代码如下：

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in);
		BotContext bot = new BotContext();
		for (;;) {
			System.out.print("> ");
			String input = scanner.nextLine();
			String output = bot.chat(input);
			System.out.println(output.isEmpty() ? "(no reply)" : "< " + output);
		}
	}
}


class BotContext {
	private State state = new DisconnectedState();
	private boolean busy = false;
	
// 	定义工作线程时，记得在线程结束时将busy设置成false
//	private Thread work;
	
	public void setState(State state) {
		this.state = state;
	}
	
	public boolean isBusy() {
		return busy;
	}
	
	public void setBusy(boolean arg) {
		busy = arg;
	}

	public String chat(String input) {
		return state.reply(input, this);
	}
}


interface State {
	String init();
	String reply(String input,BotContext bot);
}


class DisconnectedState implements State {
	@Override
	public String init() {
		return "Bye!";
	}

	@Override
	public String reply(String input, BotContext bot) {
		if ("hello".equalsIgnoreCase(input)) {
			State nextState = new ConnectedState();
			bot.setState(nextState);
			return nextState.init();
		} else {
			return "";
		}
	}
}


class ConnectedState implements State {
	@Override
	public String init() {
		return "Hello, I'm Bob.";
	}

	@Override
	public String reply(String input, BotContext bot) {
		if ("bye".equalsIgnoreCase(input)) {
			State nextState = new DisconnectedState();
			bot.setState(nextState);
			return nextState.init();
		} else if ("Go to work".equalsIgnoreCase(input)) {
			// 可在此以多线程方式开启任务，并将BotContext保存到Work中以便在线程结束时切换状态
			// Thread t = new Work(bot); t.start(); bot.setWork(t);
			bot.setBusy(true);
			State nextState = new BusyState();
			bot.setState(nextState);
			return nextState.init();
		}
		else {
			if (input.endsWith("?")) {
				return "Yes. " + input.substring(0, input.length() - 1) + "!";
			}
			if (input.endsWith(".")) {
				return input.substring(0, input.length() - 1) + "!";
			}
			return input.substring(0, input.length() - 1) + "?";
		}
	}
}


class BusyState implements State {
	@Override
	public String init() {
		return "Yes Sir. I'm working...";
	}

	@Override
	public String reply(String input, BotContext bot) {
		if ("Stop".equalsIgnoreCase(input)) {
			// 可在此中断线程，强制切换回在线状态
			// bot.getWork().interrupt();  
			bot.setBusy(false);
			bot.setState(new ConnectedState());
			return "Ok, I have stopped working.";
		} else if (!bot.isBusy()) {
			// 工作正常结束时，可通过此分支切换回在线状态并回复消息
			State nextState = new ConnectedState();
			bot.setState(nextState);
			return nextState.reply(input, bot);
		} else {
			return "Sorry, I'm busy now!";
		}
	}
}