状态模式

定义：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。

通俗的讲，状态模式主要解决当控制一个对象状态转换的条件表达式（判断过多）过于复杂时的情况。把状态的判断逻辑转移到表示不同状态的一系列类当中，可以把复杂的判断逻辑简化。

抽象状态类：

public abstract class State {
    public abstract void Handle(Context context);
}

具体状态类A：

public class ConcreteStateA extends State {
    @Override
    public void Handle(Context context) {
        context.state = new ConcreteStateB();
    }
}

具体状态类B：

public class ConcreteStateB extends State {
    @Override
    public void Handle(Context context) {
        context.state = new ConcreteStateA();
    }
}

Context类，维护一个ConcreteState子类的实例，这个实例定义当前的状态

public class Context {
    public State state;

    public Context(State state){
        this.state = state;
    }

    public void request(){
        state.Handle(this);
    }
}

Main：

public class Main {
    public static void main(String[] args) {
        Context context1 = new Context(new ConcreteStateA());
        context1.request();
    }

}

通过状态模式可以将与特定状态相关的行为局部化，并且将不同状态的行为分割开来。将特定状态相关的行为放在一个对象中，可以消除庞大的条件分支语句。

 

以工作加班为例来表示状态模式的运用

 

public abstract class State {
    public abstract void writeProgram(Work w);
}

//上午工作状态
public class ForenoonState extends State {
    @Override
    public void writeProgram(Work w) {
        if(w.hour < 12){
            System.out.println("当前时间:"+w.hour);
        }else {
            w.setState(new NoonState());
            w.writeProgram();
        }
    }
}

//中午工作状态
public class NoonState extends State {
    @Override
    public void writeProgram(Work w) {
        if(w.hour < 13){
            System.out.println("当前时间:"+w.hour);
        }else {
            w.setState(new AfternoonState());
            w.writeProgram();
        }
    }
}

public class AfternoonState extends State {
    @Override
    public void writeProgram(Work w) {
        if(w.hour < 17){
            System.out.println("当前时间:"+w.hour);
        }else {
            w.setState(new EveningState());
            w.writeProgram();
        }
    }
}

public class EveningState extends State {
    @Override
    public void writeProgram(Work w) {
        if(w.taskFinished){
            w.setState(new RestState());
            w.writeProgram();
        }else {
            if (w.hour<21){
                System.out.println("当前时间:"+w.hour);
            }else {
                w.setState(new SleepingState());
                w.writeProgram();
            }
        }
    }
}

public class SleepingState extends State {
    @Override
    public void writeProgram(Work w) {
        System.out.println("当前时间"+w.hour);
    }
}

public class RestState extends State {
    @Override
    public void writeProgram(Work w) {
        System.out.println("当前时间："+w.hour);
    }
}

public class Work {
    private State current;
    public Work(){
        current = new ForenoonState();
    }
    public double hour;

    public boolean finish = false;

    public boolean taskFinished;

    public void setState(State s){
        current = s;
    }

    public void writeProgram(){
        current.writeProgram(this);
    }

}

public class Main {
    public static void main(String[] args) {
        Work work = new Work();
        work.hour = 9;
        work.writeProgram();
        work.hour = 10;
        work.writeProgram();
        work.hour = 12;
        work.writeProgram();
        work.hour = 13;
        work.writeProgram();
        work.hour = 14;
        work.writeProgram();
        work.hour = 17;
        work.writeProgram();

        work.taskFinished = false;

        work.hour = 19;
        work.writeProgram();
        work.hour = 22;
        work.writeProgram();
    }
}