Builder模式

定義

將一個複雜對象的構建與它的表象分離，使得一樣的構建能夠建立不一樣的表示。bash

使用場景

相同的方法，不一樣的執行順序，產生不一樣的事件結果時markdown

多個部件或零件，均可以裝配到一個對象中，可是產生的運行結果又不相同時app

產品類很是複雜，或者產品類中的調用順序產生不一樣做用時ide

初始化一個對象特別複雜時ui

實現

經典方案

經典的Builder模式通常擁有4個角色this

Product 產品類或產品抽象類
Builder 抽象類
ConcreteBuilder 具體Builder實現類
Director 組裝類

public class Meal {
    private  String food;
    private  String drink;

    public String getFood() {
        return food;
    }

    public void setFood(String food) {
        this.food = food;
    }

    public String getDrink() {
        return drink;
    }

    public void setDrink(String drink) {
        this.drink = drink;
    }
}

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菜單類Meal在這裏充當Product的角色，提供了事物food和飲料drink供客人選擇。spa

public abstract class MealBuilder {

    protected Meal meal = new Meal();

    public abstract void buildFood();

    public abstract void buildDrink();

    public Meal getMeal() {
        return meal;
    }
}

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MealBuilder是一個抽象Builder，提供了2個抽象方法buildFood（）和buildDrink（）供給客人點餐，而食物的品種咱們並不知道，具體的食物和套餐是交給子類去實現的。code

public class SubMealBuilderA extends MealBuilder {
    @Override
    public void buildFood() {
        meal.setFood("一個雞腿堡");
    }

    @Override
    public void buildDrink() {
        meal.setDrink("一杯可樂");
    }
}

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SubMealBuilderA 是做爲ConcreteBuilder的，他給出了一個具體實現方案---套餐A( 包含一個雞腿堡和一杯可樂）orm

public class SubMealBuilderB extends MealBuilder {
    @Override
    public void buildFood() {
        meal.setFood("一個雞肉卷");
    }

    @Override
    public void buildDrink() {
        meal.setDrink("一杯果汁");
    }
}

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SubMealBuilderB 也是做爲ConcreteBuilder的，他給出了一個具體實現方案---套餐B( 包含一個雞肉卷和一杯果汁）對象

public class Waiter {
    private MealBuilder mealBuilder;

    public void setMealBuilder(MealBuilder mealBuilder) {
        this.mealBuilder = mealBuilder;
    }

    public Meal construct() {
        mealBuilder.buildDrink();
        mealBuilder.buildFood();
        return mealBuilder.getMeal();
    }
}

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Waiter類充當Director的角色，客人將選好的套餐告訴服務員後，服務員將客人選擇的食物組裝起來而且通知後廚開始生產。

public class Test {

  public static void main(String[] args) {

    Waiter waiter = new Waiter();
    waiter.setMealBuilder(new SubMealBuilderA());
    Meal meal = waiter.construct();

    log(meal);

    waiter.setMealBuilder(new SubMealBuilderB());
    meal = waiter.construct();

    log(meal);
  }

  private static void log(Meal meal) {
    System.out.println(meal.getClass() + ":" + meal.getFood() + ":" + meal.getDrink());
  }
}

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運行結果

class com.example.jc.myapplication.builder.custom.Meal:一個雞腿堡:一杯可樂
class com.example.jc.myapplication.builder.custom.Meal:一個雞肉卷:一杯果汁
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改進方案

在實際中咱們更常見的Builder模式是有對經典模式又必定修改的。如AlertDialog的Builer模式，它的AlertDialog.Builer同時扮演了Builder、 ConcreteBuilder、 Director的角色，簡化了Builer的使用。同時這種形式的Builer也更常見。

public class WaiterX {
  private MenuController mWaiter;

  WaiterX() {
    mWaiter = new MenuController();
  }

  public static class Builder {
    private MenuController.MenuParams p;

    public Builder() {
      p = new MenuController.MenuParams();
    }

    public Builder setFood(String food) {
      p.food = food;
      return this;
    }

    public Builder setDrink(String drink) {
      p.drink = drink;
      return this;
    }

    public Builder setCount(int count) {
      p.count = count;
      return this;
    }

    public Builder setType(int type) {
      p.type = type;
      return this;
    }

    public Builder setRemark(String remark) {
      p.remark = remark;
      return this;
    }

    public WaiterX create() {
      WaiterX waiter = new WaiterX();
      p.apply(waiter.mWaiter);
      return waiter;
    }

  }

  @Override public String toString() {
    return "WaiterX{" + "mWaiter=" + mWaiter + '}';
  }
}

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在WaiterX中經過一個靜態內部類Builder來負責調度產品的組件，再經過create()方法將組裝後的總體返回。而具體的裝細節被隱藏了，經過交由MenuController.MenuParams去專門處理細節信息。

public class MenuController {

  private String food;
  private String drink;
  /**
   * 點單分量
   */
  private int count;
  /**
   * 類型  外帶，食堂
   */
  private int type;
  /**
   * 備註
   */
  private String remark;

  public void setFood(String food) {
    this.food = food;
  }

  public void setDrink(String drink) {
    this.drink = drink;
  }

  public void setCount(int count) {
    this.count = count;
  }

  public void setType(int type) {
    this.type = type;
  }

  public void setRemark(String remark) {
    this.remark = remark;
  }

  public static class MenuParams {

    public String food;
    public String drink;
    public int count;
    public int type;
    public String remark;

    public void apply(MenuController waiter) {
      if (drink != null) {
        waiter.setDrink(drink);
      }
      if (food != null) {
        waiter.setFood(food);
      }
      if (count > 1) {
        waiter.setCount(count);
      } else {
        waiter.setCount(1);
      }
      if (type > 0) {
        waiter.setType(type);
      } else {
        waiter.setType(0);
      }

      if (remark != null) {
        waiter.setRemark(remark);
      }
    }
  }

  @Override public String toString() {
    return "MenuController{" + (food != null ? "food='" + food + '\'' : "") + (drink != null ? ", drink='" + drink + '\'' : "") + (count > 0 ? ", count='" + count + '\'' : "")

        + (type > 0 ? ", type='" + type + '\'' : "") + (remark != null ? ", remark='" + remark + '\'' : "") + '}'; } } 複製代碼

MenuController負責餐點的具體細節處理，用於記錄客戶具體點了什麼食物、分量、需求等信息

public class Test {
  public static void main(String[] args) {
    WaiterX waiterX = new WaiterX.Builder().setFood("香辣雞腿堡")
        .setDrink("可樂")
        .setType(2)
        .setRemark("少鹽")
        .setCount(3)
        .create();
    System.out.println(waiterX.toString());

  }
}

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結果

WaiterX{mWaiter=MenuController{food='香辣雞腿堡', drink='可樂', count='3', type='2', remark='少鹽'}}
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經過這種形式，咱們在調用的時候就能採用鏈式調用的方式來處理了，使用起來更加方便。

缺點是當有新的元素或者新的屬性須要添加時候，須要對多個類進行修改。如要添加一個字段取餐時間time，則WaiterX和MenuController中都必須對這個字段time進行支持，這就務必要對2個類進行修改了。