パターンについて

ツリー構造や配列構造のように複雑な構造で保持されているオブジェクトがあるときに、
対象オブジェクトの構造を意識しない簡単な操作で対象オブジェクトの構造を扱うことでできる。

HeadFirstデザインパターンでの定義

内部表現を公開することなく、アグリゲートオブジェクトの
要素に順次アクセスする方法を提供する

パターン構造

iterator_pattern
構成要素
Aggregate : createIteratorの抽象操作を定義します
ConcreteAggregate : ArrayListやHashMap、自作イタレーターをcreateIteratorで実装します
iterator : イタレータの抽象操作を定義します
Concreteiterator : イタレータの抽象操作を実装します
Client : createIteratorを利用します

サンプル

データの持ち方が異なるメニュー(コレクションクラス)に順次アクセスしていく場合

・メニューインタフェース(Aggregate)

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import java.util.Iterator;
public interface Menu {
public Iterator<MenuItem> createIterator();
}

・メニュークラス

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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;
}
}

・パンケーキメニュー(ConcreteAggregate)(ArrayListイタレータ)

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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, and blueberry syrup",
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<MenuItem> createIterator() {
return menuItems.iterator();
}
}

・カフェメニュー(ConcreteAggregate)(HashMapイタレータ)

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public class CafeMenu implements Menu {
HashMap<String, MenuItem> menuItems = new HashMap<String, MenuItem>();
public CafeMenu() {
addItem("Veggie Burger and Air Fries",
"Veggie burger on a whole wheat bun, lettuce, tomato, and fries",
true, 3.99);
addItem("Soup of the day",
"A cup of the soup of the day, with a side salad",
false, 3.69);
addItem("Burrito",
"A large burrito, with whole pinto beans, salsa, guacamole",
true, 4.29);
}
public void addItem(String name, String description,
boolean vegetarian, double price)
{
MenuItem menuItem = new MenuItem(name, description, vegetarian, price);
menuItems.put(menuItem.getName(), menuItem);
}
public Map<String, MenuItem> getItems() {
return menuItems;
}
public Iterator<MenuItem> createIterator() {
return menuItems.values().iterator();
}
}

・ディナーイタレーター(ConcreteIterator)

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public class DinerMenuIterator implements Iterator<MenuItem> {
MenuItem[] list;
int position = 0;
public DinerMenuIterator(MenuItem[] list) {
this.list = list;
}
public MenuItem next() {
MenuItem menuItem = list[position];
position = position + 1;
return menuItem;
}
public boolean hasNext() {
if (position >= list.length || list[position] == null) {
return false;
} else {
return true;
}
}
public void remove() {
if (position <= 0) {
throw new IllegalStateException
("You can't remove an item until you've done at least one next()");
}
if (list[position-1] != null) {
for (int i = position-1; i < (list.length-1); i++) {
list[i] = list[i+1];
}
list[list.length-1] = null;
}
}
}

・ディナーメニュー(ConcreteAggregate)(独自イタレータ)

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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",
"A medly of 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<MenuItem> createIterator() {
return new DinerMenuIterator(menuItems);
}
}

・ウェイトレス(Client)

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public class Waitress {
Menu pancakeHouseMenu;
Menu dinerMenu;
Menu cafeMenu;
public Waitress(Menu pancakeHouseMenu, Menu dinerMenu, Menu cafeMenu) {
this.pancakeHouseMenu = pancakeHouseMenu;
this.dinerMenu = dinerMenu;
this.cafeMenu = cafeMenu;
}
public void printMenu() {
Iterator<MenuItem> pancakeIterator = pancakeHouseMenu.createIterator();
Iterator<MenuItem> dinerIterator = dinerMenu.createIterator();
Iterator<MenuItem> cafeIterator = cafeMenu.createIterator();
System.out.println("MENU\n----\nBREAKFAST");
printMenu(pancakeIterator);
System.out.println("\nLUNCH");
printMenu(dinerIterator);
System.out.println("\nDINNER");
printMenu(cafeIterator);
}
private void printMenu(Iterator<MenuItem> iterator) {
while (iterator.hasNext()) {
MenuItem menuItem = iterator.next();
System.out.print(menuItem.getName() + ", ");
System.out.print(menuItem.getPrice() + " -- ");
System.out.println(menuItem.getDescription());
}
}
}

・実行コード(Client)

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public class Client {
public static void main(String args[]) {
PancakeHouseMenu pancakeHouseMenu = new PancakeHouseMenu();
DinerMenu dinerMenu = new DinerMenu();
CafeMenu cafeMenu = new CafeMenu();
Waitress waitress = new Waitress(pancakeHouseMenu, dinerMenu, cafeMenu);
waitress.printMenu();
}
}


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