Java的23種設計模式，詳細講解（二）

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1.責任鏈（Chain Of Responsibility）

Intent

使多個對象都有機會處理請求，從而避免請求的發(fā)送者和接收者之間的耦合關系。將這些對象連成一條鏈，并沿著這條鏈發(fā)送該請求，直到有一個對象處理它為止。

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Class Diagram

• Handler：定義處理請求的接口，并且實現(xiàn)后繼鏈（successor）

Implementation

public?abstract?class?Handler?{

????protected?Handler?successor;


????public?Handler(Handler?successor)?{
????????this.successor?=?successor;
????}


????protected?abstract?void?handleRequest(Request?request);}public?class?ConcreteHandler1?extends?Handler?{

????public?ConcreteHandler1(Handler?successor)?{
????????super(successor);
????}


????@Override
????protected?void?handleRequest(Request?request)?{
????????if?(request.getType()?==?RequestType.TYPE1)?{
????????????System.out.println(request.getName()?+?"?is?handle?by?ConcreteHandler1");
????????????return;
????????}
????????if?(successor?!=?null)?{
????????????successor.handleRequest(request);
????????}
????}}public?class?ConcreteHandler2?extends?Handler?{

????public?ConcreteHandler2(Handler?successor)?{
????????super(successor);
????}


????@Override
????protected?void?handleRequest(Request?request)?{
????????if?(request.getType()?==?RequestType.TYPE2)?{
????????????System.out.println(request.getName()?+?"?is?handle?by?ConcreteHandler2");
????????????return;
????????}
????????if?(successor?!=?null)?{
????????????successor.handleRequest(request);
????????}
????}}public?class?Request?{

????private?RequestType?type;
????private?String?name;


????public?Request(RequestType?type,?String?name)?{
????????this.type?=?type;
????????this.name?=?name;
????}


????public?RequestType?getType()?{
????????return?type;
????}


????public?String?getName()?{
????????return?name;
????}}public?enum?RequestType?{
????TYPE1,?TYPE2}public?class?Client?{

????public?static?void?main(String[]?args)?{

????????Handler?handler1?=?new?ConcreteHandler1(null);
????????Handler?handler2?=?new?ConcreteHandler2(handler1);

????????Request?request1?=?new?Request(RequestType.TYPE1,?"request1");
????????handler2.handleRequest(request1);

????????Request?request2?=?new?Request(RequestType.TYPE2,?"request2");
????????handler2.handleRequest(request2);
????}}request1?is?handle?by?ConcreteHandler1request2?is?handle?by?ConcreteHandler2

JDK

• java.util.logging.Logger#log()

• Apache Commons Chain

• javax.servlet.Filter#doFilter()

2. 命令（Command）

Intent

將命令封裝成對象中，具有以下作用：

• 使用命令來參數(shù)化其它對象

• 將命令放入隊列中進行排隊

• 將命令的操作記錄到日志中

• 支持可撤銷的操作

Class Diagram

• Command：命令

• Receiver：命令接收者，也就是命令真正的執(zhí)行者

• Invoker：通過它來調(diào)用命令

• Client：可以設置命令與命令的接收者

Implementation

設計一個遙控器，可以控制電燈開關。

public?interface?Command?{
????void?execute();}public?class?LightOnCommand?implements?Command?{
????Light?light;

????public?LightOnCommand(Light?light)?{
????????this.light?=?light;
????}

????@Override
????public?void?execute()?{
????????light.on();
????}}public?class?LightOffCommand?implements?Command?{
????Light?light;

????public?LightOffCommand(Light?light)?{
????????this.light?=?light;
????}

????@Override
????public?void?execute()?{
????????light.off();
????}}public?class?Light?{

????public?void?on()?{
????????System.out.println("Light?is?on!");
????}

????public?void?off()?{
????????System.out.println("Light?is?off!");
????}}/**?*?遙控器?*/public?class?Invoker?{
????private?Command[]?onCommands;
????private?Command[]?offCommands;
????private?final?int?slotNum?=?7;

????public?Invoker()?{
????????this.onCommands?=?new?Command[slotNum];
????????this.offCommands?=?new?Command[slotNum];
????}

????public?void?setOnCommand(Command?command,?int?slot)?{
????????onCommands[slot]?=?command;
????}

????public?void?setOffCommand(Command?command,?int?slot)?{
????????offCommands[slot]?=?command;
????}

????public?void?onButtonWasPushed(int?slot)?{
????????onCommands[slot].execute();
????}

????public?void?offButtonWasPushed(int?slot)?{
????????offCommands[slot].execute();
????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????Invoker?invoker?=?new?Invoker();
????????Light?light?=?new?Light();
????????Command?lightOnCommand?=?new?LightOnCommand(light);
????????Command?lightOffCommand?=?new?LightOffCommand(light);
????????invoker.setOnCommand(lightOnCommand,?0);
????????invoker.setOffCommand(lightOffCommand,?0);
????????invoker.onButtonWasPushed(0);
????????invoker.offButtonWasPushed(0);
????}}

