No class error even though the .class files are present

Getting a Error: Could not find or load main class KnockKnockServer, when running the below code using java KnockKnockServer 4444, even though the class file KnockKnockServer.class is present in the directory.

I was following this link to about learn the server-client working.

 import java.net.*;
 import java.io.*;

public class KnockKnockServer {
public static void main(String[] args) throws IOException {

    ServerSocket serverSocket = null;
    try {
        serverSocket = new ServerSocket(4444);
    } catch (IOException e) {
        System.err.println("Could not listen on port: 4444.");
        System.exit(1);
    }

    Socket clientSocket = null;
    try {
        clientSocket = serverSocket.accept();
    } catch (IOException e) {
        System.err.println("Accept failed.");
        System.exit(1);
    }

    PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
    BufferedReader in = new BufferedReader(
            new InputStreamReader(
            clientSocket.getInputStream()));
    String inputLine, outputLine;
    KnockKnockProtocol kkp = new KnockKnockProtocol();

    outputLine = kkp.processInput(null);
    out.println(outputLine);

    while ((inputLine = in.readLine()) != null) {
         outputLine = kkp.processInput(inputLine);
         out.println(outputLine);
         if (outputLine.equals("Bye."))
            break;
    }
    out.close();
    in.close();
    clientSocket.close();
    serverSocket.close();
}
}

If you don't pass the classpath on the commandline, it considers the CLASSPATH environment variable. And only if that one is not set either, it considers the default one, being "."

Therefore, either unset the CLASSPATH environment variable or pass -cp . on the command line.