橋接模式 Bridge Pattern - LinZiyou Dev Blog

目的#

將實作體系與抽象體系分開，讓兩者能各自更動。

適用#

想避免將抽象和實體綁死在一起。需要一個統一的介面來交換層次結構。抽象體和實作體都能用子類別去個別擴充。實作體有變但抽象體沒變時，不應影響client。隱藏實作方式。能在多個物件之間共享實作碼，但不影響client。

結構及成員#

Collaborations: Abstraction將Client的訊息請求轉傳給對應的Implementor去處理。

Abstraction: like the Window抽象體的介面。持有Implementor。RefinedAbstraction: like the PopupWindow, IconWindow.擴充Abstraction。Implementor: like the WindowImp, ColorImp, FontImp定義實作類別之共同介面，可以和Abstraction相去甚遠。通常只含最基本的操作，讓Abstraction組出更高階的操作。ConcreteImplementor: like the XWindowImp, PMWindowImpImplementor 介面的實體實作。

影響結果#

好處#

將介面與實作隔離開來，在Run-Time，實作不會被綁死，可動態設定、抽換，系統的高階部分只需要知道Abstraction和Implementor即可。易於擴充，Abstraction和Implementor均可個別擴充。隱藏內部實作細節。

壞處#

一種尺寸適合所有（one-size-fits-all）Abstraction 和 Implementor 的 interfaces。

實作#

只有一種Implementor#

如果實作體只會有一種，那抽象類別Implementor就不必存在（如：DBMgr），但如果希望類別內容不會影響Client，那對Bridge仍有幫助。

建立正確的Implementor物件#

若Abstraction能認得所有的ConcreteImplementor，便可在建構子依參數決定要實體化哪一個。若有大小容器，可以先預設用小的容器實作，超過臨界值再用大的容器實作。也可以將決定權妥託給其他物件，如Abstract Factory。

Example#

DrawAPI#

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public abstract class Shape {
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   protected DrawAPI drawAPI;
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   protected Shape(DrawAPI drawAPI){
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      this.drawAPI = drawAPI;
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   }
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   public abstract void draw();
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}

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public class Circle extends Shape {
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   private int x, y, radius;
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   public Circle(int x, int y, int radius, DrawAPI drawAPI) {
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      super(drawAPI);
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      this.x = x;
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      this.y = y;
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      this.radius = radius;
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   }
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   public void draw() {
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      drawAPI.drawCircle(radius,x,y);
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   }
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}

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public interface DrawAPI {
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   public void drawCircle(int radius, int x, int y);
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}

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public class RedCircle implements DrawAPI {
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   @Override
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   public void drawCircle(int radius, int x, int y) {
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      System.out.println("Drawing Circle[ color: red, radius: " + radius + ", x: " + x + ", " + y + "]");
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   }
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}

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public class GreenCircle implements DrawAPI {
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   @Override
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   public void drawCircle(int radius, int x, int y) {
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      System.out.println("Drawing Circle[ color: green, radius: " + radius + ", x: " + x + ", " + y + "]");
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   }
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}

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public class BridgePatternDemo {
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   public static void main(String[] args) {
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      Shape redCircle = new Circle(100,100, 10, new RedCircle());
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      Shape greenCircle = new Circle(100,100, 10, new GreenCircle());
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      redCircle.draw();
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      greenCircle.draw();
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   }
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}

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Result:
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Drawing Circle[ color: red, radius: 10, x: 100, 100]
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Drawing Circle[  color: green, radius: 10, x: 100, 100]

Car Remote Controller#

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//Class: Car
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package car.remotecontrol;
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public abstract class Car {
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  private final Product product;
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  private final String carType;
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  public Car(Product product, String carType){
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    this.product = product;
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    this.carType = carType;
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  }
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  public abstract void assemble();
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  public abstract void produceProduct();
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  public void printDetails(){
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    System.out.println("Car: "+carType+", Product:"+product.productName());
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  }
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}