JDK

• java.lang.Runnable

• Netflix Hystrix

• javax.swing.Action

3. 解釋器（Interpreter）

Intent

為語言創(chuàng)建解釋器，通常由語言的語法和語法分析來定義。

Class Diagram

• TerminalExpression：終結(jié)符表達式，每個終結(jié)符都需要一個 TerminalExpression。

• Context：上下文，包含解釋器之外的一些全局信息。

Implementation

以下是一個規(guī)則檢驗器實現(xiàn)，具有 and 和 or 規(guī)則，通過規(guī)則可以構建一顆解析樹，用來檢驗一個文本是否滿足解析樹定義的規(guī)則。

例如一顆解析樹為 D And (A Or (B C))，文本 "D A" 滿足該解析樹定義的規(guī)則。

這里的 Context 指的是 String。

public?abstract?class?Expression?{
????public?abstract?boolean?interpret(String?str);}public?class?TerminalExpression?extends?Expression?{

????private?String?literal?=?null;

????public?TerminalExpression(String?str)?{
????????literal?=?str;
????}

????public?boolean?interpret(String?str)?{
????????StringTokenizer?st?=?new?StringTokenizer(str);
????????while?(st.hasMoreTokens())?{
????????????String?test?=?st.nextToken();
????????????if?(test.equals(literal))?{
????????????????return?true;
????????????}
????????}
????????return?false;
????}}public?class?AndExpression?extends?Expression?{

????private?Expression?expression1?=?null;
????private?Expression?expression2?=?null;

????public?AndExpression(Expression?expression1,?Expression?expression2)?{
????????this.expression1?=?expression1;
????????this.expression2?=?expression2;
????}

????public?boolean?interpret(String?str)?{
????????return?expression1.interpret(str)?&&?expression2.interpret(str);
????}}public?class?OrExpression?extends?Expression?{
????private?Expression?expression1?=?null;
????private?Expression?expression2?=?null;

????public?OrExpression(Expression?expression1,?Expression?expression2)?{
????????this.expression1?=?expression1;
????????this.expression2?=?expression2;
????}

????public?boolean?interpret(String?str)?{
????????return?expression1.interpret(str)?||?expression2.interpret(str);
????}}public?class?Client?{

????/**?????*?構建解析樹?????*/
????public?static?Expression?buildInterpreterTree()?{
????????//?Literal????????Expression?terminal1?=?new?TerminalExpression("A");
????????Expression?terminal2?=?new?TerminalExpression("B");
????????Expression?terminal3?=?new?TerminalExpression("C");
????????Expression?terminal4?=?new?TerminalExpression("D");
????????//?B?C????????Expression?alternation1?=?new?OrExpression(terminal2,?terminal3);
????????//?A?Or?(B?C)????????Expression?alternation2?=?new?OrExpression(terminal1,?alternation1);
????????//?D?And?(A?Or?(B?C))????????return?new?AndExpression(terminal4,?alternation2);
????}

????public?static?void?main(String[]?args)?{
????????Expression?define?=?buildInterpreterTree();
????????String?context1?=?"D?A";
????????String?context2?=?"A?B";
????????System.out.println(define.interpret(context1));
????????System.out.println(define.interpret(context2));
????}}truefalse

JDK

• java.util.Pattern

• java.text.Normalizer

• All subclasses of?java.text.Format

• javax.el.ELResolver

4. 迭代器（Iterator）

Intent

提供一種順序訪問聚合對象元素的方法，并且不暴露聚合對象的內(nèi)部表示。

Class Diagram

• Aggregate 是聚合類，其中 createIterator() 方法可以產(chǎn)生一個 Iterator；

• Iterator 主要定義了 hasNext() 和 next() 方法。

• Client 組合了 Aggregate，為了迭代遍歷 Aggregate，也需要組合 Iterator。

Implementation

public?interface?Aggregate?{
????Iterator?createIterator();}public?class?ConcreteAggregate?implements?Aggregate?{

????private?Integer[]?items;

????public?ConcreteAggregate()?{
????????items?=?new?Integer[10];
????????for?(int?i?=?0;?i?<?items.length;?i++)?{
????????????items[i]?=?i;
????????}
????}

????@Override
????public?Iterator?createIterator()?{
????????return?new?ConcreteIterator<Integer>(items);
????}}public?interface?Iterator<Item>?{

????Item?next();

????boolean?hasNext();}public?class?ConcreteIterator<Item>?implements?Iterator?{

????private?Item[]?items;
????private?int?position?=?0;

????public?ConcreteIterator(Item[]?items)?{
????????this.items?=?items;
????}

????@Override
????public?Object?next()?{
????????return?items[position++];
????}

????@Override
????public?boolean?hasNext()?{
????????return?position?<?items.length;
????}}public?class?Client?{

