设计模式- 状态模式

Posted 2020-11-17 levcon

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状态模式

当一个对象的内在状态改变时允许改变其行为，这个对象看起来像是改变了其类。

在平常开发自己也会遇到好多方法过长，里面的判断语句太多，导致后续修改十分麻烦。今天看到状态模式的介绍，觉得很受益。下面通过一段代码错误的显示一遍没有使用状态模式的代码

public class Work {
    private int hour;
    private boolean finished;
    private boolean taskFinished;
    
    public void workProgram() {
        if (hour < 12) {
            System.out.println("current time: "+new Date() + "  working in the morning");
        } else if (hour < 13) {
            System.out.println("current time: "+new Date() + "  have a break!");
        } else if (hour < 17) {
            System.out.println("current time: "+new Date() + "  working in the afternoon");
        } else {
            if (finished) {
                System.out.println("current time: "+new Date() + " after work!");
            } else if (hour <12) {
                System.out.println("current time: "+new Date() + "  very hard!");
            }
        }
    }
    
}
从上述代码中看出，当我要修改hour的判断范围，或者在各个判断条件之后新增一些功能，都会使这段方法变得十分长，而且阅读起来也比较繁琐。
所以代码过长了就有坏味道了。在面向对象的设计中也希望做到代码的责任分解。

当一个对象状态转换的条件表达式过于复杂的时候，把状态的判断逻辑移到不同的一系列类当中，可以把复杂的判断逻辑简单化，这就是状态模式的主要作用。但并不是所有有判断的都要使用状态模式，简单的判断逻辑就不需要了。
代码改良

/**
 * 工作类 没有了判断语句
 */
public class Work {
    private int hour;

    private boolean finished;

    private boolean taskFinished;

    private State state;

    private State currentState;

    public Work() {
        currentState = new ForenoonState();

    }

    public void workProgram() {
        currentState.workProgram(this);
    }

    public int getHour() {
        return hour;
    }

    public void setHour(int hour) {
        this.hour = hour;
    }

    public boolean isFinished() {
        return finished;
    }

    public void setFinished(boolean finished) {
        this.finished = finished;
    }

    public boolean isTaskFinished() {
        return taskFinished;
    }

    public void setTaskFinished(boolean taskFinished) {
        this.taskFinished = taskFinished;
    }

    public State getState() {
        return state;
    }

    public void setState(State state) {
        this.currentState = state;
    }
}

/**
 * morning
 */
public class ForenoonState extends State {
    public void workProgram(Work work) {
        if (work.getHour() < 12) {
            System.out.println("current time: "+new Date() + "  working in the morning");
        } else {
            work.setState(new NoonState());
        }
    }
}

/**
 * noon
 */
public class NoonState extends State {
    public void workProgram(Work work) {
        if (work.getHour() < 13) {
            System.out.println("current time: "+new Date() + "  have a break!");
        } else {
            work.setState(new AfternoonState());
        }
    }
}

/**
 * afternoon
 */
public class AfternoonState extends State {
    public void workProgram(Work work) {
        if (work.getHour() < 17) {
            System.out.println("current time: "+new Date() + "  working in the afternoon");
        } else {
            //todo .......
        }
    }
}

/**
 * Client
 */
public class Client {

    public static void main(String[] args) {
        Work work = new Work();
        work.setHour(12);
        work.workProgram();

        work.setHour(13);
        work.workProgram();
    }
}

从上术代码中，在客户端代码不需要做任何的修改，程序可以变得非常灵活，当程序还需要添加一个额外的工作状态时，再添加一个其它状态类的代码，也不影响其它状态类的代码。