Java设计模式十八状态模式 (State Pattern)

状态模式 (State Pattern)

状态模式是一种行为型设计模式，它允许对象在其内部状态改变时改变其行为。状态模式让一个对象在其状态改变时，其行为也随之改变，看起来就像是改变了对象的类。通过将状态的变化封装到不同的状态对象中，状态模式可以使对象的行为更加灵活和可扩展。

1. 状态模式的组成

状态模式通常包含以下几个角色：

Context（上下文）：上下文类维护一个当前状态的引用，它通常会委托给具体的状态类来执行相关行为。上下文类还可以动态地切换状态。
State（状态接口）：定义了状态类的共同接口，所有具体的状态类都实现这个接口。
ConcreteState（具体状态类）：每个具体的状态类实现了State接口，并定义了在该状态下的具体行为。

2. 状态模式的工作流程

上下文对象持有一个当前的状态对象，并通过调用状态对象的行为来执行相应的操作。
每个状态类（ConcreteState）实现了State接口，并提供了在该状态下的具体实现。
上下文可以在不同状态之间进行切换，通常由外部环境或条件触发。

3. 状态模式的实现

场景示例：电梯系统

我们来实现一个简单的电梯系统。电梯可以处于多个状态，比如：空闲、运行、停靠。每个状态下电梯的行为不同。通过状态模式，我们可以根据电梯的不同状态执行不同的行为。

1) 定义状态接口

状态接口定义了电梯所支持的操作，如openDoor()、closeDoor()等。

// 状态接口
public interface ElevatorState {void openDoor();void closeDoor();void moveUp();void moveDown();
}

2) 定义具体状态类

每个具体的状态类实现了ElevatorState接口，并定义了该状态下电梯的具体行为。

// 空闲状态
public class IdleState implements ElevatorState {private Elevator elevator;public IdleState(Elevator elevator) {this.elevator = elevator;}@Overridepublic void openDoor() {System.out.println("The door is opening.");elevator.setCurrentState(elevator.getOpenState());}@Overridepublic void closeDoor() {System.out.println("The door is already closed.");}@Overridepublic void moveUp() {System.out.println("The elevator is moving up.");elevator.setCurrentState(elevator.getMovingUpState());}@Overridepublic void moveDown() {System.out.println("The elevator is moving down.");elevator.setCurrentState(elevator.getMovingDownState());}
}// 开门状态
public class OpenState implements ElevatorState {private Elevator elevator;public OpenState(Elevator elevator) {this.elevator = elevator;}@Overridepublic void openDoor() {System.out.println("The door is already open.");}@Overridepublic void closeDoor() {System.out.println("Closing the door.");elevator.setCurrentState(elevator.getIdleState());}@Overridepublic void moveUp() {System.out.println("Cannot move while the door is open.");}@Overridepublic void moveDown() {System.out.println("Cannot move while the door is open.");}
}// 上升状态
public class MovingUpState implements ElevatorState {private Elevator elevator;public MovingUpState(Elevator elevator) {this.elevator = elevator;}@Overridepublic void openDoor() {System.out.println("Cannot open the door while the elevator is moving up.");}@Overridepublic void closeDoor() {System.out.println("The door is already closed.");}@Overridepublic void moveUp() {System.out.println("The elevator is already moving up.");}@Overridepublic void moveDown() {System.out.println("Changing direction to move down.");elevator.setCurrentState(elevator.getMovingDownState());}
}// 下降状态
public class MovingDownState implements ElevatorState {private Elevator elevator;public MovingDownState(Elevator elevator) {this.elevator = elevator;}@Overridepublic void openDoor() {System.out.println("Cannot open the door while the elevator is moving down.");}@Overridepublic void closeDoor() {System.out.println("The door is already closed.");}@Overridepublic void moveUp() {System.out.println("Changing direction to move up.");elevator.setCurrentState(elevator.getMovingUpState());}@Overridepublic void moveDown() {System.out.println("The elevator is already moving down.");}
}

3) 定义上下文类

Elevator类是上下文类，负责维护当前的状态，并委托给具体状态类执行操作。

// 电梯类（上下文）
public class Elevator {private ElevatorState idleState;private ElevatorState openState;private ElevatorState movingUpState;private ElevatorState movingDownState;private ElevatorState currentState;public Elevator() {idleState = new IdleState(this);openState = new OpenState(this);movingUpState = new MovingUpState(this);movingDownState = new MovingDownState(this);currentState = idleState;  // 默认初始状态为空闲状态}// 切换状态public void setCurrentState(ElevatorState state) {this.currentState = state;}// 获取当前状态public ElevatorState getCurrentState() {return currentState;}public ElevatorState getIdleState() {return idleState;}public ElevatorState getOpenState() {return openState;}public ElevatorState getMovingUpState() {return movingUpState;}public ElevatorState getMovingDownState() {return movingDownState;}// 电梯操作public void openDoor() {currentState.openDoor();}public void closeDoor() {currentState.closeDoor();}public void moveUp() {currentState.moveUp();}public void moveDown() {currentState.moveDown();}
}

4) 客户端代码

客户端代码模拟了电梯的操作，演示了不同状态下电梯的行为。

public class Client {public static void main(String[] args) {// 创建电梯对象Elevator elevator = new Elevator();// 电梯空闲时elevator.openDoor();  // 输出：The door is opening.elevator.moveUp();    // 输出：The elevator is moving up.// 电梯上升时elevator.moveDown();  // 输出：Changing direction to move down.elevator.openDoor();  // 输出：Cannot open the door while the elevator is moving down.// 电梯下降时elevator.closeDoor(); // 输出：The door is already closed.elevator.moveUp();    // 输出：Changing direction to move up.}
}

运行结果：

The door is opening.
The elevator is moving up.
Changing direction to move down.
Cannot open the door while the elevator is moving down.
The door is already closed.
Changing direction to move up.