状态模式：允许对象在内部状态改变时改变它的行为，对象看起来好像修改了它的类。以下是例子：
public interface Weather {
String getWeather(); }
public class Rain implements Weather {
public String getWeather() { return "下雨"; } }
public class Sunshine implements Weather {
public String getWeather() { return "阳光"; } }
public class Context {
private Weather weather;
public void setWeather(Weather weather){
this.weather = weather; }
public Weather getWeather(){
return weather; }
public String weatherMessage(){
return weather.getWeather(); } }
//测试类
public class Test {
public static void main(String[] args) {
Context ctx1 = new Context();
ctx1.setWeather(new Sunshine());
System.out.println(ctx1.weatherMessage());
System.out.println("==============");
Context ctx2 = new Context();
ctx2.setWeather(new Rain());
System.out.println(ctx2.weatherMessage()); } }