建構者模式 Builder Pattern

目的

將建構邏輯從物件轉移到建構器。從複雜物件的佈局中抽取出生成程序，以便用同一個生成程序製造各種不同的物件佈局。

Move the construction logic from the object to a builder: Separate the construction of a complex object from its representation so that the same construction process can create different representations.

適用

  1. 欲將「建造複雜物件的演算法」與物件的零件及組裝方式保持獨立時。
  2. 想讓同一個物件生成程序能夠產生數種不同佈局形式的物件時。
  3. 若一個產品生產程序太過複雜，Concrete Factory可以把產品生成責任委託給Builder。
  4. 設計流程和製造流程可以隨產品而變化。

結構及成員

Collaborations:
Client 先建立 Director物件，並將組態設定成想要的 Builder 物件。
Director 會在需要建立 Product 的各個零件時通知 Builder。
Builder 處理 Director 送來的命令，將零件一一加進 Product 裡。
最後 Client 從 Builder 手中取得 Product。

  • Builder: like the TextConverter
    • 抽象介面，用來生成Product的各零件。
  • ConcreteBuilder: like the ASCIIConverter, TeXConverter, and TextWidgetConverter
    • Builder介面的具體實作，負責建構及組裝Product的各零件。
    • 定義並記載它所造出的物件之佈局形式。
    • 存取Product的介面（如：GetASCIIText、GetTextWidget）
  • Director: like the RTFReader
    • 利用Builder介面來建構物件。
  • Product: like the ASCIIText, TeXText, TextWidget.
    • 欲產生的最終複雜物件。它的內部佈局形式以及組裝程序，全都是由ConcreteBuilder負責定義。
    • 包含各零件的類別，類別裡又含有可將這些零件組裝成最終結果的介面。

影響結果

好處

  • 能夠改變成品物件的內部佈局形式。只要定義新的Builder，就能改變成品物件的內部構造。
  • 將生成程序與內部佈局的程式碼隔離開來。Client不必知道成品物件內部結構是由哪些類別所定義的，因為這些類別名稱都沒有出現在Builder的外顯介面裡。
  • 對生成程序掌控得更細緻。成品物件是在Director的監控下一步步建出來的，等到完全建好之後Director才向Builder索取成品物件。所以Builder的面比其他生成模式更著重在建構成品物件的細步程序。

實作

組裝及建構介面

Builder類別的介面必須一般化到能滿足各種ConcreteBuilder所需的生成程序。

為什麼成品物件沒有抽象介面？

  • 一般而言，ConcreteBuilder所建的成品物件內部佈局方式差異相當大，並不值得費力抽出共同的父類別。
  • Client通常都會用適切的ConcreteBuilder去設定Director物態，它很清楚自己正在用哪一種Builder實體子類別，能夠正確處理成品物件。

　　可能有些人會覺得，同樣都是建立物件，那直接 Abstract Factory Pattern 來建立物件不就好了？這裡要注意的是，Builder 著重在隱藏複雜物件生成的步驟，且生成的物件（通常是複雜物件）彼此會有「部份」（Part of）的概念。而 Abstract Factory 則是著重在管理有關聯性的物件，但這些物件不會有「部份」（Part of）的概念。實務上這個模式還滿常被使用到的，如 JAVA SDK 裡的 StringBuilder，StringBuffer，以及 Android SDK 裡的 AlertDialog.Builder，有興趣的人可以參考看看。

　　Builder Pattern的特點就是將複雜物件的產生過程隱藏起來，使用者無法碰到，且允許物件用多個步驟建立（跟 Factory Method 只有一個步驟不同），因為它的特性，因此經常用來建立合成結構。但對於使用者而言，要是不知道有哪些 setXXX() 方法可以用，也無法建立出想要的物件，這是要注意的地方。

某些時候，會看到有些寫法會將 Builder 的各個 Setter() 回傳 Builder 物件本身（即：public XXBuilder setCompA(…)）。以便能以Fluent Interface的風格進行Builder的設定（如：Product p = builder.setCompA(…).setCompB(…).setC().build()）。