????public?static?void?main(String[]?args)?{
????????Aggregate?aggregate?=?new?ConcreteAggregate();
????????Iterator<Integer>?iterator?=?aggregate.createIterator();
????????while?(iterator.hasNext())?{
????????????System.out.println(iterator.next());
????????}
????}}

JDK

• java.util.Iterator

• java.util.Enumeration

5. 中介者（Mediator）

Intent

集中相關對象之間復雜的溝通和控制方式。

Class Diagram

• Mediator：中介者，定義一個接口用于與各同事（Colleague）對象通信。

• Colleague：同事，相關對象

Implementation

Alarm（鬧鐘）、CoffeePot（咖啡壺）、Calendar（日歷）、Sprinkler（噴頭）是一組相關的對象，在某個對象的事件產(chǎn)生時需要去操作其它對象，形成了下面這種依賴結(jié)構：

使用中介者模式可以將復雜的依賴結(jié)構變成星形結(jié)構：

public?abstract?class?Colleague?{
????public?abstract?void?onEvent(Mediator?mediator);}public?class?Alarm?extends?Colleague?{

????@Override
????public?void?onEvent(Mediator?mediator)?{
????????mediator.doEvent("alarm");
????}

????public?void?doAlarm()?{
????????System.out.println("doAlarm()");
????}}public?class?CoffeePot?extends?Colleague?{
????@Override
????public?void?onEvent(Mediator?mediator)?{
????????mediator.doEvent("coffeePot");
????}

????public?void?doCoffeePot()?{
????????System.out.println("doCoffeePot()");
????}}public?class?Calender?extends?Colleague?{
????@Override
????public?void?onEvent(Mediator?mediator)?{
????????mediator.doEvent("calender");
????}

????public?void?doCalender()?{
????????System.out.println("doCalender()");
????}}public?class?Sprinkler?extends?Colleague?{
????@Override
????public?void?onEvent(Mediator?mediator)?{
????????mediator.doEvent("sprinkler");
????}

????public?void?doSprinkler()?{
????????System.out.println("doSprinkler()");
????}}public?abstract?class?Mediator?{
????public?abstract?void?doEvent(String?eventType);}public?class?ConcreteMediator?extends?Mediator?{
????private?Alarm?alarm;
????private?CoffeePot?coffeePot;
????private?Calender?calender;
????private?Sprinkler?sprinkler;

????public?ConcreteMediator(Alarm?alarm,?CoffeePot?coffeePot,?Calender?calender,?Sprinkler?sprinkler)?{
????????this.alarm?=?alarm;
????????this.coffeePot?=?coffeePot;
????????this.calender?=?calender;
????????this.sprinkler?=?sprinkler;
????}

????@Override
????public?void?doEvent(String?eventType)?{
????????switch?(eventType)?{
????????????case?"alarm":
????????????????doAlarmEvent();
????????????????break;
????????????case?"coffeePot":
????????????????doCoffeePotEvent();
????????????????break;
????????????case?"calender":
????????????????doCalenderEvent();
????????????????break;????????????default:
????????????????doSprinklerEvent();
????????}
????}

????public?void?doAlarmEvent()?{
????????alarm.doAlarm();
????????coffeePot.doCoffeePot();
????????calender.doCalender();
????????sprinkler.doSprinkler();
????}

????public?void?doCoffeePotEvent()?{
????????//?...????}

????public?void?doCalenderEvent()?{
????????//?...????}

????public?void?doSprinklerEvent()?{
????????//?...????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????Alarm?alarm?=?new?Alarm();
????????CoffeePot?coffeePot?=?new?CoffeePot();
????????Calender?calender?=?new?Calender();
????????Sprinkler?sprinkler?=?new?Sprinkler();
????????Mediator?mediator?=?new?ConcreteMediator(alarm,?coffeePot,?calender,?sprinkler);
????????//?鬧鐘事件到達，調(diào)用中介者就可以操作相關對象????????alarm.onEvent(mediator);
????}}doAlarm()doCoffeePot()doCalender()doSprinkler()

JDK

• All scheduleXXX() methods of?java.util.Timer

• java.util.concurrent.Executor#execute()

• submit() and invokeXXX() methods of?java.util.concurrent.ExecutorService

• scheduleXXX() methods of?java.util.concurrent.ScheduledExecutorService

• java.lang.reflect.Method#invoke()

6. 備忘錄（Memento）

Intent

在不違反封裝的情況下獲得對象的內(nèi)部狀態(tài)，從而在需要時可以將對象恢復到最初狀態(tài)。

Class Diagram

• Originator：原始對象

• Caretaker：負責保存好備忘錄

• Menento：備忘錄，存儲原始對象的的狀態(tài)。備忘錄實際上有兩個接口，一個是提供給 Caretaker 的窄接口：它只能將備忘錄傳遞給其它對象；一個是提供給 Originator 的寬接口，允許它訪問到先前狀態(tài)所需的所有數(shù)據(jù)。理想情況是只允許 Originator 訪問本備忘錄的內(nèi)部狀態(tài)。

