原理或定義
迭代器模式又叫做遊標(Cursor)模式。GOF給出的定義:提供一種方法訪問一個容器(container)對象中的各個元素,而又不暴露該對象的內部細節。
結構
迭代器角色(Iterator):
負責定義訪問和遍歷元素的接口。
具體迭代器角色(Concrete Iterator):實現迭代器接口,並要記錄遍歷中的當前位置。
容器角色(Aggregate):
負責提供創建具體迭代器角色的接口。
具體容器角色(ConcreteAggregate):實現創建具體迭代器角色的接口,
這個具體迭代器角色與該容器的結構相關。
類圖
案例與代碼
本模式使用兩個菜館合併後的菜單問題來作爲案例
蛋糕店與餐廳合併後,怎麼統一管理菜單項目
問題:一個用ArrayList管理菜單,一個用數組管理
通用菜單類:
public class MenuItem {
private String name,description;
private boolean vegetable;
private float price;
public MenuItem(String name,String description,boolean vegetable,float price)
{
this.name=name;
this.description=description;
this.vegetable=vegetable;
this.price=price;
}
public String getName()
{
return name;
}
public String getDescription()
{
return description;
}
public float getPrice()
{
return price;
}
public boolean isVegetable()
{
return vegetable;
}
}
餐廳類:
public class CakeHouseMenu {
private ArrayList<MenuItem> menuItems;
public CakeHouseMenu() {
menuItems = new ArrayList<MenuItem>();
addItem("KFC Cake Breakfast","boiled eggs&toast&cabbage",true,3.99f);
addItem("MDL Cake Breakfast","fried eggs&toast",false,3.59f);
addItem("Stawberry Cake","fresh stawberry",true,3.29f);
addItem("Regular Cake Breakfast","toast&sausage",true,2.59f);
}
private void addItem(String name, String description, boolean vegetable,
float price) {
MenuItem menuItem = new MenuItem(name, description, vegetable, price);
menuItems.add(menuItem);
}
public ArrayList<MenuItem> getMenuItems() {
return menuItems;
}
//其他功能代碼
}
public class DinerMenu {
private final static int Max_Items=5;
public int numberOfItems=0;
private MenuItem[] menuItems;
public DinerMenu()
{
menuItems=new MenuItem[Max_Items] ;
addItem("vegetable Blt","bacon&lettuce&tomato&cabbage",true,3.58f);
addItem("Blt","bacon&lettuce&tomato",false,3.00f);
addItem("bean soup","bean&potato salad",true,3.28f);
addItem("hotdog","onions&cheese&bread",false,3.05f);
}
private void addItem(String name, String description, boolean vegetable,
float price) {
MenuItem menuItem = new MenuItem(name, description, vegetable, price);
if(numberOfItems>=Max_Items)
{
System.err.println("sorry,menu is full!can not add another item");
}else{
menuItems[numberOfItems]=menuItem;
numberOfItems++;
}
}
public MenuItem[] getMenuItems() {
return menuItems;
}
}
public class Waitress {
private CakeHouseMenu mCakeHouseMenu;
private DinerMenu mDinerMenu;
private ArrayList<MenuItem> cakeitems;
private MenuItem[] dineritems;
public Waitress() {
mCakeHouseMenu = new CakeHouseMenu();
cakeitems = mCakeHouseMenu.getMenuItems();
mDinerMenu = new DinerMenu();
dineritems = mDinerMenu.getMenuItems();
}
public void printMenu() {
MenuItem menuItem;
for (int i = 0, len = cakeitems.size(); i < len; i++) {
menuItem = cakeitems.get(i);
System.out.println(menuItem.getName() + "***"
+menuItem.getPrice()+"***"+ menuItem.getDescription());
}
for (int i = 0, len = mDinerMenu.numberOfItems; i < len; i++) {
menuItem = dineritems[i];
System.out.println(menuItem.getName() + "***"
+menuItem.getPrice()+"***"+ menuItem.getDescription());
}
}
public void printBreakfastMenu() {
}
public void printLunchMenu() {
}
public void printVegetableMenu() {
}
}
顧客 / 測試類:
public class MainTest {
public static void main(String[] args) {
Waitress mWaitress=new Waitress();
//服務員展示2家餐廳合併的菜單
mWaitress.printMenu();
}
}
此設計違法單一責任原則,當添加新餐廳,打印菜單的方法需要改變,而且複雜度和新餐廳的數據結構的複雜度成正比。
迭代器模式的設計方案:
通用菜單類不變。
迭代器角色(Iterator):
public interface Iterator {
public boolean hasNext();
public Object next();
}
餐廳類:
public class CakeHouseMenu { private ArrayList<MenuItem> menuItems; public CakeHouseMenu() { menuItems = new ArrayList<MenuItem>(); addItem("KFC Cake Breakfast","boiled eggs&toast&cabbage",true,3.99f); addItem("MDL Cake Breakfast","fried eggs&toast",false,3.59f); addItem("Stawberry Cake","fresh stawberry",true,3.29f); addItem("Regular Cake Breakfast","toast&sausage",true,2.59f); } private void addItem(String name, String description, boolean vegetable, float price) { MenuItem menuItem = new MenuItem(name, description, vegetable, price); menuItems.add(menuItem); } public Iterator getIterator() { return new CakeHouseIterator() ; } class CakeHouseIterator implements Iterator { private int position=0; public CakeHouseIterator() { position=0; } @Override public boolean hasNext() { // TODO Auto-generated method stub if(position<menuItems.size()) { return true; } return false; } @Override public Object next() { // TODO Auto-generated method stub MenuItem menuItem =menuItems.get(position); position++; return menuItem; }}; //其他功能代碼 }
public class DinerMenu {
private final static int Max_Items = 5;
private int numberOfItems = 0;
private MenuItem[] menuItems;
public DinerMenu() {
menuItems = new MenuItem[Max_Items];
addItem("vegetable Blt", "bacon&lettuce&tomato&cabbage", true, 3.58f);
addItem("Blt", "bacon&lettuce&tomato", false, 3.00f);
addItem("bean soup", "bean&potato salad", true, 3.28f);
addItem("hotdog", "onions&cheese&bread", false, 3.05f);
}
private void addItem(String name, String description, boolean vegetable,
float price) {
MenuItem menuItem = new MenuItem(name, description, vegetable, price);
if (numberOfItems >= Max_Items) {
System.err.println("sorry,menu is full!can not add another item");
} else {
menuItems[numberOfItems] = menuItem;
numberOfItems++;
}
}
public Iterator getIterator() {
return new DinerIterator();
}
class DinerIterator implements Iterator {
private int position;
public DinerIterator() {
position = 0;
}
@Override
public boolean hasNext() {
// TODO Auto-generated method stub
if (position < numberOfItems) {
return true;
}
return false;
}
@Override
public Object next() {
// TODO Auto-generated method stub
MenuItem menuItem = menuItems[position];
position++;
return menuItem;
}
};
}
服務員類:
public class Waitress {
private ArrayList<Iterator> iterators=new ArrayList<Iterator>();
public Waitress() {
}
public void addIterator(Iterator iterator)
{
iterators.add(iterator);
}
public void printMenu() {
Iterator iterator;
MenuItem menuItem;
for (int i = 0, len = iterators.size(); i < len; i++) {
iterator = iterators.get(i);
while(iterator.hasNext())
{
menuItem=(MenuItem) iterator.next();
System.out.println(menuItem.getName() + "***"
+menuItem.getPrice()+"***"+ menuItem.getDescription());
}
}
}
}
測試類:
public class MainTest {
public static void main(String[] args) {
Waitress mWaitress=new Waitress();
CakeHouseMenu mCakeHouseMenu = new CakeHouseMenu();
DinerMenu mDinerMenu = new DinerMenu();
mWaitress.addIterator(mCakeHouseMenu.getIterator());
mWaitress.addIterator(mDinerMenu.getIterator());
mWaitress.printMenu();
}
}
該案例可以直接使用JDK內置的迭代器接口作爲迭代器角色:Iterator
單一責任原則:一個類應該只有一個引起變化的原因.
使用場景
1. 訪問一個聚合對象的內容而無需暴露它的內部表示
2.支持對聚合對象的多種遍歷
3.爲遍歷不同的聚合結構提供一個統一的接口
優缺點
主要優點有:
1. 簡化了遍歷方式。
2.可以提供多種遍歷方式,比如說對有序列表,我們可以根據需要提供正序遍歷,倒序遍歷兩種迭代器。
3.封裝性良好,用戶只需要得到迭代器就可以遍歷,而對於遍歷算法則不用去關心。
缺點主要有:
對於比較簡單的遍歷(像數組或者有序列表),使用迭代器方式遍歷較爲繁瑣,大家可能都有感覺,像ArrayList,我們寧可願意使用for循環和get方法來遍歷集合。
迭代器模式是與集合共生共死的,一般來說,我們只要實現一個集合,就需要同時提供這個集合的迭代器,就像java中的Collection,List、Set、Map等,這些集合都有自己的迭代器。假如我們要實現一個這樣的新的容器,當然也需要引入迭代器模式,給我們的容器實現一個迭代器。