序

事物的发展总是伴随着状态的转换，犹如人会经历婴幼儿到青壮年。四季一年，有春的盎然，夏的炽烈，秋的多彩，冬的洁白。第三篇就从这可爱的四季开始吧。

状态模式的核心：允许对象在内部状态发生改变时改变它的行为，对象看起来好像修改了它的类。

优点：封装了转换规则。缺点：状态类之间的耦合性比较强。

实现

上下文管理类

public class Context 
{    
    private State curState;    
    public  void process(int month) 
    {        
        this.curState.handle(month, this);    
    }    
    public void setState(State newState)    
    {        
        this.curState = newState;    
    }
}

状态接口类

public interface State 
{    
    public void handle(int age, Context ctx);
}

具体实现类

春

public class Spring implements State
{    
    @Override    public void handle(int month, Context ctx) 
    {        
        System.out.println("Spring: Green");        
        if(month >= SeasonDefine.SUMMER_BEGIN_MONTH - 1)        
        {            
            System.out.println("Change State: Spring -> Summer, Sweet Flower and SunShine");            
            ctx.setState(new Summer());        
        }    
    }
}

夏

public class Summer implements State{    
    @Override    
    public void handle(int month, Context ctx) 
    {        
        System.out.println("Summer: Red");        
        if(month >= SeasonDefine.AUTUMN_BEGIN_MONTH - 1)        
        {            
            System.out.println("Change State: Summer -> Autumn, Bright Colors");                     ctx.setState(new Autumn());        
        }    
    }
}

秋

public class Autumn implements State
{    
    @Override    
    public void handle(int month, Context ctx) 
    {        
        System.out.println("autumn, yellow");        
        if(month >= SeasonDefine.WINNER_BEGIN_MONTH - 1)        
        {            
            System.out.println("Change State: Autumn -> Winner, magic snow world");                   ctx.setState(new Winner());        
        }    
    }
}

冬

public class Winner implements State 
{    
    @Override    
    public void handle(int month, Context ctx) 
    {        
        System.out.println("Winner, White");        
        if(month >= SeasonDefine.WINNER_OVER_MONTH) 
        {            
            System.out.println("Change State: Winnder -> Spring, Happy New Year");                   ctx.setState(new Spring());        
        }    
    }
}

测试

public class StatePatternDemo 
{    
    public static void main(String[] args) 
    {        
        Context ctx = new Context();        
        ctx.setState(new Spring());        
        for(int month = 1; month <= 12; month++)        
        {            
            System.out.println("month : " + month);            
            ctx.process(month);      
        }    
    }
}

附

public class SeasonDefine 
{    
    public static final int SPRING_BEGIN_MONTH = 1;    
    public static final int SUMMER_BEGIN_MONTH = 4;    
    public static final int AUTUMN_BEGIN_MONTH = 7;    
    public static final  int WINNER_BEGIN_MONTH = 10;    
    public static final  int WINNER_OVER_MONTH = 12;
}