這屬于它的功能，然后調用被裝飾者的方法實現，從而也保留了被裝飾者的功能。可以看到，具體組件應當是裝飾層次的最低層，因為只有具體組件的方法實現不需要依賴于其它對象。

Implementation

設計不同種類的飲料，飲料可以添加配料，比如可以添加牛奶，并且支持動態添加新配料。每增加一種配料，該飲料的價格就會增加，要求計算一種飲料的價格。

下圖表示在 DarkRoast 飲料上新增新添加 Mocha 配料，之后又添加了 Whip 配料。DarkRoast 被 Mocha 包裹，Mocha 又被 Whip 包裹。它們都繼承自相同父類，都有 cost() 方法，外層類的 cost() 方法調用了內層類的 cost() 方法。

public?interface?Beverage?{
????double?cost();}public?class?DarkRoast?implements?Beverage?{
????@Override
????public?double?cost()?{
????????return?1;
????}}public?class?HouseBlend?implements?Beverage?{
????@Override
????public?double?cost()?{
????????return?1;
????}}public?abstract?class?CondimentDecorator?implements?Beverage?{
????protected?Beverage?beverage;}public?class?Milk?extends?CondimentDecorator?{

????public?Milk(Beverage?beverage)?{
????????this.beverage?=?beverage;
????}

????@Override
????public?double?cost()?{
????????return?1?+?beverage.cost();
????}}public?class?Mocha?extends?CondimentDecorator?{

????public?Mocha(Beverage?beverage)?{
????????this.beverage?=?beverage;
????}

????@Override
????public?double?cost()?{
????????return?1?+?beverage.cost();
????}}public?class?Client?{

????public?static?void?main(String[]?args)?{
????????Beverage?beverage?=?new?HouseBlend();
????????beverage?=?new?Mocha(beverage);
????????beverage?=?new?Milk(beverage);
????????System.out.println(beverage.cost());
????}}3.0

設計原則

類應該對擴展開放，對修改關閉：也就是添加新功能時不需要修改代碼。飲料可以動態添加新的配料，而不需要去修改飲料的代碼。

不可能把所有的類設計成都滿足這一原則，應當把該原則應用于最有可能發生改變的地方。

JDK

• java.io.BufferedInputStream(InputStream)

• java.io.DataInputStream(InputStream)

• java.io.BufferedOutputStream(OutputStream)

• java.util.zip.ZipOutputStream(OutputStream)

• java.util.Collections#checkedList|Map|Set|SortedSet|SortedMap

5. 外觀（Facade）

Intent

提供了一個統一的接口，用來訪問子系統中的一群接口，從而讓子系統更容易使用。

Class Diagram

Implementation

觀看電影需要操作很多電器，使用外觀模式實現一鍵看電影功能。

public?class?SubSystem?{
????public?void?turnOnTV()?{
????????System.out.println("turnOnTV()");
????}

????public?void?setCD(String?cd)?{
????????System.out.println("setCD(?"?+?cd?+?"?)");
????}

????public?void?startWatching(){
????????System.out.println("startWatching()");
????}}public?class?Facade?{
????private?SubSystem?subSystem?=?new?SubSystem();

????public?void?watchMovie()?{
????????subSystem.turnOnTV();
????????subSystem.setCD("a?movie");
????????subSystem.startWatching();
????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????Facade?facade?=?new?Facade();
????????facade.watchMovie();
????}}

設計原則

最少知識原則：只和你的密友談話。也就是說客戶對象所需要交互的對象應當盡可能少。

6. 享元（Flyweight）

Intent

利用共享的方式來支持大量細粒度的對象，這些對象一部分內部狀態是相同的。

Class Diagram

• Flyweight：享元對象

• IntrinsicState：內部狀態，享元對象共享內部狀態

• ExtrinsicState：外部狀態，每個享元對象的外部狀態不同

Implementation

public?interface?Flyweight?{
????void?doOperation(String?extrinsicState);}public?class?ConcreteFlyweight?implements?Flyweight?{

????private?String?intrinsicState;

????public?ConcreteFlyweight(String?intrinsicState)?{
????????this.intrinsicState?=?intrinsicState;
????}

????@Override
????public?void?doOperation(String?extrinsicState)?{
????????System.out.println("Object?address:?"?+?System.identityHashCode(this));
????????System.out.println("IntrinsicState:?"?+?intrinsicState);
????????System.out.println("ExtrinsicState:?"?+?extrinsicState);
????}}public?class?FlyweightFactory?{

