Java设计模式(十)：迭代器设计模式

野性酷女 2022-03-30 09:30 295阅读 0赞

# 1.应用场景 #

(1)访问一个聚合对象的内容而无需暴露它的内部表示

(2)支持对聚合对象的多种遍历

(3)为遍历不同的聚合结构提供一个统一的接口

# 2.概念 #

提供一种顺序访问聚合对象元素的方法，并且不暴露聚合对象的内部表示。这个模式给使用者提供了一种方法，可以顺序访问一个聚集对象中的元素，而又不用知道内部是如何表示的。

# 3. Class Digram #

*  Aggregate 是聚合类，其中 createIterator() 方法可以产生一个 Iterator；
 *  Iterator 主要定义了 hasNext() 和 next() 方法。
 *  Client 组合了 Aggregate，为了迭代遍历 Aggregate，也需要组合 Iterator。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIxMTI1MTgz_size_16_color_FFFFFF_t_70]

# 4. Implementation #

public interface Menu { 
    	public Iterator createIterator();
    }
    
    public interface Iterator { 
    	boolean hasNext();
    	MenuItem next();
    }
    
    public class MenuItem { 
    	String name;
    	String description;
    	boolean vegetarian;
    	double price;
     
    	public MenuItem(String name, 
    	                String description, 
    	                boolean vegetarian, 
    	                double price) 
    	{ 
    		this.name = name;
    		this.description = description;
    		this.vegetarian = vegetarian;
    		this.price = price;
    	}
      
    	public String getName() { 
    		return name;
    	}
      
    	public String getDescription() { 
    		return description;
    	}
      
    	public double getPrice() { 
    		return price;
    	}
      
    	public boolean isVegetarian() { 
    		return vegetarian;
    	}
    	public String toString() { 
    		return (name + ", $" + price + "\n " + description);
    	}
    }
    
    public class PancakeHouseMenu implements Menu { 
    	ArrayList<MenuItem> menuItems;
     
    	public PancakeHouseMenu() { 
    		menuItems = new ArrayList<MenuItem>();
        
    		addItem("K&B's Pancake Breakfast", 
    			"Pancakes with scrambled eggs, and toast", 
    			true,
    			2.99);
     
    		addItem("Regular Pancake Breakfast", 
    			"Pancakes with fried eggs, sausage", 
    			false,
    			2.99);
     
    		addItem("Blueberry Pancakes",
    			"Pancakes made with fresh blueberries",
    			true,
    			3.49);
     
    		addItem("Waffles",
    			"Waffles, with your choice of blueberries or strawberries",
    			true,
    			3.59);
    	}
    
    	public void addItem(String name, String description,
    	                    boolean vegetarian, double price)
    	{ 
    		MenuItem menuItem = new MenuItem(name, description, vegetarian, price);
    		menuItems.add(menuItem);
    	}
     
    	public ArrayList<MenuItem> getMenuItems() { 
    		return menuItems;
    	}
      
    	public Iterator createIterator() { 
    		return new PancakeHouseMenuIterator(menuItems);
    	}
      
    	public String toString() { 
    		return "Objectville Pancake House Menu";
    	}
    
    	// other menu methods here
    }
    
    public class PancakeHouseMenuIterator implements Iterator { 
    	ArrayList<MenuItem> items;
    	int position = 0;
     
    	public PancakeHouseMenuIterator(ArrayList<MenuItem> items) { 
    		this.items = items;
    	}
     
    	public MenuItem next() { 
    		MenuItem item = items.get(position);
    		position = position + 1;
    		return item;
    	}
     
    	public boolean hasNext() { 
    		if (position >= items.size()) { 
    			return false;
    		} else { 
    			return true;
    		}
    	}
    }
    
    public class DinerMenu implements Menu { 
    	static final int MAX_ITEMS = 6;
    	int numberOfItems = 0;
    	MenuItem[] menuItems;
      
    	public DinerMenu() { 
    		menuItems = new MenuItem[MAX_ITEMS];
     