Example: Car

Class: Car

package builder.car;

public class Car {
	
	private String bodyStyle;
	private String power;
	private String engine;
	private String breaks;
	private String seats;
	private String windows;
	private String fuelType;
	private String carType;
	
	public Car (String carType){
		this.carType = carType;
	}
	
	public String getBodyStyle() {
		return bodyStyle;
	}
	public void setBodyStyle(String bodyStyle) {
		this.bodyStyle = bodyStyle;
	}
	public String getPower() {
		return power;
	}
	public void setPower(String power) {
		this.power = power;
	}
	public String getEngine() {
		return engine;
	}
	public void setEngine(String engine) {
		this.engine = engine;
	}
	public String getBreaks() {
		return breaks;
	}
	public void setBreaks(String breaks) {
		this.breaks = breaks;
	}
	public String getSeats() {
		return seats;
	}
	public void setSeats(String seats) {
		this.seats = seats;
	}
	public String getWindows() {
		return windows;
	}
	public void setWindows(String windows) {
		this.windows = windows;
	}
	public String getFuelType() {
		return fuelType;
	}
	public void setFuelType(String fuelType) {
		this.fuelType = fuelType;
	}
	
	@Override
	public String toString(){
		StringBuilder sb = new StringBuilder();
		sb.append("--------------"+carType+"--------------------- \n");
		sb.append(" Body: ");
		sb.append(bodyStyle);
		sb.append("\n Power: ");
		sb.append(power);
		sb.append("\n Engine: ");
		sb.append(engine);
		sb.append("\n Breaks: ");
		sb.append(breaks);
		sb.append("\n Seats: ");
		sb.append(seats);
		sb.append("\n Windows: ");
		sb.append(windows);
		sb.append("\n Fuel Type: ");
		sb.append(fuelType);
		
		return sb.toString();
	}
}
Class: CarDirector

package builder.car;

public class CarDirector {
	
	private CarBuilder carBuilder;
	
	public CarDirector(CarBuilder carBuilder){
		this.carBuilder = carBuilder;
	}

	public void build(){
		carBuilder.buildBodyStyle();
		carBuilder.buildPower();
		carBuilder.buildEngine();
		carBuilder.buildBreaks();
		carBuilder.buildSeats();
		carBuilder.buildWindows();
		carBuilder.buildFuelType();
	}
}
Interface: CarBuilder


package builder.car;

public interface CarBuilder {
	
	public void buildBodyStyle();
	public void buildPower();
	public void buildEngine();
	public void buildBreaks();
	public void buildSeats();
	public void buildWindows();
	public void buildFuelType();
	public Car getCar();
}
Class: SedanCarBuilder

package builder.car;

public class SedanCarBuilder implements CarBuilder{
	
	private final Car car = new Car("SEDAN");
	
	@Override
	public void buildBodyStyle() {
		car.setBodyStyle("External dimensions: overall length (inches): 202.9, " +
"overall width (inches): 76.2, overall height (inches): 60.7, wheelbase (inches): 112.9," + " front track (inches): 65.3, rear track (inches): 65.5 and curb to curb turning circle (feet): 39.5");
	}
	
	@Override
	public void buildPower(){
		car.setPower("285 hp @ 6,500 rpm; 253 ft lb of torque @ 4,000 rpm");
	}

	@Override
	public void buildEngine() {
		car.setEngine("3.5L Duramax V 6 DOHC");
	}

	@Override
	public void buildBreaks() {
		car.setBreaks("Four-wheel disc brakes: two ventilated. Electronic brake distribution");
	}

	@Override
	public void buildSeats() {
		car.setSeats("Front seat center armrest.Rear seat center armrest.Split-folding rear seats");
	}

	@Override
	public void buildWindows() {
		car.setWindows("Laminated side windows.Fixed rear window with defroster");
		
