物件導向-設計模式-行為型

物件導向-設計模式-行為型

 

　　　　　　日暮鄉關何處是？煙波江上使人愁。

 

簡介：物件導向-設計模式-行為型。

一、概述

何謂設計模式：

設計模式(Design Pattern)是一套被反覆使用、多數人知曉的、經過分類的、程式碼設計經驗的總結。

設計模式的好處&學習目的：

1、為了程式碼可重用行、讓程式碼更易被他人理解、保證程式碼的可靠性、使程式碼編寫真正實現工程化；

2、設計模式便於我們維護專案，增強系統的健壯性和可擴充套件性；

3、設計模式還可以鍛鍊碼農的設計思維、昇華程式碼質量等。

二、行為型

責任鏈模式、命令模式、直譯器模式、迭代器模式、中介者模式、備忘錄模式、觀察者模式、狀態模式、策略模式、模板方法模式、訪問者模式、空物件模式。

1. 責任鏈（Chain Of Responsibility）

Intent

使多個物件都有機會處理請求，從而避免請求的傳送者和接收者之間的耦合關係。將這些物件連成一條鏈，並沿著這條鏈傳送該請求，直到有一個物件處理它為止。

Class Diagram

• Handler：定義處理請求的介面，並且實現後繼鏈（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);
    }
}

輸出：
request1 is handle by ConcreteHandler1
request2 is handle by ConcreteHandler2

2. 命令（Command）

Intent

將命令封裝成物件中，具有以下作用：

• 使用命令來引數化其它物件
• 將命令放入佇列中進行排隊
• 將命令的操作記錄到日誌中
• 支援可撤銷的操作

Class Diagram

• Command：命令
• Receiver：命令接收者，也就是命令真正的執行者
• Invoker：通過它來呼叫命令
• Client：可以設定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);
    }
}

3. 直譯器（Interpreter）

Intent

為語言建立直譯器，通常由語言的語法和語法分析來定義。

Class Diagram

• TerminalExpression：終結符表示式，每個終結符都需要一個 TerminalExpression。
• Context：上下文，包含直譯器之外的一些全域性資訊。

Implementation

以下是一個規則檢驗器實現，具有 and 和 or 規則，通過規則可以構建一顆解析樹，用來檢驗一個文字是否滿足解析樹定義的規則。

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

這裡的 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));  // true
        System.out.println(define.interpret(context2));  // false
    }
}

4. 迭代器（Iterator）

Intent

提供一種順序訪問聚合物件元素的方法，並且不暴露聚合物件的內部表示。

Class Diagram

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

5. 中介者（Mediator）

Intent

集中相關物件之間複雜的溝通和控制方式。

Class Diagram

• Mediator：中介者，定義一個介面用於與各同事（Colleague）物件通訊。
• Colleague：同事，相關物件。

Implementation

Alarm（鬧鐘）、CoffeePot（咖啡壺）、Calendar（日曆）、Sprinkler（噴頭）是一組相關的物件，在某個物件的事件產生時需要去操作其它物件，形成了下面這種依賴結構：

使用中介者模式可以將複雜的依賴結構變成星形結構：

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，呼叫中介者就可以操作相關物件
        alarm.onEvent(mediator);
    }
}

輸出：
doAlarm()
doCoffeePot()
doCalender()
doSprinkler()

6. 備忘錄（Memento）

Intent

在不違反封裝的情況下獲得物件的內部狀態，從而在需要時可以將物件恢復到最初狀態。

Class Diagram

• Originator：原始物件
• Caretaker：負責儲存好備忘錄
• Memento：備忘錄，儲存原始物件的狀態。備忘錄實際上有兩個介面，一個是提供給 Caretaker 的窄介面：它只能將備忘錄傳遞給其它物件；一個是提供給 Originator 的寬介面，允許它訪問到先前狀態所需的所有資料。理想情況是隻允許 Originator 訪問本備忘錄的內部狀態。

Implementation

以下實現了一個簡單計算器程式，可以輸入兩個值，然後計算這兩個值的和。備忘錄模式允許將這兩個值儲存起來，然後在某個時刻用儲存的狀態進行恢復。

/**
 * 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());
    }
}

輸出：
110
-273
110

7. 觀察者（Observer）

Intent

定義物件之間的一對多依賴，當一個物件狀態改變時，它的所有依賴都會收到通知並且自動更新狀態。

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

Class Diagram

主題（Subject）具有註冊和移除觀察者、並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這些操作的。

