（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，全部依赖于它的对象都收到通知而且自己主动更新，观察者与被观察者分开。比如邮件订阅、RSS订阅，假设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
     
       vector = new Vector<>();	public void add(Observers ob){		vector.add(ob);	}	public void delete(Observers ob){		vector.remove(ob);	}	public void notifyObservers(){		Enumeration
      
        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訪问集合元素提供一种方式。以下是自己的一个集合，当然仅仅能放 int ，简单的实现了 迭代器的功能，当然实际的Java迭代器比这要复杂非常多。

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;	}	@Override	public Iterator iterator() {		return new MyIterator(this);	}	@Override	public Object get(int i) {		return num[i];	}	@Override	public int size() {		return num.length;	}}class MyIterator implements Iterator{	private Collection collection;	private int pos = -1;	public MyIterator(Collection collection){		this.collection = collection;	}	@Override	public Object previous() {		if(pos > 0){			pos--;		}		return collection.get(pos);	}	@Override	public Object next() {		if(pos < collection.size() - 1){			pos ++;		}		return collection.get(pos);	}	@Override	public boolean hasNext() {		if(pos < collection.size() - 1){			return true;		}else{			return false;		}	}	@Override	public 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());		}	}}