	}

	@Override
	public void buildFuelType() {
		car.setFuelType("Gasoline 19 MPG city, 29 MPG highway, 23 MPG combined and 437 mi. range");
		
	}
	
	@Override
	public Car getCar(){
		return car;
	}
	
}
Class: SportsCarBuilder

package builder.car;

public class SportsCarBuilder implements CarBuilder{
	
	private final Car car = new Car("SPORTS");
	
	@Override
	public void buildBodyStyle() {
		car.setBodyStyle("External dimensions: overall length (inches): 192.3," +
" overall width (inches): 75.5, overall height (inches): 54.2, wheelbase (inches): 12.3," + " front track (inches): 63.7, rear track (inches): 64.1 and curb to curb turning circle (feet): 37.7");
	}
	
	@Override
	public void buildPower(){
		car.setPower("323 hp @ 6,800 rpm; 278 ft lb of torque @ 4,800 rpm");
	}

	@Override
	public void buildEngine() {
		car.setEngine("3.6L V 6 DOHC and variable valve timing");
	}

	@Override
	public void buildBreaks() {
		car.setBreaks("Four-wheel disc brakes: two ventilated. Electronic brake distribution. StabiliTrak stability control");
		
	}

	@Override
	public void buildSeats() {
		car.setSeats("Driver sports front seat with one power adjustments manual height, front passenger seat sports front seat with one power adjustments");
	}

	@Override
	public void buildWindows() {
		car.setWindows("Front windows with one-touch on two windows");
		
	}

	@Override
	public void buildFuelType() {
		car.setFuelType("Gasoline 17 MPG city, 28 MPG highway, 20 MPG combined and 380 mi. range");
	}
	
	@Override
	public Car getCar(){
		return car;
	}

}
Class: TestBuilderPattern

package builder.car;

public class TestBuilderPattern {

	public static void main(String[] args) {
		CarBuilder carBuilder = new SedanCarBuilder();
		CarDirector director = new CarDirector(carBuilder);
		director.build();
		Car car = carBuilder.getCar();
		System.out.println(car);
		
		carBuilder = new SportsCarBuilder();
		director = new CarDirector(carBuilder);
		director.build();
		car = carBuilder.getCar();
		System.out.println(car);
	}

}
Result:

--------------SEDAN--------------------- 
 Body: External dimensions: overall length (inches): 202.9, overall width (inches): 76.2, overall height (inches): 60.7, wheelbase (inches): 112.9, front track (inches): 65.3, rear track (inches): 65.5 and curb to curb turning circle (feet): 39.5
 Power: 285 hp @ 6,500 rpm; 253 ft lb of torque @ 4,000 rpm
 Engine: 3.5L Duramax V 6 DOHC
 Breaks: Four-wheel disc brakes: two ventilated. Electronic brake distribution
 Seats: Front seat center armrest.Rear seat center armrest.Split-folding rear seats
 Windows: Laminated side windows.Fixed rear window with defroster
 Fuel Type: Gasoline 19 MPG city, 29 MPG highway, 23 MPG combined and 437 mi. range
--------------SPORTS--------------------- 
 Body: External dimensions: overall length (inches): 192.3, overall width (inches): 75.5, overall height (inches): 54.2, wheelbase (inches): 112.3, front track (inches): 63.7, rear track (inches): 64.1 and curb to curb turning circle (feet): 37.7
 Power: 323 hp @ 6,800 rpm; 278 ft lb of torque @ 4,800 rpm
 Engine: 3.6L V 6 DOHC and variable valve timing
 Breaks: Four-wheel disc brakes: two ventilated. Electronic brake distribution. StabiliTrak stability control
 Seats: Driver sports front seat with one power adjustments manual height, front passenger seat sports front seat with one power adjustments
 Windows: Front windows with one-touch on two windows
 Fuel Type: Gasoline 17 MPG city, 28 MPG highway, 20 MPG combined and 380 mi. range
2020 年 11 月 2 日 by Java 物件導向 設計模式

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Jerry Lin
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