Turn Server into a sealed interface

Sealed interfaces are useful in switch exprs to enumerate possible values
so that it becomes a compile error to introduce a new server type
without implementing an appropriate arm for the switch block.

clientserver/src/main/java/net/rptools/clientserver/simple/server/AbstractServer.java

@@ -20,7 +20,7 @@
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
-public abstract class AbstractServer implements Server {
+public abstract class AbstractServer {
 
   private static final Logger log = LogManager.getLogger(AbstractServer.class);
 

clientserver/src/main/java/net/rptools/clientserver/simple/server/NilServer.java

@@ -15,7 +15,7 @@
 package net.rptools.clientserver.simple.server;
 
 /** A server implementation that never receives connections */
-public class NilServer extends AbstractServer {
+public final class NilServer extends AbstractServer implements Server {
   @Override
   public void start() {}
 

clientserver/src/main/java/net/rptools/clientserver/simple/server/Server.java

@@ -16,7 +16,7 @@
 
 import java.io.IOException;
 
-public interface Server extends AutoCloseable {
+public sealed interface Server extends AutoCloseable permits NilServer, SocketServer, WebRTCServer {
   void start() throws IOException;
 
   void close();

clientserver/src/main/java/net/rptools/clientserver/simple/server/SocketServer.java

@@ -25,7 +25,7 @@
 /**
  * @author drice
  */
-public class SocketServer extends AbstractServer {
+public final class SocketServer extends AbstractServer implements Server {
 
   private static final Logger log = LogManager.getLogger(SocketServer.class);
   private final int port;

clientserver/src/main/java/net/rptools/clientserver/simple/server/WebRTCServer.java

@@ -29,7 +29,7 @@
 import org.java_websocket.client.WebSocketClient;
 import org.java_websocket.handshake.ServerHandshake;
 
-public class WebRTCServer extends AbstractServer {
+public final class WebRTCServer extends AbstractServer implements Server {
   private static final Logger log = LogManager.getLogger(WebRTCServer.class);
 
   public interface Listener {