Java设计模式——原型模式

原型模式是一种创建模式，它在考虑性能的同时创建重复对象。
当直接创建对象成本高时，使用原形模式。它定义一个原型接口，这个接口会帮助返回克隆对象。
例如，一个对象将在一个代价高昂的数据库操作之后创建。我们可以缓存对象，在下一次请求时返回其克隆，并在需要时更新数据库，从而减少数据库调用。

举例说明：

1. 创建一个抽象类Shape以及它的子类（Circle、Rectangle、Square）;
2. 创建ShapeCache类，它将shape对象存放在哈希表中，并在得到请求时返回一个克隆对象。

1、创建抽象类Shape，需要实现Clonable接口

public abstract class Shape implements Cloneable {
   
   private String id;
   protected String type;
   
   abstract void draw();
   
   public String getType(){
      return type;
   }
   
   public String getId() {
      return id;
   }
   
   public void setId(String id) {
      this.id = id;
   }
   
   public Object clone() {
      Object clone = null;
      
      try {
         clone = super.clone();
         
      } catch (CloneNotSupportedException e) {
         e.printStackTrace();
      }
      
      return clone;
   }
}

2、创建子类

public class Rectangle extends Shape {

   public Rectangle(){
     type = "Rectangle";
   }

   @Override
   public void draw() {
      System.out.println("Inside Rectangle::draw() method.");
   }
}

public class Square extends Shape {

   public Square(){
     type = "Square";
   }

   @Override
   public void draw() {
      System.out.println("Inside Square::draw() method.");
   }
}

public class Circle extends Shape {

   public Circle(){
     type = "Circle";
   }

   @Override
   public void draw() {
      System.out.println("Inside Circle::draw() method.");
   }
}

3、创建ShapeCache类，用于将对象存放到哈希表中

import java.util.Hashtable;

public class ShapeCache {
	
   private static Hashtable<String, Shape> shapeMap  = new Hashtable<String, Shape>();

   public static Shape getShape(String shapeId) {
      Shape cachedShape = shapeMap.get(shapeId);
      return (Shape) cachedShape.clone();
   }

   // for each shape run database query and create shape
   // shapeMap.put(shapeKey, shape);
   // for example, we are adding three shapes
   
   public static void loadCache() {
      Circle circle = new Circle();
      circle.setId("1");
      shapeMap.put(circle.getId(),circle);

      Square square = new Square();
      square.setId("2");
      shapeMap.put(square.getId(),square);

      Rectangle rectangle = new Rectangle();
      rectangle.setId("3");
      shapeMap.put(rectangle.getId(), rectangle);
   }
}

4、通过ShapeCache，返回克隆对象

public class PrototypePatternDemo {
   public static void main(String[] args) {
      ShapeCache.loadCache();

      Shape clonedShape = (Shape) ShapeCache.getShape("1");
      System.out.println("Shape : " + clonedShape.getType());		

      Shape clonedShape2 = (Shape) ShapeCache.getShape("2");
      System.out.println("Shape : " + clonedShape2.getType());		

      Shape clonedShape3 = (Shape) ShapeCache.getShape("3");
      System.out.println("Shape : " + clonedShape3.getType());		
   }
}

5、结果

Shape : Circle
Shape : Square
Shape : Rectangle