    		addItem("Vegetarian BLT",
    			"(Fakin') Bacon with lettuce & tomato on whole wheat", true, 2.99);
    		addItem("BLT",
    			"Bacon with lettuce & tomato on whole wheat", false, 2.99);
    		addItem("Soup of the day",
    			"Soup of the day, with a side of potato salad", false, 3.29);
    		addItem("Hotdog",
    			"A hot dog, with saurkraut, relish, onions, topped with cheese",
    			false, 3.05);
    		addItem("Steamed Veggies and Brown Rice",
    			"Steamed vegetables over brown rice", true, 3.99);
    		addItem("Pasta",
    			"Spaghetti with Marinara Sauce, and a slice of sourdough bread",
    			true, 3.89);
    	}
      
    	public void addItem(String name, String description, 
    	                     boolean vegetarian, double price) 
    	{ 
    		MenuItem menuItem = new MenuItem(name, description, vegetarian, price);
    		if (numberOfItems >= MAX_ITEMS) { 
    			System.err.println("Sorry, menu is full! Can't add item to menu");
    		} else { 
    			menuItems[numberOfItems] = menuItem;
    			numberOfItems = numberOfItems + 1;
    		}
    	}
     
    	public MenuItem[] getMenuItems() { 
    		return menuItems;
    	}
      
    	public Iterator createIterator() { 
    		return new DinerMenuIterator(menuItems);
    		// To test Alternating menu items, comment out above line,
    		// and uncomment the line below.
    		//return new AlternatingDinerMenuIterator(menuItems);
    	}
     
    	// other menu methods here
    }
    
    public class DinerMenuIterator implements Iterator { 
        private MenuItem[] menuItem;
    
        int position=0;
    
        public DinerMenuIterator(MenuItem[] menuItem) { 
            this.menuItem = menuItem;
        }
    
        @Override
        public boolean hasNext() { 
            return position < menuItem.length && menuItem[position] != null;
        }
    
        @Override
        public MenuItem next() { 
            MenuItem menuItem = this.menuItem[position];
            position++;
            return menuItem;
        }
    }
    
    public class Waiterss { 
        PancakeHouseMenu pancakeHouseMenu;
        DinerMenu dinerMenu;
    
        public Waiterss(PancakeHouseMenu pancakeHouseMenu, DinerMenu dinerMenu) { 
            this.pancakeHouseMenu = pancakeHouseMenu;
            this.dinerMenu = dinerMenu;
        }
    
        public void printMenu() { 
            Iterator pancakeIterator = pancakeHouseMenu.createIterator();
            Iterator dinerIterator = dinerMenu.createIterator();
    
            System.out.println("MENU\n----\nBREAKFAST");
            printMenu(pancakeIterator);
            System.out.println("\nLUNCH");
            printMenu(dinerIterator);
    
        }
    
        private void printMenu(Iterator iterator) { 
            while (iterator.hasNext()) { 
                MenuItem menuItem = iterator.next();
                System.out.print(menuItem.getName() + ", ");
                System.out.print(menuItem.getPrice() + " -- ");
                System.out.println(menuItem.getDescription());
            }
        }
    }
    
    public class MenuTestDriver { 
        public static void main(String[] args) { 
            DinerMenu dinerMenu=new DinerMenu();
            PancakeHouseMenu pancakeHouseMenu=new PancakeHouseMenu();
            Waiterss waiterss=new Waiterss(pancakeHouseMenu,dinerMenu);
            waiterss.printMenu();
            Iterator iterator = pancakeHouseMenu.createIterator();
            while (iterator.hasNext()){ 
                MenuItem next = iterator.next();
                System.out.println(next);
            }
    
            System.out.println();
    
            Iterator dinerMenuIterator = dinerMenu.createIterator();
            while (dinerMenuIterator.hasNext()){ 
                MenuItem next = dinerMenuIterator.next();
                System.out.println(next);
            }
        }
    }

# 5. JDK #

*  [java.util.Iterator][]
 *  [java.util.Enumeration][]

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[java.util.Iterator]: http://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
[java.util.Enumeration]: http://docs.oracle.com/javase/8/docs/api/java/util/Enumeration.html