Implementation

以下實現(xiàn)了一個簡單計算器程序，可以輸入兩個值，然后計算這兩個值的和。備忘錄模式允許將這兩個值存儲起來，然后在某個時刻用存儲的狀態(tài)進行恢復。

實現(xiàn)參考：Memento Pattern - Calculator Example - Java Sourcecode

/**?*?Originator?Interface?*/public?interface?Calculator?{

????//?Create?Memento????PreviousCalculationToCareTaker?backupLastCalculation();

????//?setMemento????void?restorePreviousCalculation(PreviousCalculationToCareTaker?memento);

????int?getCalculationResult();

????void?setFirstNumber(int?firstNumber);

????void?setSecondNumber(int?secondNumber);}/**?*?Originator?Implementation?*/public?class?CalculatorImp?implements?Calculator?{

????private?int?firstNumber;
????private?int?secondNumber;

????@Override
????public?PreviousCalculationToCareTaker?backupLastCalculation()?{
????????//?create?a?memento?object?used?for?restoring?two?numbers????????return?new?PreviousCalculationImp(firstNumber,?secondNumber);
????}

????@Override
????public?void?restorePreviousCalculation(PreviousCalculationToCareTaker?memento)?{
????????this.firstNumber?=?((PreviousCalculationToOriginator)?memento).getFirstNumber();
????????this.secondNumber?=?((PreviousCalculationToOriginator)?memento).getSecondNumber();
????}

????@Override
????public?int?getCalculationResult()?{
????????//?result?is?adding?two?numbers????????return?firstNumber?+?secondNumber;
????}

????@Override
????public?void?setFirstNumber(int?firstNumber)?{
????????this.firstNumber?=?firstNumber;
????}

????@Override
????public?void?setSecondNumber(int?secondNumber)?{
????????this.secondNumber?=?secondNumber;
????}}/**?*?Memento?Interface?to?Originator?*?*?This?interface?allows?the?originator?to?restore?its?state?*/public?interface?PreviousCalculationToOriginator?{
????int?getFirstNumber();
????int?getSecondNumber();}/**?*??Memento?interface?to?CalculatorOperator?(Caretaker)?*/public?interface?PreviousCalculationToCareTaker?{
????//?no?operations?permitted?for?the?caretaker}/**?*?Memento?Object?Implementation?*?<p>?*?Note?that?this?object?implements?both?interfaces?to?Originator?and?CareTaker?*/public?class?PreviousCalculationImp?implements?PreviousCalculationToCareTaker,
????????PreviousCalculationToOriginator?{

????private?int?firstNumber;
????private?int?secondNumber;

????public?PreviousCalculationImp(int?firstNumber,?int?secondNumber)?{
????????this.firstNumber?=?firstNumber;
????????this.secondNumber?=?secondNumber;
????}

????@Override
????public?int?getFirstNumber()?{
????????return?firstNumber;
????}

????@Override
????public?int?getSecondNumber()?{
????????return?secondNumber;
????}}/**?*?CareTaker?object?*/public?class?Client?{

????public?static?void?main(String[]?args)?{
????????//?program?starts????????Calculator?calculator?=?new?CalculatorImp();

????????//?assume?user?enters?two?numbers????????calculator.setFirstNumber(10);
????????calculator.setSecondNumber(100);

????????//?find?result????????System.out.println(calculator.getCalculationResult());

????????//?Store?result?of?this?calculation?in?case?of?error????????PreviousCalculationToCareTaker?memento?=?calculator.backupLastCalculation();

????????//?user?enters?a?number????????calculator.setFirstNumber(17);

????????//?user?enters?a?wrong?second?number?and?calculates?result????????calculator.setSecondNumber(-290);

????????//?calculate?result????????System.out.println(calculator.getCalculationResult());

????????//?user?hits?CTRL?+?Z?to?undo?last?operation?and?see?last?result????????calculator.restorePreviousCalculation(memento);

????????//?result?restored????????System.out.println(calculator.getCalculationResult());
????}}110-273110

JDK

• java.io.Serializable

7. 觀察者（Observer）

Intent

定義對象之間的一對多依賴，當一個對象狀態(tài)改變時，它的所有依賴都會收到通知并且自動更新狀態(tài)。

主題（Subject）是被觀察的對象，而其所有依賴者（Observer）稱為觀察者。

Class Diagram

主題（Subject）具有注冊和移除觀察者、并通知所有觀察者的功能，主題是通過維護一張觀察者列表來實現(xiàn)這些操作的。

觀察者（Observer）的注冊功能需要調(diào)用主題的 registerObserver() 方法。

Implementation

天氣數(shù)據(jù)布告板會在天氣信息發(fā)生改變時更新其內(nèi)容，布告板有多個，并且在將來會繼續(xù)增加。

public?interface?Subject?{
????void?registerObserver(Observer?o);

????void?removeObserver(Observer?o);

