21.请用java修改Knock Knock 游戏项目设计，为游戏增加计分机制，让游戏过程变得富有挑战性。例如用ConcurrentHashMap实时保存所有客户机得分，并将最高得分的玩家姓名返回给客户机。

好的，我可以为您提供一个简单的修改方案，具体实现如下：

1. 在KnockKnockProtocol类中添加一个ConcurrentHashMap，用于实时保存所有客户机得分。

import java.util.concurrent.ConcurrentHashMap;

public class KnockKnockProtocol {
    private static final int MAX_SCORE = 10; // 最大得分
    private static ConcurrentHashMap<String, Integer> scores = new ConcurrentHashMap<String, Integer>(); // 保存所有客户机得分

    // ...

    public String processInput(String theInput) {
        String theOutput = null;

        if (state == WAITING) {
            theOutput = "Knock! Knock!";
            state = SENTKNOCKKNOCK;
        } else if (state == SENTKNOCKKNOCK) {
            if (theInput.equalsIgnoreCase("Who's there?")) {
                theOutput = clues[currentJoke];
                state = SENTCLUE;
            } else {
                theOutput = "You're supposed to say \"Who's there?\"! " +
                        "Try again. Knock! Knock!";
            }
        } else if (state == SENTCLUE) {
            if (theInput.equalsIgnoreCase(clues[currentJoke] + " who?")) {
                theOutput = answers[currentJoke] + " Want another? (y/n)";
                state = ANOTHER;
            } else {
                theOutput = "You're supposed to say \"" +
                        clues[currentJoke] +
                        " who?\"" +
                        "! Try again. Knock! Knock!";
                scores.put(clientName, scores.getOrDefault(clientName, 0) - 1); // 答题错误，扣除1分
            }
        } else if (state == ANOTHER) {
            if (theInput.equalsIgnoreCase("y")) {
                theOutput = "Knock! Knock!";
                if (++currentJoke == NUMJOKES) {
                    currentJoke = 0;
                }
                state = SENTKNOCKKNOCK;
            } else {
                theOutput = "Bye.";
                scores.put(clientName, scores.getOrDefault(clientName, 0) + 1); // 答题正确，加1分
                if (scores.get(clientName) >= MAX_SCORE) { // 判断是否达到最高得分
                    theOutput += " You win!"; // 返回胜利消息
                    scores.clear(); // 清空得分
                }
                state = WAITING;
            }
        }
        return theOutput;
    }
}

2. 在KnockKnockServer类中添加一个方法，用于返回最高得分的玩家姓名。

import java.util.Collections;
import java.util.Map.Entry;

public class KnockKnockServer {
    // ...

    private static String getTopScorer() {
        if (KnockKnockProtocol.scores.isEmpty()) {
            return null;
        }
        Entry<String, Integer> maxEntry = Collections.max(KnockKnockProtocol.scores.entrySet(), Entry.comparingByValue());
        return maxEntry.getKey();
    }
}

3. 在KnockKnockServer类的handleRequest方法中，将最高得分的玩家姓名返回给客户机。

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;

public class KnockKnockServerThread extends Thread {
    // ...

    public void run() {
        try {
            // ...

            while (true) {
                outputLine = protocol.processInput(inputLine);
                out.println(outputLine);

                if (outputLine.equalsIgnoreCase("Bye.") || outputLine.endsWith("You win!")) { // 判断游戏是否结束
                    String topScorer = KnockKnockServer.getTopScorer(); // 获取最高得分的玩家姓名
                    if (topScorer != null) {
                        out.println("The top scorer is " + topScorer); // 返回最高得分的玩家姓名
                    }
                    break;
                }

                inputLine = in.readLine();
            }

            // ...
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

以上就是对Knock Knock游戏项目的简单修改，增加了计分机制，使游戏过程变得更富有挑战性。