????private?HashMap<String,?Flyweight>?flyweights?=?new?HashMap<>();

????Flyweight?getFlyweight(String?intrinsicState)?{
????????if?(!flyweights.containsKey(intrinsicState))?{
????????????Flyweight?flyweight?=?new?ConcreteFlyweight(intrinsicState);
????????????flyweights.put(intrinsicState,?flyweight);
????????}
????????return?flyweights.get(intrinsicState);
????}}public?class?Client?{

????public?static?void?main(String[]?args)?{
????????FlyweightFactory?factory?=?new?FlyweightFactory();
????????Flyweight?flyweight1?=?factory.getFlyweight("aa");
????????Flyweight?flyweight2?=?factory.getFlyweight("aa");
????????flyweight1.doOperation("x");
????????flyweight2.doOperation("y");
????}}Object?address:?1163157884IntrinsicState:?aaExtrinsicState:?xObject?address:?1163157884IntrinsicState:?aaExtrinsicState:?y

JDK

Java 利用緩存來加速大量小對象的訪問時間。

• java.lang.Integer#valueOf(int)

• java.lang.Boolean#valueOf(boolean)

• java.lang.Byte#valueOf(byte)

• java.lang.Character#valueOf(char)

7. 代理（Proxy）

Intent

控制對其它對象的訪問。

Class Diagram

代理有以下四類：

• 遠程代理（Remote Proxy）：控制對遠程對象（不同地址空間）的訪問，它負責將請求及其參數進行編碼，并向不同地址空間中的對象發送已經編碼的請求。

• 虛擬代理（Virtual Proxy）：根據需要創建開銷很大的對象，它可以緩存實體的附加信息，以便延遲對它的訪問，例如在網站加載一個很大圖片時，不能馬上完成，可以用虛擬代理緩存圖片的大小信息，然后生成一張臨時圖片代替原始圖片。

• 保護代理（Protection Proxy）：按權限控制對象的訪問，它負責檢查調用者是否具有實現一個請求所必須的訪問權限。

• 智能代理（Smart Reference）：取代了簡單的指針，它在訪問對象時執行一些附加操作：記錄對象的引用次數；當第一次引用一個對象時，將它裝入內存；在訪問一個實際對象前，檢查是否已經鎖定了它，以確保其它對象不能改變它。

Implementation

以下是一個虛擬代理的實現，模擬了圖片延遲加載的情況下使用與圖片大小相等的臨時內容去替換原始圖片，直到圖片加載完成才將圖片顯示出來。

public?interface?Image?{
????void?showImage();}public?class?HighResolutionImage?implements?Image?{

????private?URL?imageURL;
????private?long?startTime;
????private?int?height;
????private?int?width;

????public?int?getHeight()?{
????????return?height;
????}

????public?int?getWidth()?{
????????return?width;
????}

????public?HighResolutionImage(URL?imageURL)?{
????????this.imageURL?=?imageURL;
????????this.startTime?=?System.currentTimeMillis();
????????this.width?=?600;
????????this.height?=?600;
????}

????public?boolean?isLoad()?{
????????//?模擬圖片加載，延遲?3s?加載完成????????long?endTime?=?System.currentTimeMillis();
????????return?endTime?-?startTime?>?3000;
????}

????@Override
????public?void?showImage()?{
????????System.out.println("Real?Image:?"?+?imageURL);
????}}public?class?ImageProxy?implements?Image?{

????private?HighResolutionImage?highResolutionImage;

????public?ImageProxy(HighResolutionImage?highResolutionImage)?{
????????this.highResolutionImage?=?highResolutionImage;
????}

????@Override
????public?void?showImage()?{
????????while?(!highResolutionImage.isLoad())?{
????????????try?{
????????????????System.out.println("Temp?Image:?"?+?highResolutionImage.getWidth()?+?"?"?+?highResolutionImage.getHeight());
????????????????Thread.sleep(100);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????}
????????highResolutionImage.showImage();
????}}public?class?ImageViewer?{

????public?static?void?main(String[]?args)?throws?Exception?{
????????String?image?=?"http://image.jpg";
????????URL?url?=?new?URL(image);
????????HighResolutionImage?highResolutionImage?=?new?HighResolutionImage(url);
????????ImageProxy?imageProxy?=?new?ImageProxy(highResolutionImage);
????????imageProxy.showImage();
????}}

JDK

• java.lang.reflect.Proxy

• RMI