Why is my method undefined for the type object?

Change your main to:

public static void main(String[] args) {
    EchoServer echoServer = new EchoServer();
    echoServer.listen();
}

When you declare Object EchoServer0; you have a few mistakes.

1. EchoServer0 is of type Object, therefore it doesn’t have the method listen().
2. You will also need to create an instance of it with new.
3. Another problem, this is only regarding naming conventions, you should call your variables starting by lower case letters, echoServer0 instead of EchoServer0. Uppercase names are usually for class names.
4. You should not create a variable with the same name as its class. It is confusing.