状态模式简介及使用示例

状态模式简介

状态模式(State Pattern)：允许一个对象在内部状态改变时改变行为，看起来似乎修改了类。

类图：
通过将状态封装成类，将状态流转和不同状态下的行为模式独立出来实现解耦，但同时产生了大量的类，一般状态类不应超过5个。

状态模式使用示例

public class Context {
    private State state;

    public Context() {
        State01 state = new State01();
        state.setContext(this);
        this.state = state;
    }

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

    public void back(){
        this.state.back();
    }

    public void fore(){
        this.state.fore();
    }
}

public abstract class State {
    private Context context;

    public void setContext(Context context) {
        this.context = context;
    }
    public Context getContext() {
        return context;
    }

    public abstract void back();
    public abstract void fore();
}

public class State01 extends State {
    @Override
    public void back() {
        System.out.println("State01 back");
        System.out.println("State01 --> State03");
        State03 state03 = new State03();
        state03.setContext(super.getContext());
        super.getContext().setState(state03);
    }

    @Override
    public void fore() {
        System.out.println("State01 fore");
        System.out.println("State01 --> State02");
        State02 state02 = new State02();
        state02.setContext(super.getContext());
        super.getContext().setState(state02);
    }
}

public class State02 extends State {
    @Override
    public void back() {
        System.out.println("State02 back");
        System.out.println("State02 --> State01");
        State01 state01 = new State01();
        state01.setContext(super.getContext());
        super.getContext().setState(state01);
    }

    @Override
    public void fore() {
        System.out.println("State02 fore");
        System.out.println("State02 --> State03");
        State03 state03 = new State03();
        state03.setContext(super.getContext());
        super.getContext().setState(state03);
    }
}

public class State03 extends State {
    @Override
    public void back() {
        System.out.println("State03 back");
        System.out.println("State03 --> state01");
        State02 state02 = new State02();
        state02.setContext(super.getContext());
        super.getContext().setState(state02);
    }

    @Override
    public void fore() {
        System.out.println("State03 fore");
        System.out.println("State03 --> State01");
        State01 state01 = new State01();
        state01.setContext(super.getContext());
        super.getContext().setState(state01);
    }
}

public class Client {
    public static void main(String[] args) {
        Context context = new Context();
        context.back();
        System.out.println("=======");
        context.fore();
        System.out.println("=======");
        context.fore();
        System.out.println("=======");
        context.back();
    }
}

结果输出：

State01 back
State01 --> State03
=======
State03 fore
State03 --> State01
=======
State01 fore
State01 --> State02
=======
State02 back
State02 --> State01

扩展学习资料

其他设计模式