java - 更改状态时添加附加内容的正确方法是什么

Question

我有一个这样的类(class):

public class Machine {
    enum State {
        STARTED, STOPPED
    }

    private State state;
    private String whyIsItStopped; // only used in STOPPED state

    ...
}

我想知道执行此操作的正确方法是什么。我有一个更复杂的类，如果我这样做，它似乎会变得一团糟。

问题出在我的架构上，你能给我一些建议吗？

更新

使用状态模式，我得到了一个几乎像这样的解决方案:

public interface State {

    public String whyIsItStopped();

    public class Started implements State {
        @Override
        public String whyIsItStopped() {
            return null;
        }
    }

    public class Stopped implements State {
        private final String reason;

        public Stopped(String reason) {
            this.reason = reason;
        }

        @Override
        public String whyIsItStopped() {
            return reason;
        }
    }
}

public class Machine {

    private State state = new State.Started();

    public String whyIsItStopped(){
        state.whyIsItStopped();
    }

    // setState etc ...
}

状态模式对于改变同一方法的行为似乎很好，但对于附加字段/数据来说很奇怪。

问题是它并不是真正的同一个对象，多态性并不能真正发挥作用。我必须在获取其内容之前测试机器是否已停止(作为实例的方式)

if (machine.isStopped()){
     println(machine.whyIsItStopped());
}

另一种方法可能是仅在 Stopped 状态下设置 getter

public interface State {

    public class Started implements State {}

    public class Stopped implements State {
        private final String reason;

        public String whyIsItStopped() {
            return reason;
        }
    }
}

并检索消息:

if (machine.isStopped()){
    println(((State.Stopped)machine.getState()).whyIsItStopped());
}
// Yeah that's not really beautiful

无论是在架构上还是在使用上都很奇怪。

你还有比instanceof更好的其他解决方案吗？

Answer 1

您的问题似乎是 State Pattern 的明显例子。它将允许您实现一个可以使用新状态进行扩展的解决方案，而无需修改现有代码。

The state pattern is a behavioral software design pattern that implements a state machine in an object-oriented way. With the state pattern, a state machine is implemented by implementing each individual state as a derived class of the state pattern interface, and implementing state transitions by invoking methods defined by the pattern's superclass.