（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;
}
@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());
}
}
}