????void?notifyObserver();}public?class?WeatherData?implements?Subject?{
????private?List<Observer>?observers;
????private?float?temperature;
????private?float?humidity;
????private?float?pressure;

????public?WeatherData()?{
????????observers?=?new?ArrayList<>();
????}

????public?void?setMeasurements(float?temperature,?float?humidity,?float?pressure)?{
????????this.temperature?=?temperature;
????????this.humidity?=?humidity;
????????this.pressure?=?pressure;
????????notifyObserver();
????}

????@Override
????public?void?registerObserver(Observer?o)?{
????????observers.add(o);
????}

????@Override
????public?void?removeObserver(Observer?o)?{
????????int?i?=?observers.indexOf(o);
????????if?(i?>=?0)?{
????????????observers.remove(i);
????????}
????}

????@Override
????public?void?notifyObserver()?{
????????for?(Observer?o?:?observers)?{
????????????o.update(temperature,?humidity,?pressure);
????????}
????}}public?interface?Observer?{
????void?update(float?temp,?float?humidity,?float?pressure);}public?class?StatisticsDisplay?implements?Observer?{

????public?StatisticsDisplay(Subject?weatherData)?{
????????weatherData.reisterObserver(this);
????}

????@Override
????public?void?update(float?temp,?float?humidity,?float?pressure)?{
????????System.out.println("StatisticsDisplay.update:?"?+?temp?+?"?"?+?humidity?+?"?"?+?pressure);
????}}public?class?CurrentConditionsDisplay?implements?Observer?{

????public?CurrentConditionsDisplay(Subject?weatherData)?{
????????weatherData.registerObserver(this);
????}

????@Override
????public?void?update(float?temp,?float?humidity,?float?pressure)?{
????????System.out.println("CurrentConditionsDisplay.update:?"?+?temp?+?"?"?+?humidity?+?"?"?+?pressure);
????}}public?class?WeatherStation?{
????public?static?void?main(String[]?args)?{
????????WeatherData?weatherData?=?new?WeatherData();
????????CurrentConditionsDisplay?currentConditionsDisplay?=?new?CurrentConditionsDisplay(weatherData);
????????StatisticsDisplay?statisticsDisplay?=?new?StatisticsDisplay(weatherData);

????????weatherData.setMeasurements(0,?0,?0);
????????weatherData.setMeasurements(1,?1,?1);
????}}CurrentConditionsDisplay.update:?0.0?0.0?0.0StatisticsDisplay.update:?0.0?0.0?0.0CurrentConditionsDisplay.update:?1.0?1.0?1.0StatisticsDisplay.update:?1.0?1.0?1.0

JDK

• java.util.Observer

• java.util.EventListener

• javax.servlet.http.HttpSessionBindingListener

• RxJava

8. 狀態(tài)（State）

Intent

允許對象在內(nèi)部狀態(tài)改變時改變它的行為，對象看起來好像修改了它所屬的類。

Class Diagram

Implementation

糖果銷售機有多種狀態(tài)，每種狀態(tài)下銷售機有不同的行為，狀態(tài)可以發(fā)生轉(zhuǎn)移，使得銷售機的行為也發(fā)生改變。

public?interface?State?{
????/**?????*?投入?25?分錢?????*/
????void?insertQuarter();

????/**?????*?退回?25?分錢?????*/
????void?ejectQuarter();

????/**?????*?轉(zhuǎn)動曲柄?????*/
????void?turnCrank();

????/**?????*?發(fā)放糖果?????*/
????void?dispense();}public?class?HasQuarterState?implements?State?{

????private?GumballMachine?gumballMachine;

????public?HasQuarterState(GumballMachine?gumballMachine)?{
????????this.gumballMachine?=?gumballMachine;
????}

????@Override
????public?void?insertQuarter()?{
????????System.out.println("You?can't?insert?another?quarter");
????}

????@Override
????public?void?ejectQuarter()?{
????????System.out.println("Quarter?returned");
????????gumballMachine.setState(gumballMachine.getNoQuarterState());
????}

????@Override
????public?void?turnCrank()?{
????????System.out.println("You?turned...");
????????gumballMachine.setState(gumballMachine.getSoldState());
????}

????@Override
????public?void?dispense()?{
????????System.out.println("No?gumball?dispensed");
????}}public?class?NoQuarterState?implements?State?{

????GumballMachine?gumballMachine;

????public?NoQuarterState(GumballMachine?gumballMachine)?{
????????this.gumballMachine?=?gumballMachine;
????}

????@Override
????public?void?insertQuarter()?{
????????System.out.println("You?insert?a?quarter");
????????gumballMachine.setState(gumballMachine.getHasQuarterState());
????}

????@Override
????public?void?ejectQuarter()?{
????????System.out.println("You?haven't?insert?a?quarter");
????}

????@Override
????public?void?turnCrank()?{
????????System.out.println("You?turned,?but?there's?no?quarter");
????}

