java:字符串索引超出范围

我正在尝试学习如何使用套接字，我编写了一个小型echo服务器。我在eclipse上运行代码，然后转到命令提示符并将telnet发送到端口，它应该会回显，但它会给出以下错误：

Exception in thread "Thread-0" java.lang.StringIndexOutOfBoundsException: String index out of range: 20
    at java.lang.String.getChars(Unknown Source)
    at java.io.BufferedWriter.write(Unknown Source)
    at echoserver.run(echoserver.java:40)

这是我的密码：

public final class echoserver extends Thread {
private static final int PORT = 8889;
public static void main(String args[]){
    echoserver Echoserver = new echoserver();
    if (Echoserver != null){
        Echoserver.start();
    }
}
public void run (){
    try {

    ServerSocket server = new ServerSocket(PORT , 1);
    while (true) {
        Socket client =server.accept();
        System.out.println("Client connected");
        while (true){
            BufferedReader reader =
                    new BufferedReader(new InputStreamReader(
                        client.getInputStream()));
            System.out.println("Read from client");
            String textLine = reader.readLine() + "\n";
            if (textLine.equalsIgnoreCase("Exit\n")){
                System.out.println("exit invoked, closing client");
                break;
            }
            BufferedWriter writer = new BufferedWriter(
                    new OutputStreamWriter(
                            client.getOutputStream()));
            System.out.println("Echo input to client");
            writer.write("echo from server:"
                    + textLine, 0 , textLine.length() + 18);
            writer.flush();
        }
        client.close();
    }
}   catch(IOException e){
        System.err.println(e);
    }
}
}

如何修复此错误？

这就是问题所在：

writer.write("echo from server:" + textLine, 0 , textLine.length() + 18);

您的“额外文本”实际上是17个字符，而不是18个字符。不过，您不需要指定偏移量和长度-您可以使用：

writer.write("echo from server:" + textLine);