行为型模式：状态模式

定义一个自行车类，自行车可以通过变挡来提速（变速车，假设一共三档）

public class Bike {
    GearState gearState;
    public Bike() {
        gearState = new FirstGear(this);
    }

    // 提速
    public void gearUp(){
        gearState.gearUp();
    }
    // 降速
    public void gearDown(){
        gearState.gearDown();
    }
}

定义一个挡位提速和减速的状态抽象类

public abstract class GearState {

    Bike bike;
    public GearState(Bike bike) {
        this.bike = bike;
    }

    public abstract void gearUp();
    public abstract void gearDown();

}

下面是三个挡位的实现：

public class FirstGear extends GearState{

    public FirstGear(Bike bike) {
        super(bike);
    }

    @Override
    public void gearUp() {
        System.out.println("提速：1---->2");
        bike.gearState = new SecondGear(bike);
    }

    @Override
    public void gearDown() {
        System.out.println("挡速不能再低了！");
    }
}

public class SecondGear extends GearState{
    public SecondGear(Bike bike) {
        super(bike);
    }

    @Override
    public void gearUp() {
        System.out.println("提速：2---->3");
        bike.gearState = new ThirdGear(bike);
    }

    @Override
    public void gearDown() {
        System.out.println("降速：2---->1");
        bike.gearState = new FirstGear(bike);
    }
}

public class ThirdGear extends GearState{
    public ThirdGear(Bike bike) {
        super(bike);
    }

    @Override
    public void gearUp() {
        System.out.println("挡速不能再高了！最高3挡！");
    }

    @Override
    public void gearDown() {
        System.out.println("降速：3---->2");
        bike.gearState = new SecondGear(bike);
    }
}

新建一个客户端类测试

public class Client {
    public static void main(String[] args) {
        Bike bike = new Bike();
        //bike.gearDown();
        bike.gearUp();
        bike.gearUp();
        bike.gearUp();
        bike.gearUp();
        bike.gearDown();
        bike.gearDown();
        bike.gearDown();
    }
}

打印结果：

提速：1---->2
提速：2---->3
挡速不能再高了！最高3挡！
挡速不能再高了！最高3挡！
降速：3---->2
降速：2---->1
挡速不能再低了！