The State Pattern is a behavioral design pattern that allows an object to alter(改变) its behavior when its internal(内部) state changes. This pattern is used to encapsulate(封装) varying(变化) behavior for the same routine(例行程序) based on an object's state object. It's essentially(本质上) a way to model state transitions(过渡) in an object-oriented(面向对象) way.

Intent

The State Pattern is used to allow an object to change its behavior when its internal state changes. This pattern can be thought of as(be thought of as 被认为是) a state machine implemented through(通过) object-oriented principles(原则).

How it works

  • Context: The class that has a state that needs to change its behavior based on that state.
    上下文：需要根据状态改变行为的状态类。
  • State Interface: This defines a common interface for all concrete states. It usually declares methods that correspond to different behaviors.
    状态接口：它为所有具体状态定义了一个通用接口。它通常会声明与不同行为相对应的方法。
  • Concrete State Classes: Implementations of the State interface that perform specific behaviors associated with a state of the Context.
    具体状态类：状态接口的实现，可执行与上下文状态相关的特定行为。

Use Case Example

Consider a simple example of a document with states (Draft, Moderation, Published). The document's ability to be edited, approved, or published depends on its current state. Each state would have specific behavior for the same operations (e.g., an edit operation might be allowed in the Draft and Moderation states but not in the Published state).
举一个简单的例子，说明文档的状态（草稿、审核、发布）。文档能否被编辑、批准或发布取决于其当前状态。对于相同的操作，每种状态都有特定的行为（例如，在草稿和审核状态下可能允许编辑操作，但在发布状态下则不允许）。

Let's dive deeper with a simple example to illustrate the State Pattern in Java. We'll model a basic Traffic Light system where the traffic light can be in one of three states: Green, Yellow, or Red. Each state will change to the next one after some operation, mimicking the behavior of a real traffic light.
让我们通过一个简单的示例来深入说明 Java 中的状态模式。我们将模拟一个基本的交通灯系统，交通灯可以处于三种状态之一：绿灯、黄灯或红灯。每个状态都会在某些操作后切换到下一个状态，模仿真实交通灯的行为。

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State Interface

First, we define a TrafficLightState interface with a method changeLight(TrafficLightContext context) that all concrete states will implement to transition to the next state.
首先，我们定义了一个 TrafficLightState 接口和一个 changeLight(TrafficLightContext context) 方法，所有具体状态都将通过该方法过渡到下一个状态。

public interface TrafficLightState {
    void changeLight(TrafficLightContext context);
}

Concrete States

Next, we create concrete states implementing the TrafficLightState interface.
接下来，我们创建实现 TrafficLightState 接口的具体状态。

public class GreenState implements TrafficLightState {
    @Override
    public void changeLight(TrafficLightContext context) {
        System.out.println("Green light - Go!");
        context.setState(new YellowState());
    }
}

public class YellowState implements TrafficLightState {
    @Override
    public void changeLight(TrafficLightContext context) {
        System.out.println("Yellow light - Prepare to stop.");
        context.setState(new RedState());
    }
}

public class RedState implements TrafficLightState {
    @Override
    public void changeLight(TrafficLightContext context) {
        System.out.println("Red light - Stop.");
        context.setState(new GreenState());
    }
}

Context Class

The TrafficLightContext class holds a reference to the current state and allows it to be changed via the changeLight() method.
TrafficLightContext 类保存当前状态的引用，并允许通过 changeLight() 方法对其进行更改。

public class TrafficLightContext {
    private TrafficLightState state;

    public TrafficLightContext(TrafficLightState state) {
        this.state = state;
    }

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

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

Using the Pattern

Finally, we can use our state pattern to model the traffic light changing states.
最后，我们可以使用状态模式来模拟红绿灯的变化状态。

public class TrafficLightDemo {
    public static void main(String[] args) {
        TrafficLightContext context = new TrafficLightContext(new GreenState());

        for (int i = 0; i < 5; i++) {
            context.changeLight();
        }
    }
}

This example shows a simple usage of the State Pattern where the traffic light changes from green, to yellow, to red, and then back to green again. Each state knows what the next state should be and transitions the context (the TrafficLightContext class) to that next state.
本示例展示了状态模式的一个简单用法，即交通信号灯从绿色变为黄色，再变为红色，然后再次变回绿色。每个状态都知道下一个状态是什么，并将上下文（ TrafficLightContext 类）转换到下一个状态。

This pattern is useful for managing complex state transitions in a way that encapsulates the state-specific behaviors and transitions within state objects, making the system easier to understand and maintain.
这种模式有助于管理复杂的状态转换，将特定状态的行为和转换封装在状态对象中，使系统更容易理解和维护。

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