觀察者（Observer）的註冊功能需要呼叫主題的 registerObserver() 方法。

Implementation

天氣資料佈告板會在天氣資訊發生改變時更新其內容，佈告板有多個，並且在將來會繼續增加。

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.registerObserver(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);
    }
}

輸出：
CurrentConditionsDisplay.update: 0.0 0.0 0.0
StatisticsDisplay.update: 0.0 0.0 0.0
CurrentConditionsDisplay.update: 1.0 1.0 1.0
StatisticsDisplay.update: 1.0 1.0 1.0

8. 狀態（State）

Intent

允許物件在內部狀態改變時改變它的行為，物件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，每種狀態下銷售機有不同的行為，狀態可以發生轉移，使得銷售機的行為也發生改變。

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

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

    /**
     * 轉動曲柄
     */
    void turnCrank();

    /**
     * 發放糖果
     */
    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();
    }
}

輸出：
You insert a quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
Quarter returned
You turned, but there's no quarter
You need to pay first
You insert a quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
You turned...
A gumball comes rolling out the slot...
You haven't insert a quarter
You insert a quarter
You can't insert another quarter
You turned...
A gumball comes rolling out the slot...
You insert a quarter
You turned...
A gumball comes rolling out the slot...
Oops, out of gumballs
You can't insert a quarter, the machine is sold out
You turned, but there are no gumballs
No gumball dispensed

9. 策略（Strategy）

Intent

定義一系列演算法，封裝每個演算法，並使它們可以互換。

策略模式可以讓演算法獨立於使用它的客戶端。

Class Diagram

• Strategy 介面定義了一個演算法族，它們都實現了 behavior() 方法。
• Context 是使用到該演算法族的類，其中的 doSomething() 方法會呼叫 behavior()，setStrategy(Strategy) 方法可以動態地改變 strategy 物件，也就是說能動態地改變 Context 所使用的演算法。

與狀態模式的比較

狀態模式的類圖和策略模式類似，並且都是能夠動態改變物件的行為。但是狀態模式是通過狀態轉移來改變 Context 所組合的 State 物件，而策略模式是通過 Context 本身的決策來改變組合的 Strategy 物件。所謂的狀態轉移，是指 Context 在執行過程中由於一些條件發生改變而使得 State 物件發生改變，注意必須要是在執行過程中。

狀態模式主要是用來解決狀態轉移的問題，當狀態發生轉移了，那麼 Context 物件就會改變它的行為；而策略模式主要是用來封裝一組可以互相替代的演算法族，並且可以根據需要動態地去替換 Context 使用的演算法。

Implementation

設計一個鴨子，它可以動態地改變叫聲。這裡的演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();
    }
}

輸出：
squeak!
quack!

10. 模板方法（Template Method）

Intent

定義演算法框架，並將一些步驟的實現延遲到子類。

通過模板方法，子類可以重新定義演算法的某些步驟，而不用改變演算法的結構。

Class Diagram

Implementation

衝咖啡和沖茶都有類似的流程，但是某些步驟會有點不一樣，要求複用那些相同步驟的程式碼。

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();
    }
}

輸出：
boilWater
Coffee.brew
pourInCup
Coffee.addCondiments
-----------
boilWater
Tea.brew
pourInCup
Tea.addCondiments

11. 訪問者（Visitor）

Intent

為一個物件結構（比如組合結構）增加新能力。

Class Diagram

• Visitor：訪問者，為每一個 ConcreteElement 宣告一個 visit 操作
• ConcreteVisitor：具體訪問者，儲存遍歷過程中的累計結果
• ObjectStructure：物件結構，可以是組合結構，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();
    }
}

輸出：
customer1
order1
item1
order2
item1
order3
item1
customer2
order_a
item_a1
item_a2
item_a3
Number of customers: 2
Number of orders:    4
Number of items:     6

12. 空物件（Null）

Intent

使用什麼都不做的空物件來代替 NULL。

一個方法返回 NULL，意味著方法的呼叫端需要去檢查返回值是否是 NULL，這麼做會導致非常多的冗餘的檢查程式碼。並且如果某一個呼叫端忘記了做這個檢查返回值，而直接使用返回的物件，那麼就有可能丟擲空指標異常。

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();
    }
}

 

 

 

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