设计模式之迭代器与组合模式（四）
因为这系列篇幅较长，所以在这里也不进行任何铺垫，直奔主题去啦。
利用组合设计菜单
我们要如何在菜单上应用组合模式呢？一开始，我们需要创建一个组件接口来作为菜单和菜单项的共同接口，让我们能够用统一的做法来处理菜单和菜单项。换句话说，我们可以针对菜单或菜单项调用相同的方法。
让我们从头来看看如何让菜单能够符合组合模式的结构：
实现菜单组件
好了，我们开始编写菜单组件的抽象类；请记住，菜单组件的角色是为叶节点和组合节点提供一个共同的接口。
public abstract class MenuComponent {
public void add(MenuComponent menuComponent) {
throw new UnsupportedOperationException();
}
public void remove(MenuComponent menuComponent) {
throw new UnsupportedOperationException();
}
public MenuComponent getChild(int i) {
throw new UnsupportedOperationException();
}
public String getName() {
throw new UnsupportedOperationException();
}
public String getDescription() {
throw new UnsupportedOperationException();
}
public double getPrice() {
throw new UnsupportedOperationException();
}
public boolean isVegetarian() {
throw new UnsupportedOperationException();
}
public abstract Iterator<MenuComponent> createIterator();
public void print() {
throw new UnsupportedOperationException();
}
}
让我们来看菜单类。别忘了，这是组合类图里的叶类，它实现组合内元素的行为。
public class MenuItem extends MenuComponent {
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 Iterator<MenuComponent> createIterator() {
return new NullIterator();
}
public void print() {
System.out.print(" " + getName());
if (isVegetarian()) {
System.out.print("(v)");
}
System.out.println(", " + getPrice());
System.out.println(" -- " + getDescription());
}
}
我们已经有了菜单项，还需要组合类，这就是我们叫做菜单的。别忘了，此组合类可以持有菜单项或其他菜单。
public class Menu extends MenuComponent {
Iterator<MenuComponent> iterator = null;
ArrayList<MenuComponent> menuComponents = new ArrayList<MenuComponent>();
String name;
String description;
public Menu(String name, String description) {
this.name = name;
this.description = description;
}
public void add(MenuComponent menuComponent) {
menuComponents.add(menuComponent);
}
public void remove(MenuComponent menuComponent) {
menuComponents.remove(menuComponent);
}
public MenuComponent getChild(int i) {
return menuComponents.get(i);
}
public String getName() {
return name;
}
public String getDescription() {
return description;
}
public Iterator<MenuComponent> createIterator() {
if (iterator == null) {
iterator = new CompositeIterator(menuComponents.iterator());
}
return iterator;
}
public void print() {
System.out.print("\n" + getName());
System.out.println(", " + getDescription());
System.out.println("---------------------");
}
}
因为菜单是一个组合，包含了菜单项和其他的菜单，所以它的print()应该打印出它所包含的一切。如果它不这么做，我们就必须遍历整个组合的每个节点，然后将每一项打印出来。这么一来，也就失去了使用组合结构的意义。
所以，print还得进行优化，如下:
public void print() {
System.out.print("\n" + getName());
System.out.println(", " + getDescription());
System.out.println("---------------------");
Iterator<MenuComponent> iterator = menuComponents.iterator();
while (iterator.hasNext()) {
MenuComponent menuComponent = iterator.next();
menuComponent.print();
}
}
看到上面了没，我们用了迭代器。用它遍历所有菜单组件，遍历过程中，可能遇到其他菜单，或者是遇到菜单项。由于菜单和菜单项都实现了print，那我们只要调用print即可。
开始测试数据之前，我们了解一下，在运行时菜单组合是什么样的：
开始运行我们的测试程序啦:
public class MenuTestDrive {
public static void main(String args[]) {
MenuComponent pancakeHouseMenu =
new Menu("PANCAKE HOUSE MENU", "Breakfast");
MenuComponent dinerMenu =
new Menu("DINER MENU", "Lunch");
MenuComponent cafeMenu =
new Menu("CAFE MENU", "Dinner");
MenuComponent dessertMenu =
new Menu("DESSERT MENU", "Dessert of course!");
MenuComponent allMenus = new Menu("ALL MENUS", "All menus combined");
allMenus.add(pancakeHouseMenu);
allMenus.add(dinerMenu);
allMenus.add(cafeMenu);
pancakeHouseMenu.add(new MenuItem(
"K&B's Pancake Breakfast",
"Pancakes with scrambled eggs, and toast",
true,
2.99));
pancakeHouseMenu.add(new MenuItem(
"Regular Pancake Breakfast",
"Pancakes with fried eggs, sausage",
false,
2.99));
pancakeHouseMenu.add(new MenuItem(
"Blueberry Pancakes",
"Pancakes made with fresh blueberries, and blueberry syrup",
true,
3.49));
pancakeHouseMenu.add(new MenuItem(
"Waffles",
"Waffles, with your choice of blueberries or strawberries",
true,
3.59));
dinerMenu.add(new MenuItem(
"Vegetarian BLT",
"(Fakin') Bacon with lettuce & tomato on whole wheat",
true,
2.99));
dinerMenu.add(new MenuItem(
"BLT",
"Bacon with lettuce & tomato on whole wheat",
false,
2.99));
dinerMenu.add(new MenuItem(
"Soup of the day",
"A bowl of the soup of the day, with a side of potato salad",
false,
3.29));
dinerMenu.add(new MenuItem(
"Hotdog",
"A hot dog, with saurkraut, relish, onions, topped with cheese",
false,
3.05));
dinerMenu.add(new MenuItem(
"Steamed Veggies and Brown Rice",
"A medly of steamed vegetables over brown rice",
true,
3.99));
dinerMenu.add(new MenuItem(
"Pasta",
"Spaghetti with Marinara Sauce, and a slice of sourdough bread",
true,
3.89));
dinerMenu.add(dessertMenu);
dessertMenu.add(new MenuItem(
"Apple Pie",
"Apple pie with a flakey crust, topped with vanilla icecream",
true,
1.59));
dessertMenu.add(new MenuItem(
"Cheesecake",
"Creamy New York cheesecake, with a chocolate graham crust",
true,
1.99));
dessertMenu.add(new MenuItem(
"Sorbet",
"A scoop of raspberry and a scoop of lime",
true,
1.89));
cafeMenu.add(new MenuItem(
"Veggie Burger and Air Fries",
"Veggie burger on a whole wheat bun, lettuce, tomato, and fries",
true,
3.99));
cafeMenu.add(new MenuItem(
"Soup of the day",
"A cup of the soup of the day, with a side salad",
false,
3.69));
cafeMenu.add(new MenuItem(
"Burrito",
"A large burrito, with whole pinto beans, salsa, guacamole",
true,
4.29));
Waitress waitress = new Waitress(allMenus);
waitress.printVegetarianMenu();
}
}
结果这里就不附上了，请大家自行去跑代码实现吧。相信你们又对组合模式也已经有了一个大概了吧。下一篇，还有更犀利的，组合迭代器等着我们。小编马上回去搞起来，安排上。
爱生活，爱学习，爱感悟，爱挨踢
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原始发表：2019-06-26，如有侵权请联系 cloudcommunity@tencent.com 删除
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