????@Override
????public?void?dispense()?{
????????System.out.println("You?need?to?pay?first");
????}}public?class?SoldOutState?implements?State?{

????GumballMachine?gumballMachine;

????public?SoldOutState(GumballMachine?gumballMachine)?{
????????this.gumballMachine?=?gumballMachine;
????}

????@Override
????public?void?insertQuarter()?{
????????System.out.println("You?can't?insert?a?quarter,?the?machine?is?sold?out");
????}

????@Override
????public?void?ejectQuarter()?{
????????System.out.println("You?can't?eject,?you?haven't?inserted?a?quarter?yet");
????}

????@Override
????public?void?turnCrank()?{
????????System.out.println("You?turned,?but?there?are?no?gumballs");
????}

????@Override
????public?void?dispense()?{
????????System.out.println("No?gumball?dispensed");
????}}public?class?SoldState?implements?State?{

????GumballMachine?gumballMachine;

????public?SoldState(GumballMachine?gumballMachine)?{
????????this.gumballMachine?=?gumballMachine;
????}

????@Override
????public?void?insertQuarter()?{
????????System.out.println("Please?wait,?we're?already?giving?you?a?gumball");
????}

????@Override
????public?void?ejectQuarter()?{
????????System.out.println("Sorry,?you?already?turned?the?crank");
????}

????@Override
????public?void?turnCrank()?{
????????System.out.println("Turning?twice?doesn't?get?you?another?gumball!");
????}

????@Override
????public?void?dispense()?{
????????gumballMachine.releaseBall();
????????if?(gumballMachine.getCount()?>?0)?{
????????????gumballMachine.setState(gumballMachine.getNoQuarterState());
????????}?else?{
????????????System.out.println("Oops,?out?of?gumballs");
????????????gumballMachine.setState(gumballMachine.getSoldOutState());
????????}
????}}public?class?GumballMachine?{

????private?State?soldOutState;
????private?State?noQuarterState;
????private?State?hasQuarterState;
????private?State?soldState;

????private?State?state;
????private?int?count?=?0;

????public?GumballMachine(int?numberGumballs)?{
????????count?=?numberGumballs;
????????soldOutState?=?new?SoldOutState(this);
????????noQuarterState?=?new?NoQuarterState(this);
????????hasQuarterState?=?new?HasQuarterState(this);
????????soldState?=?new?SoldState(this);

????????if?(numberGumballs?>?0)?{
????????????state?=?noQuarterState;
????????}?else?{
????????????state?=?soldOutState;
????????}
????}

????public?void?insertQuarter()?{
????????state.insertQuarter();
????}

????public?void?ejectQuarter()?{
????????state.ejectQuarter();
????}

????public?void?turnCrank()?{
????????state.turnCrank();
????????state.dispense();
????}

????public?void?setState(State?state)?{
????????this.state?=?state;
????}

????public?void?releaseBall()?{
????????System.out.println("A?gumball?comes?rolling?out?the?slot...");
????????if?(count?!=?0)?{
????????????count?-=?1;
????????}
????}

????public?State?getSoldOutState()?{
????????return?soldOutState;
????}

????public?State?getNoQuarterState()?{
????????return?noQuarterState;
????}

????public?State?getHasQuarterState()?{
????????return?hasQuarterState;
????}

????public?State?getSoldState()?{
????????return?soldState;
????}

????public?int?getCount()?{
????????return?count;
????}}public?class?Client?{

????public?static?void?main(String[]?args)?{
????????GumballMachine?gumballMachine?=?new?GumballMachine(5);

????????gumballMachine.insertQuarter();
????????gumballMachine.turnCrank();

????????gumballMachine.insertQuarter();
????????gumballMachine.ejectQuarter();
????????gumballMachine.turnCrank();

????????gumballMachine.insertQuarter();
????????gumballMachine.turnCrank();
????????gumballMachine.insertQuarter();
????????gumballMachine.turnCrank();
????????gumballMachine.ejectQuarter();

????????gumballMachine.insertQuarter();
????????gumballMachine.insertQuarter();
????????gumballMachine.turnCrank();
????????gumballMachine.insertQuarter();
????????gumballMachine.turnCrank();
????????gumballMachine.insertQuarter();
????????gumballMachine.turnCrank();
????}}You?insert?a?quarterYou?turned...A?gumball?comes?rolling?out?the?slot...You?insert?a?quarterQuarter?returnedYou?turned,?but?there's?no?quarterYou?need?to?pay?firstYou?insert?a?quarterYou?turned...A?gumball?comes?rolling?out?the?slot...You?insert?a?quarterYou?turned...A?gumball?comes?rolling?out?the?slot...You?haven't?insert?a?quarterYou?insert?a?quarterYou?can't?insert?another?quarterYou?turned...A?gumball?comes?rolling?out?the?slot...You?insert?a?quarterYou?turned...A?gumball?comes?rolling?out?the?slot...Oops,?out?of?gumballsYou?can't?insert?a?quarter,?the?machine?is?sold?outYou?turned,?but?there?are?no?gumballsNo?gumball?dispensed

