What is the task of the Listener interface? Write a socket program for a file server that makes a collection of files available for transmission to clients. When a client connects to the server, the server first reads a one-line command from the client. The command string can be of the form “GET <filename>”, where <filename> is a file name. The server checks whether the requested file actually exists. If so, it first sends the word “OK” as a message to the client. Then it sends the contents of the file and closes the connection. Otherwise, it sends “ERROR” to the client as message and closes the connection. Assume that there is no sub directories.