java设计模式之状态模式

梦里梦外; 2022-12-15 02:19 121阅读 0赞

正文在下面，先打个广告：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZneWlidXBp_size_16_color_FFFFFF_t_70]

# 状态模式的定义与特点 #

对有状态的对象，把复杂的“判断逻辑”提取到不同的状态对象中，允许状态对象在其内部状态发生改变时改变其行为。

# 模式的结构 #

状态模式包含以下主要角色。

*  环境（Context）角色：也称为上下文，它定义了客户感兴趣的接口，维护一个当前状态，并将与状态相关的操作委托给当前状态对象来处理。
 *  抽象状态（State）角色：定义一个接口，用以封装环境对象中的特定状态所对应的行为。
 *  具体状态（Concrete State）角色：实现抽象状态所对应的行为。

典型类图为：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZneWlidXBp_size_16_color_FFFFFF_t_70_pic_center]

# demo #

以学生成绩为例，每次考完试，都根据成绩修改状态。  
0-59：low  
60-79: middle  
80-100: heigh

状态类：

public interface State { 
    
        void setScore(int score);
    
        String getName();
    
        int getScore();
    }

三个状态：

public class Low implements State { 
    
        private int score;
        private ScoreContext context;
    
        public Low(int score, ScoreContext context) { 
            this.score = score;
            this.context = context;
            context.setState(this);
        }
    
        @Override
        public void setScore(int score) { 
            StateUtil.modifyState(score, context);
            System.out.println("current score:" + score + " State: " + context.getState().getName());
        }
    
        @Override
        public String getName() { 
            return "low";
        }
    
        @Override
        public int getScore() { 
            return score;
        }
    }
    public class Middle implements State { 
    
        private int score;
        private ScoreContext context;
    
        public Middle(int score, ScoreContext context) { 
            this.score=score;
            this.context = context;
            context.setState(this);
        }
    
        @Override
        public void setScore(int score) { 
            StateUtil.modifyState(score, context);
            System.out.println("current score:" + score + " State: " + context.getState().getName());
        }
    
        @Override
        public String getName() { 
            return "Middle";
        }
        @Override
        public int getScore() { 
            return score;
        }
    }
    public class High implements State { 
    
        private int score;
        private ScoreContext context;
    
        public High(int score, ScoreContext context) { 
            this.score=score;
            this.context = context;
            context.setState(this);
        }
    
        @Override
        public void setScore(int score) { 
            StateUtil.modifyState(score, context);
            System.out.println("current score:" + score + " State: " + context.getState().getName());
        }
    
        @Override
        public String getName() { 
            return "High";
        }
    
    
        @Override
        public int getScore() { 
            return score;
        }
    }

状态转换工具类

public class StateUtil { 
        public static void modifyState(int score, ScoreContext context) { 
            if (score >= 60 && score < 80) { 
                new Middle(score, context);
            } else if (score >= 80) { 
                new High(score, context);
            } else { 
                new Low(score, context);
            }
        }
    }

成绩上下文类

public class ScoreContext { 
    
        private State state;
    
        public ScoreContext() { 
            new Low(0, this);
        }
    
        public State getState() { 
            return state;
        }
    
        public void setState(State state) { 
            this.state = state;
        }
    
        public void setScore(int score) { 
            state.setScore(score);
        }
    
    }

测试方法：

public class Main { 
        public static void main(String[] args) { 
            ScoreContext context = new ScoreContext();
    
            context.setScore(20);
            context.setScore(70);
            System.out.println("=======" + context.getState().getScore());
            context.setScore(90);
            context.setScore(20);
    
        }
    }

输出：

current score:20 State: low
    current score:70 State: Middle
    =======70
    current score:90 State: High
    current score:20 State: low

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