9. 策略（Strategy）

Intent

定義一系列算法，封裝每個算法，并使它們可以互換。

策略模式可以讓算法獨立于使用它的客戶端。

Class Diagram

• Strategy 接口定義了一個算法族，它們都實現(xiàn)了 behavior() 方法。

• Context 是使用到該算法族的類，其中的 doSomething() 方法會調(diào)用 behavior()，setStrategy(Strategy) 方法可以動態(tài)地改變 strategy 對象，也就是說能動態(tài)地改變 Context 所使用的算法。

與狀態(tài)模式的比較

狀態(tài)模式的類圖和策略模式類似，并且都是能夠動態(tài)改變對象的行為。但是狀態(tài)模式是通過狀態(tài)轉(zhuǎn)移來改變 Context 所組合的 State 對象，而策略模式是通過 Context 本身的決策來改變組合的 Strategy 對象。所謂的狀態(tài)轉(zhuǎn)移，是指 Context 在運行過程中由于一些條件發(fā)生改變而使得 State 對象發(fā)生改變，注意必須要是在運行過程中。

狀態(tài)模式主要是用來解決狀態(tài)轉(zhuǎn)移的問題，當狀態(tài)發(fā)生轉(zhuǎn)移了，那么 Context 對象就會改變它的行為；而策略模式主要是用來封裝一組可以互相替代的算法族，并且可以根據(jù)需要動態(tài)地去替換 Context 使用的算法。

Implementation

設計一個鴨子，它可以動態(tài)地改變叫聲。這里的算法族是鴨子的叫聲行為。

public?interface?QuackBehavior?{
????void?quack();}public?class?Quack?implements?QuackBehavior?{
????@Override
????public?void?quack()?{
????????System.out.println("quack!");
????}}public?class?Squeak?implements?QuackBehavior{
????@Override
????public?void?quack()?{
????????System.out.println("squeak!");
????}}public?class?Duck?{

????private?QuackBehavior?quackBehavior;

????public?void?performQuack()?{
????????if?(quackBehavior?!=?null)?{
????????????quackBehavior.quack();
????????}
????}

????public?void?setQuackBehavior(QuackBehavior?quackBehavior)?{
????????this.quackBehavior?=?quackBehavior;
????}}public?class?Client?{

????public?static?void?main(String[]?args)?{
????????Duck?duck?=?new?Duck();
????????duck.setQuackBehavior(new?Squeak());
????????duck.performQuack();
????????duck.setQuackBehavior(new?Quack());
????????duck.performQuack();
????}}squeak!quack!

JDK

• java.util.Comparator#compare()

• javax.servlet.http.HttpServlet

• javax.servlet.Filter#doFilter()

10. 模板方法（Template Method）

Intent

定義算法框架，并將一些步驟的實現(xiàn)延遲到子類。

通過模板方法，子類可以重新定義算法的某些步驟，而不用改變算法的結(jié)構。

Class Diagram

Implementation

沖咖啡和沖茶都有類似的流程，但是某些步驟會有點不一樣，要求復用那些相同步驟的代碼。

public?abstract?class?CaffeineBeverage?{

????final?void?prepareRecipe()?{
????????boilWater();
????????brew();
????????pourInCup();
????????addCondiments();
????}

????abstract?void?brew();

????abstract?void?addCondiments();

????void?boilWater()?{
????????System.out.println("boilWater");
????}

????void?pourInCup()?{
????????System.out.println("pourInCup");
????}}public?class?Coffee?extends?CaffeineBeverage?{
????@Override
????void?brew()?{
????????System.out.println("Coffee.brew");
????}

????@Override
????void?addCondiments()?{
????????System.out.println("Coffee.addCondiments");
????}}public?class?Tea?extends?CaffeineBeverage?{
????@Override
????void?brew()?{
????????System.out.println("Tea.brew");
????}

????@Override
????void?addCondiments()?{
????????System.out.println("Tea.addCondiments");
????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????CaffeineBeverage?caffeineBeverage?=?new?Coffee();
????????caffeineBeverage.prepareRecipe();
????????System.out.println("-----------");
????????caffeineBeverage?=?new?Tea();
????????caffeineBeverage.prepareRecipe();
????}}boilWaterCoffee.brewpourInCupCoffee.addCondiments-----------boilWaterTea.brewpourInCupTea.addCondiments

JDK

• java.util.Collections#sort()

• java.io.InputStream#skip()

• java.io.InputStream#read()

• java.util.AbstractList#indexOf()

11. 訪問者（Visitor）

Intent

為一個對象結(jié)構（比如組合結(jié)構）增加新能力。

Class Diagram

• Visitor：訪問者，為每一個 ConcreteElement 聲明一個 visit 操作

• ConcreteVisitor：具體訪問者，存儲遍歷過程中的累計結(jié)果

• ObjectStructure：對象結(jié)構，可以是組合結(jié)構，或者是一個集合。

Implementation

public?interface?Element?{
????void?accept(Visitor?visitor);}class?CustomerGroup?{

