Java状态设计模式实现对象状态转换的优雅方式

Java状态设计模式是一种处理对象状态转换的优雅方式。在这种方法中，对象的状态转换完全是由状态本身以及状态之间的转换规则驱动的，这使得代码更为清晰、易于维护和扩展。

以下是实现对象状态转换的完整攻略：

1. 定义状态和状态转换规则

首先，需要定义状态和状态转换的规则，然后将它们封装成一个状态机对象。状态机应该具有进入某个状态的方法，以及从一个状态到另一个状态的转换方法。例如：

interface State {
    void enter();
    void exit();
}

class StateA implements State {
    public void enter() {
        // 进入状态 A 的逻辑
    }

    public void exit() {
        // 离开状态 A 的逻辑
    }
}

class StateB implements State {
    public void enter() {
        // 进入状态 B 的逻辑
    }

    public void exit() {
        // 离开状态 B 的逻辑
    }
}

interface StateTransition {
    boolean canTransit(State from, State to);
}

class StateMachine {
    private State currentState;
    private Map<State, List<State>> transitionMap = new HashMap<>();
    private List<StateTransition> transitions = new ArrayList<>();

    public StateMachine(State initialState) {
        currentState = initialState;
    }

    public void addTransition(StateTransition transition) {
        transitions.add(transition);
    }

    public void addStateTransition(State from, State to) {
        List<State> toStates = transitionMap.get(from);
        if (toStates == null) {
            toStates = new ArrayList<State>();
            transitionMap.put(from, toStates);
        }
        toStates.add(to);
    }

    public void start() {
        currentState.enter();
    }

    public void transitionTo(State newState) {
        if (newState == currentState) {
            return;
        }

        for (StateTransition transition : transitions) {
            if (transition.canTransit(currentState, newState)) {
                currentState.exit();
                currentState = newState;
                currentState.enter();
                return;
            }
        }

        throw new RuntimeException("Illegal state transition from " + currentState + " to " + newState);
    }
}

2. 实现状态转换

在创建状态机对象后，需要设置状态转换规则。例如：

StateMachine machine = new StateMachine(new StateA());

machine.addStateTransition(new StateA(), new StateB());
machine.addStateTransition(new StateB(), new StateA());

machine.addTransition(new StateTransition() {
    public boolean canTransit(State from, State to) {
        return transitionMap.get(from).contains(to);
    }
});

3. 使用状态机

使用状态机时，首先需要创建一个状态机对象。然后，通过调用状态机的方法来完成状态转换。例如：

machine.start();   // 从状态 A 进入状态机
machine.transitionTo(new StateB());  // 从状态 A 转换到状态 B
machine.transitionTo(new StateA());  // 从状态 B 转换到状态 A

一个更具体的示例是一个订单状态变更的示例。

首先，我们需要定义订单的不同状态和状态间的转换规则，例如：

enum OrderState {
    CREATED,
    PAID,
    SHIPPED,
    DELIVERED,
    CANCELED,
    RETURNED
}

class Order {
    private OrderState state;
    // ...
}

interface OrderStateTransition {
    boolean canTransit(OrderState from, OrderState to);
}

class OrderStateMachine {
    private Map<OrderState, List<OrderState>> transitionMap = new HashMap<>();
    private List<OrderStateTransition> transitions = new ArrayList<>();

    public OrderStateMachine() {
        addStateTransition(OrderState.CREATED, OrderState.PAID);
        addStateTransition(OrderState.PAID, OrderState.SHIPPED);
        addStateTransition(OrderState.SHIPPED, OrderState.DELIVERED);
        addStateTransition(OrderState.CREATED, OrderState.CANCELED);
        addStateTransition(OrderState.PAID, OrderState.CANCELED);
        addStateTransition(OrderState.SHIPPED, OrderState.RETURNED);

        addTransition(new OrderStateTransition() {
            public boolean canTransit(OrderState from, OrderState to) {
                return transitionMap.get(from).contains(to);
            }
        });
    }

    public void addTransition(OrderStateTransition transition) {
        transitions.add(transition);
    }

    public void addStateTransition(OrderState from, OrderState to) {
        List<OrderState> toStates = transitionMap.get(from);
        if (toStates == null) {
            toStates = new ArrayList<>();
            transitionMap.put(from, toStates);
        }
        toStates.add(to);
    }

    public void transit(Order order, OrderState toState) {
        OrderState fromState = order.getState();
        if (fromState == toState) {
            return;
        }

        for (OrderStateTransition transition : transitions) {
            if (transition.canTransit(fromState, toState)) {
                order.setState(toState);
                return;
            }
        }

        throw new RuntimeException("Illegal state transition from " + fromState + " to " + toState);
    }
}

然后，我们将订单状态机应用到订单状态的变化中。例如：

Order order = new Order();
OrderStateMachine stateMachine = new OrderStateMachine();

stateMachine.transit(order, OrderState.PAID);
stateMachine.transit(order, OrderState.SHIPPED);
stateMachine.transit(order, OrderState.DELIVERED);

以上就是精简的Java状态设计模式实现对象状态转换的优雅方式的完整攻略，通过多条示例的讲解，希望能帮助到你。