Java设计模式（八）观察者模式 迭代器模式

（十五）观察者模式

观察者模式，定义对象间一对多关系，一个对象状态发生改变，所有依赖于它的对象都收到通知并且自动更新，观察者与被观察者分开。例如邮件订阅、RSS订阅，如果有更新就会邮件通知你。

interface Observers{public void update();}class Observer1 implements Observers{public void update(){System.out.println("observer1 has received");}}class Observer2 implements Observers{public void update(){System.out.println("observer2 has received");}}interface Subject {public void add(Observers ob);public void delete(Observers ob);public void notifyObservers();public void operation();}abstract class AbstractSubject implements Subject{private Vector<Observers> vector = new Vector<>();public void add(Observers ob){vector.add(ob);}public void delete(Observers ob){vector.remove(ob);}public void notifyObservers(){Enumeration<Observers> enumo = vector.elements();while(enumo.hasMoreElements()){enumo.nextElement().update();}}}class MySubject extends AbstractSubject{public void operation(){System.out.println("update self");notifyObservers();}}public class Observer {public static void main(String[] args){Subject sub = new MySubject();sub.add(new Observer1());sub.add(new Observer2());sub.operation();}}

（十六）迭代器模式

说到迭代器首先想到 Iterator ，使用迭代器来为顺序访问集合元素提供一种方式。下面是自己的一个集合，当然只能放 int ，简单的实现了 迭代器的功能，当然实际的Java迭代器比这要复杂很多。

interface Collection{public Iterator iterator();public Object get(int i);public int size();public void add(int n);}interface Iterator{public Object previous();public Object next();public boolean hasNext();public Object first();}class MyCollection implements Collection{private int count = 10;private int[] num = new int[count];public void add(int n){for(int i = 0;i < num.length;i++){if(num[i] == 0){num[i] = n;return;}}}public MyCollection(int count){this.count = count;}@Overridepublic Iterator iterator() {return new MyIterator(this);}@Overridepublic Object get(int i) {return num[i];}@Overridepublic int size() {return num.length;}}class MyIterator implements Iterator{private Collection collection;private int pos = -1;public MyIterator(Collection collection){this.collection = collection;}@Overridepublic Object previous() {if(pos > 0){pos--;}return collection.get(pos);}@Overridepublic Object next() {if(pos < collection.size() - 1){pos ++;}return collection.get(pos);}@Overridepublic boolean hasNext() {if(pos < collection.size() - 1){return true;}else{return false;}}@Overridepublic Object first() {pos = 0;return collection.get(pos);}}public class IteratorTest {public static void main(String[] args){Collection col = new MyCollection(10);for(int i = 0 ; i < 10;i++){col.add(i * 23);}Iterator it = col.iterator();while(it.hasNext()){System.out.println(it.next());}}}