YSMull
<-- Design Pattern

Prototype Pattern

2017/04/11

Quote

  • Prototype design pattern is used in scenarios where application needs to create a number of instances of a class, which has almost same state or differs very little.[2]

  • Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.[1]

  • 当创建给定类的实例很昂贵或很复杂时使用原型模式。—《Head First设计模式》

Participants

  • Prototype : the prototype of actual object.
  • Prototype registry : This is used as registry service to have all prototypes accessible using simple string parameters.
  • Client : Client will be responsible for using registry service to access prototype instances.

Practice

定义一个细胞Prototype，split()方法表示出分裂(clone)出一个新细胞。

interface Cell extends Cloneable {

    Cell split() throws CloneNotSupportedException;

}

分别定义动物细胞和植物细胞

public class AnimalCell implements Cell {

    @Override
    public AnimalCell split() throws CloneNotSupportedException {
        return (AnimalCell) super.clone();
    }

}

public class PlantCell implements Cell {

    @Override
    public PlantCell split() throws CloneNotSupportedException {
        return (PlantCell) super.clone();
    }

}

写一个CellRegestry，可以根据不同的细胞类型返回对应的新细胞实例。(有种简单工厂的味道吧？)

public class CellRegestry {

    public enum CellType {
        ANIMAL, PLANT
    }
    private static Map<CellType, Cell> prototypes = new HashMap<>();

    static {
        prototypes.put(CellType.ANIMAL, new AnimalCell());
        prototypes.put(CellType.PLANT, new PlantCell());
    }

    public static Cell getCell(CellType type) throws CloneNotSupportedException {
        // 先根据type获取对应实例，然后clone之
        return prototypes.get(type).split();
    }

}

Client使用:

public class Client {
    public static void main(String[] args) {
        try {
            AnimalCell cell1 = (AnimalCell) CellRegestry.getNewCell(CellType.ANIMAL);
            PlantCell cell2 = (PlantCell) CellRegestry.getNewCell(CellType.PLANT);
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
    }
}

Attention

  1. Cloneable接口是个标记接口(tagging/marker interface)，它没有方法。标记接口的唯一目的是可以用instanceof进行类型检查。
  2. 使用 Prototype Pattern 的其中一个原因是因为 clone 比 new 要快，但其实这已经过时了，参考[3]
  3. 可以使用工厂方法模式或者复制构造函数来替代该模式[4]。
  4. 需要注意深拷贝和浅拷贝的问题。

Reference