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//Class: Motoren
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package car.remotecontrol;
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public class Motoren extends Car{
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  private final Product product;
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  private final String carType;
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  public Motoren(Product product, String carType) {
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    super(product, carType);
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    this.product = product;
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    this.carType = carType;
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  }
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  @Override
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  public void assemble() {
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    System.out.println("Assembling "+product.productName()+" for "+carType);
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  }
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  @Override
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  public void produceProduct() {
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    product.produce();
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    System.out.println("Modifing product "+product.productName()+" according to "+carType);
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  }
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}

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//Class: BigWheel
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package car.remotecontrol;
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public class BigWheel extends Car{
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  private final Product product;
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  private final String carType;
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  public BigWheel(Product product, String carType) {
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    super(product, carType);
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    this.product = product;
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    this.carType = carType;
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  }
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  @Override
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  public void assemble() {
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    System.out.println("Assembling "+product.productName()+" for "+carType);
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  }
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  @Override
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  public void produceProduct() {
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    product.produce();
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    System.out.println("Modifing product "+product.productName()+" according to "+carType);
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  }
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}

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//Interface: Product
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package car.remotecontrol;
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public interface Product {
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  public String productName();
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  public void produce();
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}

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//Class: GearLocking
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package car.remotecontrol;
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public class GearLocking implements Product{
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  private final String productName;
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  public GearLocking(String productName){
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    this.productName = productName;
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  }
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  @Override
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  public String productName() {
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    return productName;
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  }
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  @Override
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  public void produce() {
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    System.out.println("Producing Gear Locking System");
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  }
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}

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//Class: CentralLocking
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package car.remotecontrol;
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public class CentralLocking implements Product{
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  private final String productName;
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  public CentralLocking(String productName){
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    this.productName = productName;
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  }
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  @Override
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  public String productName() {
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    return productName;
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  }
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  @Override
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  public void produce() {
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    System.out.println("Producing Central Locking System");
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  }
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}

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//Class: TestBridgePattern
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package car.remotecontrol;
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public class TestBridgePattern {
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  public static void main(String[] args) {
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    Product product = new CentralLocking("Central Locking System");
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    Product product2 = new GearLocking("Gear Locking System");
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    Car car = new BigWheel(product , "BigWheel xz model");
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    car.produceProduct();
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    car.assemble();
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    car.printDetails();
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    System.out.println();
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    car = new BigWheel(product2 , "BigWheel xz model");
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    car.produceProduct();
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    car.assemble();
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    car.printDetails();
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    System.out.println("-----------------------------------------------------");
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    car = new Motoren(product, "Motoren lm model");
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    car.produceProduct();
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    car.assemble();
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    car.printDetails();
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    System.out.println();
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    car = new Motoren(product2, "Motoren lm model");
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    car.produceProduct();
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    car.assemble();
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    car.printDetails();
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  }
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}

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Result:
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Producing Central Locking System
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Modifing product Central Locking System according to BigWheel xz model
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Assembling Central Locking System for BigWheel xz model
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Car: BigWheel xz model, Product:Central Locking System
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Producing Gear Locking System
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Modifing product Gear Locking System according to BigWheel xz model
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Assembling Gear Locking System for BigWheel xz model
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Car: BigWheel xz model, Product:Gear Locking System
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-----------------------------------------------------
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Producing Central Locking System
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Modifing product Central Locking System according to Motoren lm model
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Assembling Central Locking System for Motoren lm model
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Car: Motoren lm model, Product:Central Locking System
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Producing Gear Locking System
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Modifing product Gear Locking System according to Motoren lm model
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Assembling Gear Locking System for Motoren lm model
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Car: Motoren lm model, Product:Gear Locking System

目的#

適用#

結構及成員#

影響結果#

好處#

壞處#

實作#

只有一種Implementor#

建立正確的Implementor物件#

Example#

DrawAPI#

Car Remote Controller#

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