Java 中状态模式中的损坏封装

Question

我已经在 Java 中为我的套接字库实现了状态模式。我简化代码以减小大小并提高可读性只是为了演示问题。我不知道如何封装状态 classes 对 Socket class 的使用。我写了评论以更好地理解问题。所以，有一个示例代码：

package com.test;

enum ConnectionAttemptResult {
    ConnectionAttemptStarted,
    AlreadyInProgress,
    AlreadyConnected,
    UnableToCreateSocket,
    UnableToConnectSocket,
    UnableToCreateReadEvent
}

class SocketAddress {
    static SocketAddress LocalhostPort(int port) {
        // Code removed.
        return new SocketAddress();
    }
}

class Socket {
    private State currentState;

    Socket() {
        this.currentState = new NotActiveState(this);
    }

    public ConnectionAttemptResult Connect(SocketAddress remoteAddress) {
        return this.currentState.Connect(remoteAddress);
    }

    // Socket class is external and will be used by user.
    // So further methods must be private because they are internal.
    // Calling these methods by a user may broke class functionality.
    // But I can't make them private because then states couldn't access them.
    public void SwitchState(State newState) {
        this.currentState = newState;
    }

    // I removed a lot of code of this class to simplify the code.
    public boolean CreateSocket() {
        // Code removed.
        return true;
    }

    public boolean ConnectSocket(SocketAddress remoteAddress) {
        // Code removed.
        return true;
    }

    public boolean CreateReadEvent() {
        // Code removed.
        return true;
    }
}

abstract class State {
    protected Socket socket;

    State(Socket socket) {
        this.socket = socket;
    }

    public abstract ConnectionAttemptResult Connect(SocketAddress remoteAddress);
}

class NotActiveState extends State {
    NotActiveState(Socket socket) {
        super(socket);
    }

    @Override
    public ConnectionAttemptResult Connect(SocketAddress remoteAddress) {
        if (!socket.CreateSocket())
            return ConnectionAttemptResult.UnableToCreateSocket;

        if (!socket.ConnectSocket(remoteAddress))
            return ConnectionAttemptResult.UnableToConnectSocket;

        if (!socket.CreateReadEvent())
            return ConnectionAttemptResult.UnableToCreateReadEvent;

        socket.SwitchState(new ConnectingState(socket));
        return ConnectionAttemptResult.ConnectionAttemptStarted;
    }
}

class ConnectingState extends State {
    ConnectingState(Socket socket) {
        super(socket);
    }

    @Override
    public ConnectionAttemptResult Connect(SocketAddress remoteAddress) {
        return ConnectionAttemptResult.AlreadyInProgress;
    }
}

class ConnectedState extends State {
    ConnectedState(Socket socket) {
        super(socket);
    }

    @Override
    public ConnectionAttemptResult Connect(SocketAddress remoteAddress) {
        return ConnectionAttemptResult.AlreadyConnected;
    }
}

class Main {
    public static void main(String[] args) {
        Socket socket = new Socket();
        socket.Connect(SocketAddress.LocalhostPort(9898));

        // But I also can call internal methods and break the class functionality, because now they are public.
        socket.CreateSocket();
        socket.CreateReadEvent();
    }
}

我的错误在哪里？开发人员通常如何实现这种模式保存封装？希望得到您的帮助！提前致谢！

Answer 1

好吧，你可以让状态 classes 嵌套私有 classes 并且它们可以访问包含 class 的内部结构。正如我们所见，TransitiveState 可以访问 SomethingWithState#previousState 私有变量。我认为静态嵌套 classes 也可以访问封闭 class 的私有成员，但您必须将 SomethingWithState 实例传递给它们。

状态是一个实现细节，因此它们不会也不应该对外公开。

例如

public class StatePatternExample {
    public static void main(String[] args) {
        SomethingWithState something = new SomethingWithState();
        something.next();
        something.next();
    }
    
    private static class SomethingWithState {
        private State state;
        private State previousState;
        
        SomethingWithState() {
            this.previousState = null;
            this.state = new InitialState();
        }
        
        void next() {
            State next = state.next();
            System.out.println(state.name() + " --> " + next.name());
            previousState = state;
            state = next;
        }
        
        private interface State {
            State next();
            String name();
        }
        
        private class InitialState implements State {
            public State next() {
                
                return new TransitiveState();
            }
            public String name() { return "Initial"; }
        }
        
        private class TransitiveState implements State {
            public State next() {
                System.out.println("Access private previousState from TransitiveState: was " + previousState.name());
                
                return new FinalState();
            }
            public String name() { return "Transitive"; }
        }
        
        private class FinalState implements State {
            public State next() {
                throw new IllegalStateException("Already in final state.");
            }
            public String name() { return "Final"; }
        }
    }
}

Java 中状态模式中的损坏封装

Broken encapsulation in State pattern in Java

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oop

encapsulation

design-patterns

state-pattern