观察者模式

观察者模式：定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态上发生变化时，会通知所有观察者对象，让他们能够自动更新自己 。以下是例子：

public abstract class Citizen {  

List pols;

String help="normal";

public void setHelp(String help){

this.help = help; }

public String getHelp(){

return this.help; }

public abstract void sendMessage(String help);

public void setPolicemen(){

this.pols = new ArrayList(); }

public void register(Policeman pol){

this.pols.add(pol); }

public void unRegister(Policeman pol){

this.pols.remove(pol); } }

 

public class TianHeCitizen extends Citizen {

public TianHeCitizen(Policeman pol){

//创建ArrayList对象

setPolicemen();

//把Policeman变量添加到ArrayList中

register(pol); }

public void sendMessage(String help) {

setHelp(help);

for(int i=0;i<pols.size();i++){

Policeman pol = (Policeman) pols.get(i);

//通知警察行动

pol.action(this); } } }

 

public class HuangPuCitizen extends Citizen{

public HuangPuCitizen(Policeman pol){

setPolicemen();

register(pol); }

public void sendMessage(String help) {

setHelp(help);

for(int i=0;i<pols.size();i++){

Policeman pol = (Policeman) pols.get(i);

//通知警察行动

pol.action(this); } } }

 

//担当观察者 

public interface Policeman {  

void action(Citizen ci); }

 

public class TianHePoliceman implements Policeman{

public void action(Citizen ci) {

String help = ci.getHelp();

if(help.equals("normal")){

System.out.println("一切正常，不用出动"); }

if(help.equals("unnormal")){

System.out.println("有犯罪行为，天河警察出动"); } } } 

 

public class HuangPuPoliceman implements Policeman{

public void action(Citizen ci) {

String help = ci.getHelp();

if(help.equals("normal")){

System.out.println("一切正常，不用出动"); }

if(help.equals("unnormal")){

System.out.println("有犯罪行为，黄埔警察出动"); } } }

 

//测试类 

public class Test {  

public static void main(String args[]){

Policeman thPol = new TianHePoliceman();

Policeman hpPol = new HuangPuPoliceman();

Citizen citizen = new HuangPuCitizen(hpPol);

citizen.sendMessage("unnormal");

citizen.sendMessage("normal");

System.out.println("============");

citizen = new TianHeCitizen(thPol);

citizen.sendMessage("normal");

citizen.sendMessage("unnormal"); } }