????private?List<Customer>?customers?=?new?ArrayList<>();

????void?accept(Visitor?visitor)?{
????????for?(Customer?customer?:?customers)?{
????????????customer.accept(visitor);
????????}
????}

????void?addCustomer(Customer?customer)?{
????????customers.add(customer);
????}}public?class?Customer?implements?Element?{

????private?String?name;
????private?List<Order>?orders?=?new?ArrayList<>();

????Customer(String?name)?{
????????this.name?=?name;
????}

????String?getName()?{
????????return?name;
????}

????void?addOrder(Order?order)?{
????????orders.add(order);
????}

????public?void?accept(Visitor?visitor)?{
????????visitor.visit(this);
????????for?(Order?order?:?orders)?{
????????????order.accept(visitor);
????????}
????}}public?class?Order?implements?Element?{

????private?String?name;
????private?List<Item>?items?=?new?ArrayList();

????Order(String?name)?{
????????this.name?=?name;
????}

????Order(String?name,?String?itemName)?{
????????this.name?=?name;
????????this.addItem(new?Item(itemName));
????}

????String?getName()?{
????????return?name;
????}

????void?addItem(Item?item)?{
????????items.add(item);
????}

????public?void?accept(Visitor?visitor)?{
????????visitor.visit(this);

????????for?(Item?item?:?items)?{
????????????item.accept(visitor);
????????}
????}}public?class?Item?implements?Element?{

????private?String?name;

????Item(String?name)?{
????????this.name?=?name;
????}

????String?getName()?{
????????return?name;
????}

????public?void?accept(Visitor?visitor)?{
????????visitor.visit(this);
????}}public?interface?Visitor?{
????void?visit(Customer?customer);

????void?visit(Order?order);

????void?visit(Item?item);}public?class?GeneralReport?implements?Visitor?{

????private?int?customersNo;
????private?int?ordersNo;
????private?int?itemsNo;

????public?void?visit(Customer?customer)?{
????????System.out.println(customer.getName());
????????customersNo++;
????}

????public?void?visit(Order?order)?{
????????System.out.println(order.getName());
????????ordersNo++;
????}

????public?void?visit(Item?item)?{
????????System.out.println(item.getName());
????????itemsNo++;
????}

????public?void?displayResults()?{
????????System.out.println("Number?of?customers:?"?+?customersNo);
????????System.out.println("Number?of?orders:????"?+?ordersNo);
????????System.out.println("Number?of?items:?????"?+?itemsNo);
????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????Customer?customer1?=?new?Customer("customer1");
????????customer1.addOrder(new?Order("order1",?"item1"));
????????customer1.addOrder(new?Order("order2",?"item1"));
????????customer1.addOrder(new?Order("order3",?"item1"));

????????Order?order?=?new?Order("order_a");
????????order.addItem(new?Item("item_a1"));
????????order.addItem(new?Item("item_a2"));
????????order.addItem(new?Item("item_a3"));
????????Customer?customer2?=?new?Customer("customer2");
????????customer2.addOrder(order);

????????CustomerGroup?customers?=?new?CustomerGroup();
????????customers.addCustomer(customer1);
????????customers.addCustomer(customer2);

????????GeneralReport?visitor?=?new?GeneralReport();
????????customers.accept(visitor);
????????visitor.displayResults();
????}}customer1order1item1order2item1order3item1customer2order_aitem_a1item_a2item_a3Number?of?customers:?2Number?of?orders:????4Number?of?items:?????6

JDK

• javax.lang.model.element.Element and javax.lang.model.element.ElementVisitor

• javax.lang.model.type.TypeMirror and javax.lang.model.type.TypeVisitor

12. 空對象（Null）

Intent

使用什么都不做的空對象來代替 NULL。

一個方法返回 NULL，意味著方法的調(diào)用端需要去檢查返回值是否是 NULL，這么做會導致非常多的冗余的檢查代碼。并且如果某一個調(diào)用端忘記了做這個檢查返回值，而直接使用返回的對象，那么就有可能拋出空指針異常。

Class Diagram

Implementation

public?abstract?class?AbstractOperation?{
????abstract?void?request();}public?class?RealOperation?extends?AbstractOperation?{
????@Override
????void?request()?{
????????System.out.println("do?something");
????}}public?class?NullOperation?extends?AbstractOperation{
????@Override
????void?request()?{
????????//?do?nothing????}}public?class?Client?{
????public?static?void?main(String[]?args)?{
????????AbstractOperation?abstractOperation?=?func(-1);
????????abstractOperation.request();
????}

????public?static?AbstractOperation?func(int?para)?{
????????if?(para?<?0)?{
????????????return?new?NullOperation();
????????}
????????return?new?RealOperation();
????}}

網(wǎng)頁名稱：Java的23種設計模式，詳細講解（二）
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