Java的23種設計模式，詳細講解（二）
1. 責任鏈（Chain Of Responsibility）
Intent
使多個對象都有機會處理請求，從而避免請求的發送者和接收者之間的耦合關係。將這些對象連成一條鏈，並沿着這條鏈發送該請求，直到有一個對象處理它爲止。html
Class Diagram
- Handler：定義處理請求的接口，而且實現後繼鏈（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);
}
}
request1 is handle by ConcreteHandler1
request2 is handle by ConcreteHandler2
JDK
2. 命令（Command）
Intent
將命令封裝成對象中，具備如下做用：java
- 使用命令來參數化其它對象
- 將命令放入隊列中進行排隊
- 將命令的操做記錄到日誌中
- 支持可撤銷的操做
Class Diagram
- Command：命令
- Receiver：命令接收者，也就是命令真正的執行者
- Invoker：經過它來調用命令
- Client：能夠設置命令與命令的接收者
Implementation
設計一個遙控器，能夠控制電燈開關。git
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
3. 解釋器（Interpreter）
Intent
爲語言建立解釋器，一般由語言的語法和語法分析來定義。github
Class Diagram
- TerminalExpression：終結符表達式，每一個終結符都須要一個 TerminalExpression。
- Context：上下文，包含解釋器以外的一些全局信息。
Implementation
如下是一個規則檢驗器實現，具備 and 和 or 規則，經過規則能夠構建一顆解析樹，用來檢驗一個文本是否知足解析樹定義的規則。算法
例如一顆解析樹爲 D And (A Or (B C))，文本 "D A" 知足該解析樹定義的規則。express
這裏的 Context 指的是 String。apache
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));
}
}
true
false
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，而且不暴露聚合對象的內部表示。api
Class Diagram
- Aggregate 是聚合類，其中 createIterator() 方法能夠產生一個 Iterator；
- Iterator 主要定義了 hasNext() 和 next() 方法。
- Client 組合了 Aggregate，爲了迭代遍歷 Aggregate，也須要組合 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
5. 中介者（Mediator）
Intent
集中相關對象之間複雜的溝通和控制方式。併發
Class Diagram
- Mediator：中介者，定義一個接口用於與各同事（Colleague）對象通訊。
- Colleague：同事，相關對象
Implementation
Alarm（鬧鐘）、CoffeePot（咖啡壺）、Calendar（日曆）、Sprinkler（噴頭）是一組相關的對象，在某個對象的事件產生時須要去操做其它對象，造成了下面這種依賴結構：oracle
使用中介者模式能夠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，調用中介者就能夠操做相關對象
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
在不違反封裝的狀況下得到對象的內部狀態，從而在須要時能夠將對象恢復到最初狀態。
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。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，容許它訪問到先前狀態所需的全部數據。理想狀況是隻容許 Originator 訪問本備忘錄的內部狀態。
Implementation
如下實現了一個簡單計算器程序，能夠輸入兩個值，而後計算這兩個值的和。備忘錄模式容許將這兩個值存儲起來，而後在某個時刻用存儲的狀態進行恢復。
實現參考：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
-273
110
JDK
- java.io.Serializable
7. 觀察者（Observer）
Intent
定義對象之間的一對多依賴，當一個對象狀態改變時，它的全部依賴都會收到通知而且自動更新狀態。
主題（Subject）是被觀察的對象，而其全部依賴者（Observer）稱爲觀察者。
Class Diagram
主題（Subject）具備註冊和移除觀察者、並通知全部觀察者的功能，主題是經過維護一張觀察者列表來實現這些操做的。
觀察者（Observer）的註冊功能須要調用主題的 registerObserver() 方法。
Implementation
天氣數據佈告板會在天氣信息發生改變時更新其內容，佈告板有多個，而且在未來會繼續增長。
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 0.0
StatisticsDisplay.update: 0.0 0.0 0.0
CurrentConditionsDisplay.update: 1.0 1.0 1.0
StatisticsDisplay.update: 1.0 1.0 1.0
JDK
8. 狀態（State）
Intent
容許對象在內部狀態改變時改變它的行爲，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，每種狀態下銷售機有不一樣的行爲，狀態能夠發生轉移，使得銷售機的行爲也發生改變。
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();
}
}
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
定義一系列算法，封裝每一個算法，並使它們能夠互換。
策略模式可讓算法獨立於使用它的客戶端。
Class Diagram
- Strategy 接口定義了一個算法族，它們都實現了 behavior() 方法。
- Context 是使用到該算法族的類，其中的 doSomething() 方法會調用 behavior()，setStrategy(Strategy) 方法能夠動態地改變 strategy 對象，也就是說能動態地改變 Context 所使用的算法。
與狀態模式的比較
狀態模式的類圖和策略模式相似，而且都是可以動態改變對象的行爲。可是狀態模式是經過狀態轉移來改變 Context 所組合的 State 對象，而策略模式是經過 Context 自己的決策來改變組合的 Strategy 對象。所謂的狀態轉移，是指 Context 在運行過程當中因爲一些條件發生改變而使得 State 對象發生改變，注意必需要是在運行過程當中。
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 對象就會改變它的行爲；而策略模式主要是用來封裝一組能夠互相替代的算法族，而且能夠根據須要動態地去替換 Context 使用的算法。
Implementation
設計一個鴨子，它能夠動態地改變叫聲。這裏的算法族是鴨子的叫聲行爲。
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，並將一些步驟的實現延遲到子類。
經過模板方法，子類能夠從新定義算法的某些步驟，而不用改變算法的結構。
Class Diagram
Implementation
衝咖啡和沖茶都有相似的流程，可是某些步驟會有點不同，要求複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();
}
}
boilWater
Coffee.brew
pourInCup
Coffee.addCondiments
-----------
boilWater
Tea.brew
pourInCup
Tea.addCondiments
JDK
- java.util.Collections#sort()
- java.io.InputStream#skip()
- java.io.InputStream#read()
- java.util.AbstractList#indexOf()
11. 訪問者（Visitor）
Intent
爲一個對象結構（好比組合結構）增長新能力。
Class Diagram
- Visitor：訪問者，爲每個 ConcreteElement 聲明一個 visit 操做
- ConcreteVisitor：具體訪問者，存儲遍歷過程當中的累計結果
- ObjectStructure：對象結構，能夠是組合結構，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();
}
}
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
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
使用什麼都不作的空對象來代替 NULL。
一個方法返回 NULL，意味着方法的調用端須要去檢查返回值是不是 NULL，這麼作會致使很是多的冗餘的檢查代碼。而且若是某一個調用端忘記了作這個檢查返回值，而直接使用返回的對象，那麼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();
}
}
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